Blog - Page 29 of 53 - MEP Academy

How Occupancy and Vacancy Sensors Work

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October 24, 2022

How occupancy and Vacancy Sensors Work. Where should occupancy sensors be used, and which type is better, hard wired or wireless? What’s the difference between an Occupancy Sensor and a Vacancy Sensor? What does ASHRAE 90.1 require for the controllability of lights in various spaces? Lighting consumes up to 20% of the total energy in commercial buildings. By adding lighting controls considerable energy can be saved based on space usage type and the type of light source, such as incandescent, fluorescent, high intensity discharge or LED.

If you prefer to watch the YouTube video of this presentation, scroll to the bottom or click on this link. How Occupancy and Vacancy Sensors Work

Why use an Occupancy Sensor?

Occupancy sensors are used to detect motion and provide a response by turning on or off lights or HVAC equipment. By monitoring when spaces are being used, a more energy friendly support system of lights and HVAC equipment can be deployed. Various sensing technologies are used, such as Passive Infrared (PIR) and Ultrasonic. There are other technologies such as Time of Flight (ToF), microwave, and camera based technologies that are not covered here.

Hard Wired and Wireless Passive Infrared Occupancy Sensor

Energy Savings Potential

According to a Commercial Building Energy Consumption Survey, the following savings can be achieved using occupancy sensors for the various space usage types.

Energy is saved by reducing lighting levels or shutting them off when a space is not in use. By adding lighting controls, you can save up to 90% or more of the energy used depending on the space usage type.

Types of Occupancy Sensors

Communication from the sensor can be hard wired or wireless. Although most sensors are probably hard wired there are some advantages to using wireless sensors in certain applications. Wireless sensors can be battery operated or use photovoltaic (PV) powered sensors and can be easily attached to a wall or ceiling. They’re a possible option where it’s difficult or aesthetically unappealing to run electrical wiring.

The material cost for wireless occupancy sensors is usually more than hard wired, but there will be labor savings from not having to install electrical wiring. Also, the ability to easily move a wireless sensor makes more sense for spaces with frequent layout changes.

Where Should I Use an Occupancy Sensor?

The best candidates for occupancy sensors are spaces that are used intermittently, like bathrooms, meeting rooms, storage areas, classrooms, warehouses, private offices, and breakrooms. Areas to avoid putting sensors in are those that are busy or have a consistent occupancy level during a fixed schedule.

Picture of Locations for Occupancy Sensors — Locations for Occupancy Sensors

Two Lighting Control Strategies

The lighting can either automatically turn on when someone enters the room or require the occupant to manually flip a switch. These are referred to as an Occupancy or Vacancy sensor. The difference is that the vacancy sensor needs to be manually turned on, while the occupancy sensor turns on with motion. When the occupant leaves the room, both strategies will have the lights automatically shut off after a certain amount of time.

How do Occupancy Sensors Work?

The problem for the occupancy sensor is to accurately detect when occupants are present and when they have left the space. This includes the ability to recognize occupants working at their desk or on the other side of a partition. There are various occupancy sensors that accomplish these problems better than others. Here are some of the sensor technologies being used and their advantages and disadvantages. The two most used occupancy sensors are either ultrasonic sensors or passive infrared sensors.

Ultrasonic Sensors

An ultrasonic sensor will emit high-frequency sound waves that bounce around the room and objects, but are not heard by humans. The sensor will pick up any movement by noticing a change in sound wave frequency. This is based on the Doppler effect or Doppler Shift, which is the change in frequency of a wave in relation to an observer who is moving relative to the wave source.

Picture of Ultrasonic Occupancy Sensor — Ultrasonic Occupancy Sensor

Since the Ultrasonic occupancy sensor sends out a continuous signal, the electrical power consumed will be higher than the wireless passive infrared sensor. This will require the ultrasonic to most likely be hard wired to provide the constant energy for operation.

Ultrasonic sensors are good at detecting movement that may occur behind a partition or bookcase which isn’t in the direct line of sight of the sensor. This technology will pickup slight movements of an occupant sitting at their desk reading a book or typing on their computer.

The ultrasonic occupancy sensor has a greater detection range than the Passive Infrared Sensors.

Passive Infrared Sensors (PIR)

An infrared technology-based occupancy sensor is best used in smaller open spaces without obstructions that allows it to easily detect movement. These sensors require that the movement occur within their direct line of sight, as they won’t detect movement behind a partition or bookcase.

It’s called passive infrared because it doesn’t send a signal out. The sensor contains a thin film that generates electricity when exposed to heat which occurs when infrared energy is emitted from a warm object passes in front of the floor or wall in line of sight of the sensor. Because the infrared sensor doesn’t constantly send out a signal like the ultrasonic sensor, it doesn’t require the same amount of electrical power. This makes the passive infrared sensor a great option for wireless communication using a battery or PV cells for power.

The wireless passive infrared sensor will need a source of power for the controller that receives the signal from the sensor and interprets an output signal to increase or decrease lighting levels.

Wallbox sensors are made to fit in a standard electrical switch wallbox and use the voltage available at the box. This makes for an inexpensive and easy retrofit of an existing wall switch. They are limited by their range of vision.

How Long Should Lights Remain on After No Motion is Detected?

The time between no motion being detected and the shutting off or minimizing of the lights is dictated by various standards and codes. The National Electrical Manufactures Association (NEMA) has recommended that 15 minutes after no motion has been detected the interior lights should be shut off, and the 2021 IECC and ASHRAE 90.1-2019 have it at 20 minutes. Exterior lighting and parking garages have different requirements The shorter the time delay the greater the energy savings, but you also want to be sure not to have the lights going on and off too frequently.

Restrike Time

Occupancy sensors work best with light sources that have quick restrike times, that’s the amount of time it takes the light source to reach full value. Anyone who has witnessed the outage of a High Intensity Discharge light knows that it takes up to 15 minutes to reach full value. An outage occurred during a recent night-time football game, which had to be postponed long enough for the HID lights to come up to full brightness. This means that occupancy sensors are not a good choice for HID type lights but work well with quick starting light sources like LED, Incandescent and Fluorescent.

Where Should Sensors be Installed?

Sensors are best installed on ceilings or walls where interference from doors or air conditioning air flows are not a problem. Wireless sensors can be installed anywhere if motion is within the line of sight of the sensor. This allows wireless sensors to be installed where hard-wired sensors would be difficult to install.

Range of Sensors (Field of View and Coverage)

Wireless sensors need to be within 15 feet of the floor area or door to work effectively. Check the occupancy sensors manufactures literatures for specifics. If there are multiple obstructions in the space like partitions or bookcases, it may make sense to install hard-wired Ultrasonic type sensors which are better at motion detection in these situations. If wireless sensors are preferred, then more sensors will need to be installed to ensure motion is detected behind any obstructions.

Coverage or field of view can be in a narrow band like 8-degrees for use in corridors or aisles, up to a 360-degree circle, or a 180-degree semi-circle with diminishing capabilities at the extreme angles, and greater distance sensing abilities straight ahead. Sensors are also indicated by their coverage in square feet.

Multiple Levels of Lighting

In areas where there is natural sunlight, there is the option of using bi-level switching. The sensor will detect various levels of natural light and reduce the light fixtures output by half or some other percentage. Existing LED and fluorescent lights may need their ballast or drivers replaced to be compatible with the bi-level capabilities.

HVAC Controls and Occupancy Sensors

Occupancy sensors can save energy when integrated into the HVAC system control sequences. If the room is unoccupied, a signal can be sent to the HVAC equipment to reset the temperature, reduce the airflow or shut the system off. This would reduce energy consumed to condition an empty space.

How Occupancy and Vacancy Sensors work

How HVAC Split System Air Conditioners Work

HVAC Systems

MEP Academy Instructor

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October 20, 2022

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How HVAC Split System Air Conditioners Work. Using a Split System Air Conditioner or Heat Pump is common in residential and commercial applications. It’s called a split system because the indoor and outdoor components are separated or split from each other and are connected by refrigerant piping and control wiring. This differs from a self-contained rooftop packaged unit which houses both indoor and outdoor components. We’ll see different designs using split systems.

If you prefer to watch the YouTube Video of this presentation, scroll to the bottom or click on this link >> How HVAC Split System Air Conditioners Work

Picture of HVAC Split System vs Packaged Rooftop Unit — HVAC Split System vs Packaged Rooftop Unit

HVAC Split systems are convenient to use with existing buildings because it’s much easier to route small refrigerant piping to the indoor coil, then to run much larger air ducts using a rooftop packaged unit. As you can see the rooftop unit needs large openings for the supply and return ducts to enter the building. The split system uses smaller copper tubing and requires a small opening in the roof or wall.

Refrigerant Piping Size vs Ductwork Size

Refrigerant carries more heat capacity then air, which allows small refrigerant piping to easily maneuver through building structures and components as opposed to large air ducts. This is what gives split systems their advantage, except when it comes to ASHRAE 62.1, ventilation air requirements, which we’ll discuss later.

Picture of Refrigerant Piping Space required versus equivalent Ductwork Space required with equivalent tonnage capacity — Refrigerant Piping Space required versus equivalent Ductwork Space required with equivalent tonnage capacity

The split system is made up of the Outdoor unit, often called the condensing unit, because this is where the refrigerant condenses from a gas back into a liquid, and an indoor unit where the evaporator is located. The indoor unit can be called an Air Handling Unit (AHU), Fan Coil or a Furnace with Coil.

Split systems are available from less than one ton to over 100 tons of refrigeration capacity. Split systems come in two basic configurations, either as cooling only or as a Heat Pump, which we’ll explain later.

Picture of Refrigerant Cycle in a Split System HVAC unit — Refrigerant Cycle Split System HVAC

Heating in HVAC Split Systems

Heating can be in the form of a gas furnace, electric strip heater, electric heat pump, hot water, or steam. Here we show two ways of getting heat to the occupied space using a split system. First you can use a split system heat pump, which works to cool the space in summer, and heat the space in winter. See our video on How Heat Pumps Work to understand how they work.

Furnace with DX Coil

The other option is to install a furnace with a coil. The furnace will require some form of fuel such as natural gas for heating. Cooling will be accomplished by installing a evaporator coil on top of the furnace, which is connected to an outside condenser.

Picture of HVAC Split System with Furnace and DX Coil — HVAC Split System with Furnace and DX Coil

The furnace will require combustion air inlet and a means of exhaust combustion gases outdoors. It’s important that the discharge flue remain a minimum of 10 feet away from any air intake, check your local code for the proper distance.

You’ll need to install a condensate drain pipe from the cooling coil drain pan to an approved receptor, like a floor sink or the tailpipe of a sink.

Electrical will need to be installed from a breaker panel to a disconnect switch located near the equipment. The disconnect switch is a safety device that allows any technician working on the equipment to lockout the electrical power feeding the HVAC equipment.

Heating Hot Water Coil

Another option is to use a boiler to provide heating hot water to a coil located inside the air handler. The boiler will need a source of fuel for combustion, in this case natural gas. The heating hot water will need to have a pump to circulate the water to all of the Air handlers in the building.

Here we only show one air handler getting heating hot water, but it could also be dozens more in larger buildings. The heating hot water piping will need to be insulated, most likely with some form of fiberglass pipe insulation to prevent the loss of heat from the pipes. Not shown is makeup water and any other accessories like expansion tanks. This could also be a steam boiler with a steam coil in the air handler to provide heating.

Ventilation Air per ASHRAE 62.1

One of the challenges for split systems is providing the required ventilation air to each of the air handlers. Ventilation air will need to be provided to each space or indoor fan coil or air handler per ASHRAE 62.1. This requires a duct from the outside or from a DOAS unit to the space or fan coil. See our video on “Dedicated Outside Air Systems” for a better understanding.

Picture of HVAC Split System with DOAS Unit for Ventilation Air — HVAC Split System with DOAS Unit for Ventilation Air

Dedicated Outside Air System

This Dedicated Outside Air System filters and conditions the outside air before the fan sends it to each of the air handlers. The Dedicated Outside Air System handles the latent load of the ventilation air so that the air handlers won’t need to be upsized for this additional load.

Each air handler will receive the required amount of ventilation air per ASHRAE 62.1 based on the occupancy level, size of the room and space usage type.

Supply Fan with Filtration

Another option is to provide ventilation air using just filtration and a supply fan, but no conditioning of the outside air. This relieves the indoor air handlers fan from needing to pull in outside air. The disadvantage is that the air handler will need to handle the additional heating and cooling load for the ventilation air.

Picture of HVAC Split System Air Conditioner using Filtered Ventilation Air — HVAC Split System Air Conditioner using Filtered Ventilation Air

Separately Ducted Outside Air Ducts

The lowest first cost option would be to duct the ventilation air individually from each indoor air handler. This would require additional energy of the air handler fan and coil. This would require a lot of small ducts running through the building from each air handler, unless you required an economizer for your indoor air handler because it meets the threshold of your energy code, then the ducts would be much larger and may not make sense.

Picture of Split System HVAC Unit with Individually Ducted Outside Air — Split System HVAC Unit with Individually Ducted Outside Air

As shown before, each air handler has a condensate drain going to the tailpiece of the sink.

Condensing Unit

The condensing unit contains two major components, the compressor, and the condenser coil. The compressor is the heart of the unit and pumps the refrigerant around the piping circuit. The condenser coil provides a means of rejecting the heat to the outdoors using a fan blowing air over a hot coil.

In this picture we can see that this condenser hasn’t been maintained and is covered with dirt. This will reduce the capacity of the unit, so be sure to check your outdoor coil at least once or twice a year to ensure its clean.

Condenser Unit with a Dirty Condenser Coil.

We can see the hot gas discharge piping coming off the top of the compressor and feeding the condenser coil which is responsible for rejecting the heat from the building. But in this poor condition it won’t be working. Here is the filter drier that’s on the hot liquid line leaving the condenser and sending liquid to the expansion valve at the evaporator. Here is the suction piping that has arrived from the indoor evaporator section and is entering the compressor below the discharge piping.

This is the ugly insides. Here is what the condenser may look like from the outside.

Picture of HVAC Condenser (Outdoor Unit) — HVAC Condenser (Outdoor Unit)

There will also be some controls on the outdoor unit that communicates with the indoor unit and provides various safety components to protect the compressor and the other system parts. The communication from a thermostat or space sensor is typical for a residential unit, while in commercial installation there could be the addition of a BACnet card that allows communicating with a Building Automation System (BMS).

Outdoor units will contain one compressor for smaller sizes and multiple compressors for larger systems.

The condenser can also be air-cooled, water-cooled or evaporatively cooled.

Indoor Unit Air Handler (DX Fan Coil)

The Air Handler contains the indoor fan and evaporator coil. The evaporator coil is where the refrigerant absorbs the heat from the building air that is being blown over the coil. This causes the refrigerant liquid in the evaporator to evaporate while cooling down the building air, see our video on “Refrigerant Cycle 101” to better understand how a refrigeration cycle works. The expansion valve or metering device separates the high side from the low side and modulates the amount of refrigerant that passes through it in relationship to demand.

The indoor unit can be ducted or ductless.

Picture of Ductless and Ducted Air Handlers or Fan Coil Unit in a Split System HVAC Unit — Ductless and Ducted Air Handlers or Fan Coil in a Split System HVAC Unit

Residential systems are relatively simple and smaller, while the commercial versions can be very large and have many other options installed within their housing. Commercial air handlers can be packaged, custom made, or built-up with individual components selected and field erected. See our other video on “Air Handling Units” for a better understanding.

Compressors

The smaller residential units under 10 tons will most likely have 1 compressor, while commercial units 10 tons and over can have two or more compressors. Compressors vary in their ability modulate capacity. Smaller system will use a hermetically sealed compressor with a scroll compressor. Larger systems can use semi-hermetic, reciprocating, and scroll compressors in 1 or more configurations depending on size.

Most compressors use oil for lubrication and that oil travels around the refrigerant circuit continuously during operation. It’s important that the oil returns to the compressor at the same rate that it leaves, as the compressor needs to remain lubricated at all times.

Heat Pumps

In addition to the cooling only split systems, there are Heat Pumps which reverse the refrigerant cycle to provide heating also. There is a move in some states and countries to go all electric, such as in California. Heat Pump are all electric so there is no burning of a carbon-based fuel. See our other video on “How Heat Pumps Work” for a better understanding.

One of the differences between a Heat Pump and other cooling only units is that of the metering device. The metering device separates the high-side from the low-side and controls the amount of refrigerant passing through it. The Heat Pump uses a special “Reversing Valve or 4-Way Valve” that allows refrigerant to change direction based on whether in heating or cooling mode. The indoor coil is used as an evaporator to provide cooling during summer, and as a condenser to provide heating during winter. Therefore, you shouldn’t call the outdoor unit a condenser when dealing with a Heat Pump, as it acts as both a Condenser and Evaporator depending on the mode of operation (Heating or Cooling). With a cooling only Air Conditioner, the outdoor unit is always the condenser.

Split System Sizing and Mixing and Matching

Splits systems are often chosen where the outdoor and indoor unit tonnage match, but that is not always the case. Split systems can have the outdoor unit slightly larger or smaller than the indoor unit. By upsizing the outdoor unit, this will allow more latent heat for the indoor unit. The tonnage of the indoor unit can be larger than the outdoor unit to increase the sensible heat ratio and airflow (CFM). Providing an indoor unit that is one size larger or smaller than the condenser maybe safe, but check with the AC Manufacture when mixing outdoor units and indoor unit capacities. This is not allowed with most Heat Pumps.

Residential systems commonly use split systems of 5-Tons and under or multiples of these sizes for larger homes. You might find a 3-Ton unit serving the first floor and another 3-ton unit serving the 2^nd floor in a two-story home.

Refrigerant Piping and Number of Circuits

For small split systems where the indoor and outdoor units are relatively close and the pipe routing simple, the use of line sets maybe used. Line sets are available in a coil of ACR copper and are pre-charged with refrigerant, in lengths of 15, 25, 35, 50 or 100 feet. This saves on labor by avoiding field brazing of joints for fittings and couplings. Piping distances are often restricted by equipment manufactures so be sure to check the literature for these requirements if your distances exceed 100 feet or more.

Picture of Dual Refrigerant Circuits in a Split System Air Conditioner — Dual Refrigerant Circuits in a Split System Air Conditioner

Most small split system have only one circuit, but as the HAVC unit gets larger there are dual circuit units available. Dual circuits will require twice as much piping as a single circuit HVAC unit. There will be two suction lines and two liquid lines running from the indoor unit to the outdoor unit. With dual circuits the ability to control capacity is increased in addition to having a backup or redundant circuit incase a compressor burns out. Each circuit has its own compressor.

The DX coil can also be setup in either a single or dual circuit arrangement. The refrigerant can pass through the coil using one path or two. If you’re using a dual circuit condenser than you’ll have a dual circuit indoor coil. Its possible to have a single circuit condenser and a dual circuit DX coil.

Refrigerant Piping Insulation

This is where most installations that we review are lacking in code compliance and application. The proper application of insulation is important in maintaining a well-functioning HVAC split system. If insulation is poorly applied or if the wrong insulation is used, then there will be a LOSS in capacity of the system. This means that the number of tons of air conditioning that was installed will be less than that. See our video on “Refrigerant Insulation” for the proper installation and type of insulation to be used when installing refrigerant piping.

Condensate Drain

Cooling coils remove moisture from the incoming air, and that moisture condenses on the evaporator coil. The water trickles down the coil and is captured in a drain pan that sits under the cooling coil. The drain pan needs to have some way to empty this accumulation of condensate water. Piping is provided from the drain pan to approved receptor, which could be a floor sink or the tailpiece of a regular sink.

The drain piping needs to pitch in order to avoid overflowing the drain pan. If the piping can’t be pitched per the slope required by the local code, then a condensate pump will need to be added. The condensate pump will raise the level of the water in the piping so that it can meet the required slope or pump directly to the receptor. See our Video on “Condensate Drain Piping”.

Controls

The controls on most split systems are very simple, especially residential split systems. A thermostat calls for heating or cooling and the system initiates startup. For systems with more than one circuit, if the first circuit can’t satisfy the demand then the second circuits solenoid valve would open adding additional cooling capacity to the system. The circuit board for controls is usually mounted in the outdoor unit with control wiring connect the indoor unit to the outdoor unit.

Adding Building Management System (BMS) oversight or connectivity is an option usually implemented on commercial properties. This allows the building engineer or management company complete oversight using graphical interfaces to see what is happening with each system connected to the BMS.

In addition to controlling the overall unit, the system has safety controls that operate to protect the equipment. This could include high and low pressure switches to prevent excessive and minimum pressure levels respectively. There are safety devices to protect the equipment from too much electricity or excessive temperatures.

Advantages and Disadvantages of an HVAC Split System Air Conditioner

One of the biggest benefits is that they come as a packaged unit, already engineered by the manufactured with matching outdoor and indoor units.

Split systems are less invasive for remodeling projects, allowing smaller refrigerant piping to be run to a space instead of larger air ducts. This is beneficial when the indoor space is several floors below the roof, as no vertical duct shafts are required as with a Rooftop unit.

Split system outdoor units weight less than a one piece packaged unit.

Split systems require smaller openings in the structure for refrigerant piping to pass from the outdoor unit to the indoor unit, as opposed to air ducts.

Split System outdoor units are smaller than Rooftop Units, so they are easier to hide. This can be important when the building has large skylights or line of sight code compliance issues.

Split systems usually cost more on new construction projects because they require several points of connection for the electrical and refrigerant piping to connect the outdoor and indoor sections together.

Split systems will require some method of getting the required ventilation air to each space or indoor fan coil per ASHRAE 61.2

How HVAC Split System Air Conditioners Work

Top 6 HVAC Control Strategies to Save Energy

MEP Academy Instructor

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October 10, 2022

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Top 6 HVAC Control Strategies to Save Energy. In this video we’ll learn the top 6 Control Strategies for reducing energy consumption. These are control strategies to reduce cost and save energy. The control strategies are programmed into the sequence of operation for the control of HVAC systems.

We’ll learn about using trim and respond (T&R) setpoint reset logic for controlling chillers and air handlers. The energy savings from these strategies vary by geographical area and building types.

If you prefer to watch the YouTube Video of this presentation, then scroll to the bottom or click on this link. Top 6 HVAC Control Strategies to Save Energy

#6 Supply Air Temperature Reset

To save energy the supply air temperature can be reset higher when the demand is reduced. This will save on compressor energy use and allow more time for the use of the air side economizer.

Using Trim and Respond control logic if no request for cooling is sent to the air handler controller, than the supply air temperature is trimmed by a set value that is determined by the programmer. Let’s say that if no request is made the supply air temperature is reset 0.2°F, this would move our setpoint to 55.2°F. Every two minutes or whatever duration is set by the controls programmer the trimming of the supply sir temperature will occur until a request is made from a zone.

HVAC Controls Strategy - Supply Air Temperature Reset — HVAC Controls Strategy – Supply Air Temperature Reset

The VAV zone boxes can be programmed to make a request when any VAV box damper reaches 90% open. When the request is sent, the supply air temperature will respond by decreasing by a certain value, chosen by the programmer, let’s say by 0.3°F. If our temperature was trimmed up to 58°F, before a request was made, the request would decrease our supply air temperature to 57.7°F.

If multiple VAV boxes send request at the same time, then the value is multiplied by the number of request. If 4 VAV Boxes sent a request for more cooling, then the Supply Air Temperature could be reset by 4 times 0.3°F, for a total of 1.2°F decrease. The controls programmer will limit how much the supply air temperature can be decreased at any one time, no matter how many request are sent for additional cooling. This allows system stability.

Systems using an air-side economizer will have more time when the economizer is in use because of the supply temperature being set higher. This allows the outdoor air to be used instead of the compressor to provide cooling, which saves on energy.

#5 Demand Control Ventilation

The reason it’s called Demand Controlled ventilation is that it controls the amount of ventilation based on the demand. Demand changes with occupancy levels, so some form of occupancy monitoring will be used as the control strategy. Ventilation air is required by ASHRAE standard 62.1, but providing ventilation air is energy intensive and costly. Providing only what’s needed can save on energy and money.

With ASHRAE 62.1 the ventilation requirements or outdoor air is increased based on the number of occupants and square footage of the space. Demand controlled ventilation is based on providing outdoor air in proportion to the occupancy level.

Energy is wasted in the form of conditioning excessive amounts of ventilation air when the occupancy level is less than the design maximum. By using demand-controlled ventilation, the occupancy level can be monitored, and the quantity of ventilation air adjusted downward according to actual occupant levels. Tracking the occupancy level with DCV reduces energy consumption by only bringing in enough ventilation air to satisfy the current level of occupants, not the design maximum.

Using a conference room for our example. If the meeting room was designed for 125 people, but only the presenter is currently in the room waiting for attendees. The CO2 monitor would pick up the low quantity of carbon dioxide and close the outside air damper to match the occupant level. As more attendees arrive to the meeting, the level of CO2 increases, and the CO2 Monitor picks up this increase and sends a command for the Outside Air Damper to open wider, allowing more outside air into the space. As, the occupants leave the system will begin to close the outside air damper. This is a basic explanation of how DCV works, for a further explanation see our video on Demand Controlled Ventilation.

#4 Optimal Start

HVAC equipment has often been setup to use a time clock or time-of-day schedule to turn the system on and off based on a proposed occupancy schedule. The time clock will be set to start the HVAC system equipment before the occupants arrive and to shut it off at a preprogrammed time when the building is scheduled to be vacant. The schedule of when to turn on the HVAC equipment in the morning to ensure the space is comfortable for the occupants is often based on the worst summer or winter conditions. This creates an opportunity to save energy by providing an optimal start time when conditions are not at these peak conditions.

Optimal Start of HVAC Air Conditioning Equipment

With an Optimal Start strategy the building automation system will select the best, or optimal start time based on the conditions existing for that day. The buildings control system will determine how much time, or how early the HVAC equipment needs to be started to ensure the space is comfortable for the occupants when they arrive. The building automation system will prevent the HVAC equipment from starting too early, which is a waste of energy. The controls will ensure that the system is started with just enough time to have the space conditioned when occupants arrive, no sooner.

Using the capabilities of a building automation system to track historical data and use that data for current conditions is how this strategy is implemented. The controls system receives input from space sensors on the current temperature and compares this to the setpoint temperature. With historical trending data the building automation system can determine how long the HVAC equipment needs to run to make up the difference between this setpoint and the current space temperature. The greater the Delta-T, the earlier the HVAC equipment will be started.

This control strategy saves energy by optimally starting equipment and avoiding conditioning spaces to setpoint temperatures earlier than required. This saves on operational hours of the HVAC equipment, allowing the equipment to last longer.

#3 Shorten HVAC Schedule

Often HVAC schedules are set for longer time durations then the actual occupants require. Shortening the HVAC schedule works well with an optimal start and stop control strategy. This strategy avoids turning on HVAC equipment before they are needed and quickly turns them off when occupants are not scheduled to be in the building. The building automation system learns with historical trending data when it’s the best time to turn equipment on. The controls system uses outdoor air conditions, indoor temperatures and set points for heating and cooling during occupied mode.

There may be spaces within the building that operate only on specific days of the week. If this space is on the same system as the regularly scheduled equipment, it might payoff to provide a separate system or method of controlling the conditioning of this space.

HVAC Equipment operating Schedules - Separate System for irregular Scheduled Spaces — HVAC Equipment operating Schedules – Separate System for irregular Scheduled Spaces

Working with optimal start to determine the best time to start the equipment, including night setback temperatures, this strategy ensures that the equipment is only running when the building is occupied.

#2 Minimum VAV Box Damper Flow Reductions

Systems using Variable Air Volume boxes have their dampers set to a minimum open position to ensure that there is always ventilation air per ASHRAE 62.1. Often this minimum open setting is greater than the actual CFM required of the space, as the space will most likely be influenced by the internal load which will drive the damper wider open, negating the minimum setpoint. Set the VAV box damper open to the minimum mandated to meet ASHRAE standard 62.1 for ventilation and no more. See ASHRAE 62.1 Appendix “F” for a Simplified Ventilation Rate Calculation for multiple zone recirculating systems.

When the minimum damper position is set too high, then fan and cooling energy is wasted. Too much cold air is being sent into the space, requiring additional energy to reheat the air. This can be avoided by reducing the minimum setpoint, and by using this Minimum VAV Box Damper Flow Reduction strategy. This will of course depend on the space type and usage.

#1 Wider Dead Bands and Night Setbacks

On average, this was the #1 way to save energy.

DeadBand

The deadband is the temperature range when the HVAC system is neither calling for heating or cooling.

By widening the dead band, the amount of time that the HVAC system would save energy would increase. In California zonal thermostats that are used to control both heating and cooling shall be capable of providing a deadband of at least 5°F.

Night Setback

When the building is unoccupied during the nighttime hours, the HVAC system temperature can be setback in the range of 6° to 10°F. This allows the building to become warmer or cooler at night then during the day, but avoids letting it get too far from setpoint. By allowing the HVAC system to increase or decrease from the setpoint, depending on whether in cooling or heating mode, the system will come on less during the night when the building is unoccupied, thereby saving energy.

The system will come on only to avoid extreme temperature conditions. For instance, if your normal daytime setpoint for cooling is 76°F, you could program the DDC system to bring on the HVAC equipment to cool the space after hours or off schedule if the building temperature reached 86°F.

You can see that this works directly with the Optimal Start Control Strategy which is to ensure there is enough time in the morning to cool the space down to its normal setpoint. This is because it was prevented from maintaining normal setpoint temperature of 76°F during the night, hence night setback.

It’s important the system be put in the unoccupied mode; this is to ensure that the outside air dampers are closed. Bringing in ventilation air in unoccupied mode can increase energy consumption unless outdoor conditions are in the economizer range for free cooling of the space in summer.

Top 6 Control Strategies to save Energy

Water Coils Explained

MEP Academy Instructor

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October 2, 2022

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Water Coils Explained. In this presentation we’ll learn how coils are constructed and how to select coils for optimizing energy and performance. We’ll learn all the parts of the coil assembly so that you can discuss coils with anyone including engineers. We’ll look at Chilled water and Heating Hot Water coils. Coils can be found in Air Handling Units, Fan Coils, VAV Boxes and Induction coils to name a few.

If you prefer the YouTube Video of this presentation then scroll to the bottom or clink on this link. Water Coils Explained.

Coils are used to transfer heat between two mediums, often water and air. Air passes over a coil and the water passing through the coil either absorbing or giving up its heat. The greater the temperature differential between the air and water, the greater the potential heat transfer.

Coils can be installed with just one or multiple coils in a stacked or side by side configuration within HVAC equipment or as part of a built-up system. Built-up systems are used on large projects and require special structural supports to hold them in place.

First we’ll cover coil construction. Most coils used in the HVAC industry are constructed using copper tubing ranging in size from 5/16” up to 1” O.D. You may find 5/8” to be the most commonly used size. The tubes go back and forth from one end and back again, each time they go up and back that is considered 2 passes, as the coil has traversed the face of the coil twice.

Rows of Coils

The tubing can have anywhere from 1 row of coils which is common in small VAV Boxes using reheat coils, all the way up to 12 row coils for larger chilled water systems.

By adding additional rows to a coil, you can increase the heat transfer capabilities if the fins per inch remain constant. But by increasing the number of rows in a coil, the pressure drop across that coil will increase also, causing the fan to work harder, we’ll discuss more about this later.

HVAC Coil Construction - Rows of Coils and Number of Tubes on Face of Coil — HVAC Coil Construction – Rows of Coils and Number of Tubes on Face of Coil

To count the rows of coils there are several methods, first look at the end of the coil without the header and count the return bends or hairpins. The hairpins are 180-degree fittings that turn the flow back into the coil. Each hairpin accounts for two rows if aligned horizontally, so if you have two hairpin turns horizontally aligned, then you have a 4-row coil. If looking at the header, there would be one row for each header, which isn’t visible when looking directly at the end.

The coil will be housed in a casing often constructed of galvanized sheet metal. Coil casing material can be galvanized steel, carbon steel, stainless steel, copper, or aluminum. Typical casing thickness for galvanized steel is 16 or 14 gauge. The casing is the structural support frame that holds the tubing.

Coil Fins

The coil tubing is covered with fins which increase the heat transfer surface to improve energy transfer from the air to the water in the coil.

In the HVAC industry coil fins are customarily made using aluminum, but are also available in copper, and in restricted sizes for stainless steel and carbon steel. The fins surface area increases the contact area of the air passing through the coil. This increases the efficiency of the coil. The thickness of the aluminum fin material is very thin and can range from 0.0045 to 0.016”.

The tubing slides through the openings in stamped sheets of metal that comprises the fins, acting as a guide. The fins are spaced close to one another, and the quantity is referred to as the fins per inch (FPI), or fins per foot (FPF). Coils can be provided with anywhere from 4 to 20 fins per inch.

Remember the increased surface area, meaning more fins per inch, or more rows will increase the heat transfer capabilities but at the cost of fan energy. Coil manufactures software will pick the best coil, with the least number of rows to meet the project requirements.

The fins are provided in various patterns and configurations. The fins are attached by forcing a mandrel through the copper tubing, causing the tubing to expand and tighten against the fins.

This ensures a good bond between the tubing and the fin which creates good thermal conductance for heat transfer.

The fins surrounding the coils can be configured in various shapes from flat to corrugated.

Right-Handed or Left-Handed Coils

Coils are considered left-handed or right-handed. The reason this is important is that when the coil is installed you want to make sure that the piping connections to the coil are on the correct side of the equipment to avoid extra piping.

This can be determined by standing in front of the coil looking in the direction of the airflow, with the air at your back. If the connection is on your right, then it’s considered a right-handed coil connection, and if it’s on the left, than its left-handed.

Coil Headers

Headers are provided for the supply and return connections to the coil which can be provided in materials such as copper, carbon steel, stainless steel, and red brass. Piping connections can be provided in both male or female threads, grooved, welded, or brazed/sweat. Male threaded connections are common up to 2-1/2”.

Coil Header Pipe Connections (Threaded, Grooved, Welded, Brazed/Sweat)

The header serves several purposes, one is to provide a method to connect all the individual tubes in the coil to one larger pipe, that can then be used to connect the system piping to one or more inlet and outlet connections. The header allows the water to be distributed to the smaller connecting tubes.

The term coil passes refers to the number of times water traverses from one end of the coil to the other, starting at the inlet. Going up and back would be one round trip and considered a 2-pass coil, once across and then the second pass would be returning.

The face area is considered that portion of the coil which air passes through. This would exclude the casing area. To calculate the face area of a coil you would measure the height and length from the inside of the coil casing. This is the area that the air travels across and is often referenced by the square feet of face area.

Coil Circuiting

Circuiting refers to the path of travel from when the water enters the coil until it leaves the coil. Starting where the tube or circuit attaches to the supply header and then travels back and forth through the coil until it attaches to the return header is considered a circuit.

The longer the path for the water, the greater the pressure drop and the lower the flow rate, which gives the water more time to absorb or reject heat to the air blown over the coil.

Coil Air velocity

The air velocity over the coil will affect the size of the coil and the HVAC equipment cabinet that the coil is installed in. The slower the velocity, the larger the coil and cabinet that houses the coil. The reason for slower velocities might be for energy savings by reducing fan size, but there will be a slight offset in additional pump and chiller energy.

The air velocity is indicated in feet per minute (FPM) across the face area of the coil. This can easily be calculated if the CFM and the Face Area of the Coil is known. The following formula can determine the velocity.

Velocity = CFM (Ft3) / Face Area of Coil (Ft2) = FPM

Velocity = 10,000 CFM / 20 Ft2 = 500 FPM

We can also use this formula to determine the required face area of the coil. Taking our CFM and dividing it by our velocity.

Face Area of Coil (Ft2) = CFM (Ft3) / Velocity (FPM) = Ft2

10,000 CFM / 500 FPM = 20 Ft2 Face Area of Coil

Water Coil Comparisons and the effects of Air Velocity on Coil Area

If the velocity is too high then condensate can be blown downstream of the drain pan. Moisture carryover can leak out of the ductwork and cause damage to the building or within the air handler.

Coil Velocity and Flow

Water coils control capacity by modulating the flow or the temperature of the water entering the coil. When the temperature of the water is adjusted to track demand then the velocity or flow remains relativity constant. When flow is used to control capacity the velocity through the tubes will change when the control valve modulates the quantity of water through the coil. Some coil manufacturers suggest velocities from 1 to 12 feet per second.

When water velocities are too high there can be tube erosion, noise and high pressure drops. If velocities are too low, there could be tube fouling, air trapped in the coil, poor water distribution and the risk of freezing.

Less water is usually flowing in a Heating Hot Water coil as compared to a Chilled Water coil due to the different delta-t temperatures used in each system. Heating hot water systems use higher Delta-T’s allowing for lower flow volumes (GPM)

Water Coil Coatings

Coatings are provided to protect the coil from harsh environments, such as that encountered near coastal areas or offshore where the equipment is exposed to salt water. They’re also beneficial for harsh industrial environments such as oil refineries and electrical generation facilities.

Copper Tubing with Copper Fins for additional corrosion protection

After the coils are manufactured, they’re shipped to the coating factory where the coils are washed to remove any dust, dirt or residue from fabrication and transporting. Next, they’re rinsed in several baths of deionized water before they enter the coating tank. After the protective coating is applied the coil will go through several rinsing tanks or spray booths before entering a large oven to ensure that the coating is cured. The curing process ensures that the coating will adhere to the coil and prevent it from flaking off, pitting, while avoiding corrosion. The complete coil surface, fins and cracks and crevices will be covered with the coating, while still allowing air flow and heat transfer.

There is an option to provide an additional coating to protect against the UV rays of the sun, just like you might put on sunscreen before heading out for a day at the beach.

There is also the option of providing copper fins which adds another level of corrosion protection, but also adds additional cost. Another option for less sever applications would be to have just the fins precoated before assembling.

Bypass Air

A small percentage of air will bypass the coil and be untreated. This is a very small percentage that is based on how many rows and fins per inch the coil has. The amount of air that is bypassed is reduced when there is an increase in coil rows or fins per inch, or a reduction in air velocity. This accounts for less than 1% of the total air.

Coil Arrangement

Coils can be arranged within HVAC equipment or as stated previously, as part of a bult-up system. Coils can be before the supply fan which is called a Draw-Thru arrangement, or the coils can be after the fan in which it is referred to as a Blow-Thru.

Duct Mounted Coils

These coils are usually used for reheating and are installed in the ductwork feeding the zone. They can be attached to the ductwork with flanges or with a SMACNA standard S & Drive connection.

Venting and Draining

It’s important to provide a method of removing air from a coil and having the ability to drain the coil. A connection at the top of the header provides for the removal of air, while a connection at the bottom of the coil provides a method for draining.

Coil Selection

Most coil manufacturers have coil selection software to quickly provide coil options on the entered design parameters. If an increase in capacity is needed, there are four common ways to achieves this. The first would be to increase the face area of the coil, next would be to increase the number of rows of coil, another would be to increase the number of fins per inch, and lastly provide closer tube spacing.

Coil Flow (Counter Flow or Parallel Flow)

The counterflow coil arrangement is more efficient for a chilled water coil, with a supply temperature range of 42 to 44

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Rows of Coils

Coil Fins

Right-Handed or Left-Handed Coils

Coil Headers

Coil Circuiting

Coil Air velocity

Coil Velocity and Flow

Water Coil Coatings

Bypass Air

Coil Arrangement

Duct Mounted Coils

Venting and Draining

Coil Selection

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