How Gas Furnaces Work

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December 6, 2022

How Gas Furnaces Work including condensing and non-condensing furnaces

How Gas Furnaces Work. In this presentation we’ll cover how a furnace works and explain the various components, including the different types of furnaces that are used mostly in residential spaces, but also used in various commercial applications. Natural gas is the mostly used fuel for furnaces. We’ll explain the difference between a condensing furnace and a non-condensing furnace. We’ll discuss how furnaces are rated for energy efficiency and the type of flues used for each type of furnace.

If you prefer to watch the video of this presentation, then scroll to the bottom or click this link. How Gas Furnaces Work

How Gas Furnaces Work. Here is a vertical Gas Furnace with bottom return air.

Construction of a Furnace

The furnace can be installed vertically or horizontally in a closet, basement, attic, crawl space or equipment room.

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The furnace is housed in a sheet metal casing. The latest models use a variable speed fan to blow air over the heat exchanger. These types of fans have direct drive ECM motors that are much quitter and run more efficient than traditional fans. The fans don’t handle a lot of static pressure, often a maximum of 1” W.C. (0.249 kPa). Warning “Installation and Service should be performed by a qualified installer, service agency or the gas supplier”.

Gas piping will be installed from the gas meter, or from a branch off the main gas line, and connect to an internal Gas Valve and manifold in the furnace. A gas shutoff valve should be provided at the furnace for isolation of the gas. The shutoff valve needs to be in a visible location close to the furnace for ease of use in an emergency or for maintenance. Between the shutoff valve and the furnace should be a union and drip leg. The union provides for the gas piping to be removed from the furnace if required.

Gas Furnaces - Vertical and Horizontal Furnaces — Gas Furnaces – Vertical and Horizontal Furnaces

A Drip leg should be installed to trap any condensate in the gas piping and prevent it from entering the furnace. The drip leg should be located away from where it could freeze. After the piping is installed, it will need to be pressure tested to ensure there are no leaks. The pressure test can be done using compressed air or an inert gas like nitrogen. Never use oxygen to pressure test gas piping. The use of an electronic leak detector can help identify any leaks indicated by a failed pressure test. Obviously, a flame would never be used to find a leak.

Pipe size depends on the volume of gas required and the size of the furnace, but for a small residential unit the gas piping might be just a 1/2”(20mm) pipe. The gas valve controls or modulates the flow of gas to the burners. The burners convert the fuel and air into heat that enters the heat exchanger.

Furnace Heat Exchangers

With a condensing furnace there will be two heat exchangers, a primary and secondary heat exchanger. With non-condensing furnaces there is only one heat exchanger. The heat exchanger is where the room air absorbs heat from the burning fuel. By having two heat exchangers, the condensing furnace allows for additional heat transfer and lower flue gas temperatures. The secondary heat exchanger will most likely be made of stainless steel to prevent corrosion as condensate will occur at lower flue gas temperature.

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Combustion Air

The combustion air will go up through the heat exchanger and to the outdoors, while the blower fan sucks in return air and blows it through the heat exchanger, then out to the spaces to be heated. The air delivered to the spaces and the burning fuel with its combustion gases are on opposite sides of the heat exchanger wall and never encounter each other. Some manufactures provide removability of the heat exchanger for easy serviceability.

Combustion and flue exhaust will need to be provided. With a condensing furnace this is accomplished by installing a combustion inlet pipe to the outside (direct venting) or inside (non-direct venting), and a combustion flue exhaust to the outside. If using a condensing furnace, then vents can use 2” to 4” PVC as the flue gas temperatures are much lower on these types of furnaces. For non-condensing atmospheric furnaces its common to use “B” vent, check the furnace manufacture and local code authority for venting requirements.

There will be a direct spark ignition or hot surface ignition system for the lighting of the gas when heating is called for. Along with the direct spark ignition is a remote sensor that confirms that the gas is burning. This is a safety device to prevent gas from being released if no flame is sensed.

Condensate from Condensing Furnace

A condensate pipe will need to be installed for any condensing furnace installed. Condensing furnaces are more efficient, but the byproduct is condensate that needs to be routed to a drain. The drain will have to run through a neutralizing filter because of the acidic nature of the condensate. This neutralizing kit can be provided by the manufacture or bought separately.

Condensing Furnace - Condensate Neutralization Kit — Condensing Furnace – Condensate Neutralization Kit

Newer models will have integrated circuit control boards that could contain dip switches for easy setup adjustments.

Electrical

Electrical conduit and wire will be run from the breaker panel to the furnace. These furnaces require only 115 volts, single phase power to the unit. Inside the unit should be a transformer that reduces the 115 volts down to a lower control voltage.

There will be a safety switch on the cabinet door leading to the blower, so that when the door is opened the blower is shutoff. This prevents someone from getting injured from a moving fan wheel and motor during service.

Filters

Filters can be provided on the bottom or side inlet to the fan housing by adding a filter rack if one isn’t provided. Remember that the furnace should never be run without a clean filter in place. See our video on “Air Filters and Filter Housing Basics” to understand filters and how they’re rated.

Smart Thermostat

Look for furnaces that have some form of communication with a smart thermostat to provide system information and alerts to system problems. There are also mobile apps that allow for notification and setting adjustment remotely, including routine maintenance alerts. The thermostat should allow for modulation of capacity. There will be a control wire access port for the thermostat wiring coming from the conditioned space.

When the thermostat sends a signal to the furnace to provide heat, the combustion air fan will start to ensure proper air for the combustion of the gas. Then to confirm that the combustion fan is running there is some form of pressure switch that picks up pressure created by the fan which then energizes the hot surface igniter. After a preset time, the hot surface ignitor will allow gas to flow to the burners. After another preset time that the burners are operating, the blower fan will start and blow air out of the furnace into the space. If the system fails to lite the burners after various attempts, then shutoff the system and contact a service company to determine why.

Thermostats often allow for two settings. One is auto, which lets the system cycle on and off to meet demand. The other is fan only, with this setting only the fan runs continuously without any heating.

If adding a split system air conditioning unit to the installation, the DX coil will mount on the discharge of the furnace and share the same fan for air flow.

The furnace can weight from 100 to 200 pounds depending on the heating capacity of the unit.

Upflow Furnaces

With up flow, vertical furnaces, the Return air will enter the bottom of the furnace or side, and travel through a filter before entering the fan section where the air is blown through the burner and then the heat exchanger. As the air leaves the furnace it enters a plenum and then through distribution ductwork to deliver the heated air to each room.

Downflow Furnaces

Some buildings use the crawl space for the distribution ductwork and a furnace that provides a downflow configuration. The return air enters the top of the furnace with the supply air exiting at the bottom.

Horizontal Furnaces

When using a horizontal furnace, they can be installed in attics, crawl spaces or an open space. When the furnace is concealed above a ceiling or installed in an attic, there should be an area provided in front of the furnace for maintenance personnel to access the unit.

Checkout these Gas Furnaces here

Natural Gas Regulators

Gas supplied to the building is usually more than the furnace can handle. At the gas meter there should be a gas regulator that reduces the gas pressure from the utility before it enters the meter. The utility company usually sets the downstream pressure between 5 and 7” W.C. (1.2 to 1.7 kPa), or what is required for special equipment. For a gas furnace that is running the pressure can vary by make and model but should be in the 3.5” W.C. (0.87 kPa) range. There are commercial versions that have higher running pressures, be sure to check the name plate for the correct pressure. Also note the maximum pressure rating of the furnace as this value should never be exceeded. For other gas types check the manufactures literature for operating pressures.

Combustion Intake and Exhaust

A sealed combustion furnace is the best option when selecting a furnace. This is when outside air is brought directly to the combustion chamber and exhausted through the flue to the outside. This lessens the chance of combustion byproducts from entering the occupied space. With non-sealed combustion chambers, air is drawn into the unit for combustion and then exhausted through the flue. These high efficiency sealed combustion chamber units exhaust gas that is acidic and can damage old unlined chimneys. In these cases, a new flue may need to be installed or the old chimney needs to be lined.

Combustion air is required for all fuel burning appliances. If the furnace is in an enclosed space, then the proper amount of air will need to be brought into the enclosure to allow for proper combustion. No matter what the enclosure size is, there needs to be provisions for combustion air. Combustion air mixes with the fuel to provide the proper ratio for clean burning of the gas

Gas Venting

Vents are classified into one of four categories. Category I and II vents are for natural draft gas appliances that operate at a negative pressure. They depend on the natural draft of warm air. Most contractors are familiar with Type “B” double wall vent allowed under category I systems. Type “B” flue vent has an inter duct made of aluminum to resist corrosion, with an outer duct made of galvanized steel for strength with an air gap in between. Because high air rises the use of any horizontal flue pipe needs to be slopped at least a 1/4” per foot.

Category III and IV vents are operated at a positive pressure and use a fan to move flue gases out of the system. They require a tighter seal due to the positive pressure.

For condensing furnaces either direct venting or non-direct venting is used with 2” to 4” PVC piping or equivalent as approved by local codes.

Carbon Monoxide Detectors

With gas furnaces there is always the chance that the heat exchanger will leak and bypass air into the occupied space. This combustion gas is poisonous and could cause headaches, dizziness, weakness, upset stomach, vomiting, chest pain, confusion and even death.

Remember that carbon monoxide is odorless, so you won’t smell a leak. Therefore, installing a Carbon Monoxide Sensor is so important. Also, carbon monoxide can’t be seen as it’s colorless. It also can’t be heard, so your own human senses can’t be relied upon as your eyes, nose and ears won’t detect anything before the effects of the carbon monoxide could be too late, so get CO sensors installed to protect the occupants.

Gas Ignition System

Whenever there is a demand for heating, the furnace needs to ignite the gas in a safe manner. Large systems will use a two-step process, where the first step is to ignite a pilot flame and once confirmed will initiate the main burners.

Old furnaces used a standing pilot where a small amount of gas was always burning in wait for a call for heating. Current gas ignition designs use some form of automatic ignition device without the need for a standing pilot flame.

Furnace Safety Switches

Safety is paramount when supplying a combustible fuel to an appliance that serves occupied spaces. There are various safety devices contained within the furnace to provide for safe operation, in addition to installation requirements for safety.

Flame sensors ensure that the gas has ignited and that there is a flame burning. If the flame didn’t light for some reason and the gas valve had opened, the space could fill with gas and cause a possible explosion.

A Rollout switch senses if the burner flame is rolling backwards out of the heat exchanger. The rollout switch senses the flame and shuts off the flow of gas to prevent a fire.

A High Temperature Limit Switch prevents excess temperatures from occurring. Excess temperatures are an indication that something might have failed or malfunctioned. The High Limit switch will shutoff the gas if the discharge air temperature exceeds the limit. The high limit switch will have to be manually or automatically reset. If automatic, then the gas valve will be reopened when the temperature drops below the limit.

Operation and maintenance clearance requirements. Check the furnace manufacturers literature for the clearance requirements around the furnace. These clearances are for safety reason, such as preventing a fire from occurring to the surrounding construction materials. Clearances are also established for proper access to internal components for maintenance personnel.

Furnace Rating AFUE Rating

The annual fuel utilization efficiency (AFUE) indicates a furnaces efficiency. The higher the value the greater the efficiency. You can find the efficiency listed on the front of the furnace as its required by the Federal Trade Commission to be posted. The efficiency is given as a percentage, indicating how efficient the furnace is in converting the energy from fuel into useful heat on an annual basis.

The annual fuel utilization efficiency (AFUE) is a ratio indicating the total annual heat provided divided by the total fuel consumed. A ratio of 90% would indicate that 90% of the fuel was efficiently converted into heat, while 10% was wasted. This ratio is for the furnace only and not the distribution of the heat to the space through ductwork, as poorly insulated ducts will reduce the efficiency of the overall system.

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Maintenance of your Furnace

Be sure to change your filters often as clogged filters can cause airflow restrictions. With less air flowing over the heat exchanger, it could cause the heat exchanger to become too hot leading to cracking. The cracked heat exchanger would then leak carbon monoxide into the occupied space endangering the lives of the occupants.

Make sure to have your furnace inspected every year including a full-system cleaning, removing all dirt, soot, or corrosion. The inspection will help identify any potential problems with the furnace. Its important that the heat exchanger be inspected for any possible cracking or problems.

Install CO sensors close to all rooms used for sleeping. These can be battery operated or battery backed up versions.

Make sure to regularly check that the CO monitors are operating properly.

Inspect the condition of the flue vent and all connections to ensure a properly sealed system.

Clean and lubricate the fan blower.

Retrofitting Existing Furnaces

A retrofit of an existing furnace can increase it energy efficiency. The additional cost to make the retrofit needs to be evaluated against a new replacement unit. If you plan on adding air conditioning, then a new system is warranted. Remember you’re not only saving on your fuel bill but you’re reducing carbon dioxide emissions.

Before you upgrade your furnace, it’s recommended that you evaluate the potential to reduce the size of the furnace by upgrading to energy efficient windows or adding insulation to the building.

How Furnaces Work

How do Air Conditioners Work

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MEP Academy Instructor

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November 30, 2022

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Learn how Air Conditioners Work. — How do Air Conditioners Work?

How do Air Conditioners Work? The purpose of your air conditioner is to remove heat from inside your home or building and reject it to the outside, in addition to removing moisture and filtering the air. We’ll learn how an Air Conditioner does this by moving heat around using a refrigerant. We’ll also show the major components that get installed and how they’re connected together.

If you prefer to watch the video of this presentation, then scroll to the bottom or click this link. How do Air Conditioners Work

First you would install the condenser or outside section where the compressor and condenser coil are located. Next install the air handler in the attic. We show this one has a horizontal air handler with a furnace. We’ll show you a vertical unit shortly. Then the indoor and outdoor sections are connected with ACR type copper tubing. Inside the copper tubing is where the refrigerant circulates between the indoor and outdoor coils. Then we need to provide a circuit breaker that can handle 208/230 volts, single phase power, and run electrical conduit and wire from the panel to a disconnect switch and then make a connection to the outdoor unit. Then will install another circuit breaker in the electrical panel for 115 volts and fed the indoor air handler.

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Next for gas/electric units using a furnace we’ll install gas piping from the gas meter to the indoor air handler. Next, we’ll need to run a condensate drain from the indoor air handler’s DX cooling coil to a drain or the tailpipe of a sink.

Next, we’ll connect the return air ductwork to the indoor air handler and provide a filter grill in the ceiling for easy access for filter changeouts. Next we need to install the supply ductwork from the air handler to each room that will get supply air. These are the basics of how a split system air conditioner is connected.

And, here is what it might look like if you’re using a vertical unit. There is a move to go all electric and get away from burning fossil fuels, so the use of Heat Pumps seems to be increasing. If installing a heat pump there is no need for gas piping or a furnace as the Indoor and outdoor coils change to accommodate the requirement for either heating or cooling. See our video on “How Heat Pumps Work” for a better description.

How Heat Moves

If you left a hot cup of coffee on the table, it would lose its heat to the surrounding air and eventually its temperature would be close to the indoor air that surrounds the cup. This is how heat transfer works, heat always tries to equalize itself with the cooler surroundings, so any heat in the air will be attracted to the cold surface of the coil.

The 2nd Law of Thermodynamics - Heat always moves from a warmer object to a cooler one — The 2nd Law of Thermodynamics – Heat always moves from a warmer object to a cooler one

The second law of thermodynamics states that “Heat always moves from a warmer object to a cooler one, and seeks equilibrium”. One more example is if you had a glass filled with ice water sitting outside during a hot summer day, the heat from the air would transfer into the ice water causing the ice to melt and the water to reach the outdoor air temperature.

Refrigerant

A special substance called a refrigerant circulates through the piping of the air conditioner from the indoor coil to the outdoor coil. The refrigerant is designed to absorb heat from indoors and reject the heat outdoors. The refrigerant does this by changing from a liquid to a gas and then back to a liquid.

Refrigerants absorb heat when it changes from a liquid to a gas, and then gives up its heat when changing back from a gas to a liquid. The refrigerant is pumped around from the indoors to the outdoors by a compressor, while constantly changing from a gas to a liquid and then back to a gas again, repeatedly. For a better explanation of the refrigerant cycle see our video “Refrigeration Cycle 101”.

Compressor

The compressor is like a pump which uses electricity to power a motor, which causes the refrigerant to move around the piping circuit while increasing the temperature and pressure of the refrigerant before sending it to the condenser coil. There must be a difference in pressure between the high-side and the low-side for the refrigerant to move and for oil to return to the compressor. The compressor causes this difference in pressure and makes the refrigerant circulate through the system. The low-side being the indoor evaporator, and the high-side being the outdoor condenser coil. In a heat pump system these heat exchangers can be reversed and serve the opposite purpose.

The compressor raises the temperature and pressure of the refrigerant so that the heat in the refrigerant is above the outdoor design temperature. Remember that heat transfers from the warmer item to the cooler one. So, if the outdoor temperature is 95°F (35°C), then the refrigerant in the condenser needs to be above that to transfer the heat out of the refrigerant to the warm outdoor air. As the temperature of the air outdoors approaches the temperature of the refrigerant in the condenser, the heat transfer capabilities are diminished. Therefore, air conditioners struggle on super-hot days.

The condensing temperature of refrigerant R410A can be around 115 °F to 120°F (46°C to 49°C) at a pressure of 391 to 418 psig, so if the outdoor temperature gets close to this the capacity of the air conditioner will decrease.

Evaporator Coil and Supply Fan

The heat is absorbed by having a supply fan blow warm air over a cold coil located indoors, which is called the evaporator. The liquid refrigerant circulating inside the cooling coils low-pressure evaporator will turn into a gas as it absorbs the heat from the warm air passing over it, like water that boils in a pot on the stove when heat is added. Since the refrigerant is much colder, 42°F to 45°F (6°C to 7°C), this causes the heat to transfer from the warm air to the colder refrigerant in the coil, causing the air to become cooler.

Air Handler Blower Fan and "A" Coil — Air Handler Blower Fan and “A” Coil

The evaporator will also remove moisture from the air in addition to cooling the air. The change in temperature is considered sensible cooling, while the removal of moisture is considered latent cooling. Like that glass of ice water sitting outside, the air surrounding the glass meets the cold glass surface and the moisture in the air condenses out onto the glass. A similar event occurs on the cooling coil, as the warm air drops in temperature when it meets the cold coil some of the moisture or water vapor will condense on the cooling coil and become condensate. The condensate will drip off the coil into a drain pan where it is connected by a pipe leading to the sewer, drywell, or the tailpiece of a sink.

Condenser Heat Exchanger and Fan

The condenser heat exchanger is located outside, where the refrigerant will reject the heat to the outdoor air. The outdoor condenser section is where the compressor, heat exchanger and condenser fan are located, this is considered the high-side of the system.

Condenser - Outdoor Section with Compressor, Condenser Coil and Fan — Condenser – Outdoor Section with Compressor, Condenser Coil and Fan

This is called an air-cooled condenser because the fan blows air over the coil that facilitates the heat transfer , as opposed to a water-cooled condenser. Remember that heat moves from the warmer to the cooler medium. Even though the outdoor air is warm, the refrigerant is warmer, allowing heat to transfer from the condenser to the outdoor air. Remember the compressor raised the pressure and temperature of the refrigerant.

Upflow Furnace with "A" Coil (Vertical Furnace) — Upflow Furnace with “A” Coil (Vertical Furnace)

The problem comes about when it is exceptionally hot outside, then the temperature of the outdoor air and the refrigerant are getting closer to each other and the transfer of heat is slowed down, causing a loss of cooling capacity. Most condenser are rated for an outside design temperature of 95F. When the temperature outdoors exceeds this amount the system becomes derated, meaning that less cooling capacity is available. The refrigerant leaving the condenser passes through the expansion valve.

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Expansion Valve

The expansion valve controls the volume of refrigerant that passes from the outdoor condenser coil to the indoor evaporator coil.

The compressor increased the pressure of the refrigerant and now the expansion valve lowers the pressure as the refrigerant is metered through the valve from the high-side to the low-side. This is how the cooling capacity is controlled, the more refrigerant that is let through, the more heat that can be absorbed from the space.

Thermostat

The thermostat is the main controller that you use to call for cooling or heating, and where you set your requirements for space temperature.

Cooling Capacity

The air conditioners cooling capacity is rated in tons. Tons is not an indication of its weight, but of the air conditioners cooling capacity. One ton is equal to 12,000 Btuh, which is an expression of the ability of the air conditioner to remove heat. The higher the number, the more heat that can be removed from a building.

When shopping or being quoted an air conditioner, the contractor may quote the tonnage of the unit to be installed, or the btu’s per hour. Either way you can determine the value of the other as (1-Ton = 12,000 BTUH). The thermostat maybe set low to provide more cooling, but the system will be at maximum capacity if it’s on an exceptionally hot day. The capacity of the air conditioner is determined by doing a load calculation. For additional information see our video “What is a Ton of Air Conditioning”.

Load Calculation

Contractors and engineers have available to them the ability to size air conditioners using Load Calculation software. To get an accurate determination of the cooling load, which is required to size the air conditioner correctly, load calculation software should be used. The load calculation program will take into consideration the buildings orientation to the sun for a proper solar load. The determination of the composition of the walls, glass, doors, and roof are important as their “R” values determine the rate at which heat is transferred through them. The square footage of the home or building will also be considered. All of this is easily done with a load calculation program.

Air conditioners should never be sized using rule of thumb values, as there is a tendency to oversize the unit which causes more problems. A few of the problems of oversizing is the additional initial cost of the equipment and installation, and the possible poor performance of moisture removal. When air conditioners are oversized the air conditioner cools faster than normal, which causes it to frequently turn on and off. When an air conditioner cycles on and off frequently it doesn’t allow enough time for moisture removal. This could mean a cool, but damp space. Oversizing also leads to increased energy bills, additional maintenance cost and early equipment replacement cost due to an increase in the on and off cycling times. Oversized air conditioners won’t run efficiently, and energy efficiency is critical for a cost effective system.

Energy Efficiency of Air Conditioners

Air conditioners are given a grade for how efficient they convert electrical energy into useful cooling capacity. This is like a grade you might get in school, the better the grade, the smarter you are deemed to be about the subject. Air conditioners are rated by their Seasonal Energy Efficiency Ratio (SEER) value. The higher the number the better the air conditioner is at using the electrical energy consumed.

SEER and EER Efficiencies of Air Conditioning Equipment — SEER and EER Efficiency Ratios

If you live in a hot and dry climate where the peak load is critical, then you will want to look at the Energy Efficiency Ratio (EER) of the air conditioner. The EER rates the unit based on the peak load conditions, which is more likely to occur in hot climates.

How do Air Conditioners Work

How do Chilled Beams Work

By

MEP Academy Instructor

-

November 21, 2022

0

How do Chilled Beams Work? The use of chilled beams allows for a reduction in the size of Air Handlers and Chillers, as less primary air is served to the space and higher temperatures of chilled water are used for cooling.

Chilled beams are either of the active or passive type, which we’ll explain. The chilled beam will take care of the sensible requirements of the space, while the Dedicated Outside Air System or Air Handler will handle ventilation requirements.

If you prefer to watch the YouTube video of these presentation, the scroll to the bottom or click this link How do Chilled Beams Work

Cool water will circulate through the chilled beams coil and cool the air surrounding it. Cool air is heavier than warmer air, so the effect of the beam cooling the air surrounding it will naturally cause a constant flow or circulation in the space. As the cool air drops the warm air rises, causing the space to cool down. Between the two, the most commonly used one is the Active Chilled Beam.

Active Chilled Beam

An active chilled beam contains one or two coils in a sheet metal housing, suspended or recessed within the ceiling. Primary air from an air handler or DOAS unit is connected to the chilled beam sending air through nozzles that induce airflow from within the space through the coils. The use of primary air from an AHU allows for greater capacities than the passive chilled beam type. Therefore, Active Chilled Beams are more commonly used. The active chilled beams will have piping and air connections.

In most jurisdictions Ventilation air is required for all occupied spaces within a building in accordance with ASHRAE 62.1. Using an active chilled beam is when the ventilation air is brought directly to the chilled beam where it creates an induction effect. As the ventilation air blows through jets within the chilled beam housing it causes an induction effect that pulls in room air across the cold coil. This ensures that air circulates within the space in addition to the natural circulating effect caused by the differences in density between cold and warm air.

Since the cooling is being done at the space level and not back at the air handling unit, less ductwork and fan horsepower are required, including a smaller air handler. Chilled water and ventilation air will need to be brought to the space. This could also allow for a reduction in floor-to-floor height as less space is taken up in the attic for large air ducts. More energy can be carried in a small pipe than a large duct, so with the use of chilled water at the zone level, large ductwork can be avoided.

Active chilled beams come in lengths from 2 feet to 10 feet with various output values per manufacture. Outputs range based on width of unit. Common widths include 12” and 24”. Cooling capacity of active beams can range from 600 BTU/FT to 1,100 BTU’s per foot, based on make and model, while passive beams will have lower outputs in the range of 500 BTU/Ft

Passive Chilled Beam

A passive chilled beam contains one or two coils in a sheet metal housing, suspended from the ceiling.

Once again ventilation air is required for the space, except with passive chilled beams the ventilation air is delivered directly to the space, and not through the chilled beam. The passive chilled beam uses natural convection to cause air circulation. This occurs when the cool air surrounding the chilled beams coil becomes denser and drops, while the warmer air naturally rises to the ceiling. This occurs without the use of a fan, hence its passive.

Passive Chilled Beam indirectly served by a DOAS unit

The difference between active and passive chilled beams is how ventilation air is delivered to the space and how much cooling can be achieved with each chilled beam.

Two-Pipe or Four-Pipe Chilled Beams

Chilled beams have the option to have a separate heating and cooling coil, or they can share a coil with the proper control valve arrangement. A four-pipe system uses two separate coils which gets you four pipes, two chilled water and two hot water pipes. Obviously the four-pipe chilled beam will be heavier as there are two separate coils.

Air Stratification

Air stratification is when you have different levels of air temperatures. With chilled beams this involves the natural stratification of warm air versus cold air. Warm air will rise while cold air being denser will drop to the bottom.

DOAS or Central Air Handling Unit

The chilled beams are designed to handle sensile load while leaving ventilation and latent load requirements to the main DOAS or AHU. This eliminates the need for condensate drain piping at each chilled beam, as the temperature will be maintained above the dewpoint temperature. The AHU unit can be sized just to handle the ventilation requirements, thereby allowing for a smaller than normal unit.

It’s imperative that the AHU be sized to handle the dehumidification requirements as the chilled beams are designed to handle strictly sensible cooling. The primary air must be dry enough to handle any space latent load while maintaining the indoor dew point at a low enough level to prevent condensation from occurring on the chilled beam coil. If the level of humidity is not properly provided for by the AHU, then condensation can occur on the chilled beams and water could drip on the space below.

Chilled Water Supply Temperature & Humidity Levels

The chilled water temperature to a chilled beam is in the range of 55°F to 62°F (12.7°C to 16.6°C) which is higher than a traditional chilled water system of 42°F to 45°F (5.5°C to 7°C) to avoid condensation. To avoid condensation the chilled water temperature needs to be maintained above the rooms dew point temperature, the point at which condensate occurs. The dewpoint transmitter will assist in maintaining the chilled water supply temperature to the chilled beams at 3°F to 4°F (1.6°C to 2.2°C) above the room dewpoint temperature.

The temperature difference between the supply and return chilled water will be low, in the range of 5°F to 6°F (2.8°C to 3.3°C), making the return water temperature range from 60°F to 67°F (15.5°C to 19.4°C). The smaller delta-t requires a greater flow when compared to a typical VAV chilled water coil under ASHRAE 90.1 which requires a 15°F (8.3°C)temperature difference between the supply and return, with a minimum of 57°F (13.8°C) leaving water temperature. Chilled beams are excluded from the mandatory delta-t requirements of ASHRAE 90.1 as an exception 6.5.4.7 #6 states that “Coils with design entering chilled-water temperatures of 50°F (10°C) and higher” are exempt.

In order to achieve a larger capacity chilled beam, you need either greater water flow or a higher delta-t, as the equation is Q = GPM x 500 x Delta-T. By increasing flow or the delta-t you can increase the capacity. But chilled beams are limited by their size and the temperature at which the chilled water can be delivered to avoid condensation.

With an increase in the chilled water supply temperature there will be an increase in the duration for which a waterside economizer could be used.

VAV System vs Chilled Beam

The best way to see some of the benefits of a chilled beam system is to compare it to the typical VAV system. All occupied spaces require some form of ventilation air per ASHRAE 62.1, but the method of using reheat in a VAV system to control space temperature is less efficient.

Active Beam serving a Patient Room in a Hospital vs the Traditional VAV System

Capacity control for space temperature is handled differently between the two systems. The VAV system will reheat the air that has already been cooled to maintain space temperature requirements. The chilled beam system will modulate the water flow to the coil to respond to load demand, but without the wasted energy of reheating. Chilled beams use a constant volume of air, while a VAV system varies the quantity of air to the space.

Instead of the delivering cold air through large ducts, the chilled beam system uses the more energy efficient carrying capacity of piping. Chilled water piping carries over 3,500 times the volumetric heat capacity than that of air ducts. This saves on ceiling space and allows for lower floor to floor heights or higher ceilings. The VAV system increases the volume of air to satisfy the space, while the chilled beam system increases the flow of chilled water.

The ventilation air delivered to the space using the chilled beam system is the proper amount based on space usage and occupancy level. With a VAV system the volume of ventilation air is less precise as the ventilation air is mixed with return air before being delivered to the space.

Dewpoint Transmitters and Moisture Sensors

Dewpoint transmitters are used to sense space dewpoint levels to avoid condensation in the rooms where chilled beams are installed. Since chilled beams are designed to avoid condensation and are installed without condensate drain piping, the avoidance of condensation is important.

Moisture sensors can be strapped to the chilled water piping to indicate if there is the presence of moisture. This would allow a signal to be sent to the control valve to stop the flow of chilled water to the chilled beam. This is a precautionary measure if deemed necessary.

Benefits of using Active Chilled Beams

Reduced size of main supply ducts.
Reduced size of Air Handler and/or Central Fans
Elimination of reheat coils at each zone
Reduced size of Heating Hot Water System
Reduced Chiller Size
Reduction in energy use
Increased time of use for waterside economizers
Quieter than conventional systems
Reduction in floor-to-floor heights
Reduction in mechanical room size

It’s possible that a high performance VAV system will beat the energy savings of an Active Chilled Beam system. Use energy modeling software to check if the building that you’re proposing for chilled beams is as energy efficient as a high performance VAV system.

Installed Cost of a Chilled Beam

The installed cost of a chilled beam system could also be higher than a conventional VAV system due the requirement to route chilled water piping to each space in lieu of just to the air handler room. Also, chilled beams have low capacities due to the higher chilled water temperature and may require lots of them to achieve the load demand of the space. The smaller Delta-T will require more flow to meet the demand if we look back at our formula Q = GPM x 500 x Delta-T, if the delta-t is lower than the GPM must be higher to achieve the same capacity. More capacity can also be added by adding more coil area, which adds more cost.

How do Chilled Beams Work

Why is my Air Conditioner not Cooling

By

MEP Academy Instructor

-

November 18, 2022

0

Why is my Air Conditioner not Cooling? There are many things that can cause your air conditioner to underperform when you need it most. Here are a few things to check before calling your local air conditioning company unless you prefer a professional to do this.

If you prefer to watch the YouTube version this presentation, then scroll to the bottom or click this link Why is my Air Conditioner not Cooling

Dirty Filters

One of the most common reason for less cooling is clogged filters. You should replace your filters seasonally or more frequently depending on the usage of your air conditioner. Dirty filters will reduce the air flow to the rooms, causing a loss in cooling.

Replace Dirty Air Filters Seasonally or Based on Usage

Dirty Coils

The condenser unit sits outside exposed to the elements, including wind, rain, snow, dust, and dirt to name a few. It’s important that the condenser coil be cleaned to allow for proper heat rejection. A dirty coil will reduce the cooling capacity of the system.

Split System Condenser with Dirty Coil — Split System Condenser

The indoor evaporator coil should also be checked but is less exposed than the condenser coil. The evaporator coil should be mostly protected by a properly maintained filter. When cleaning the coil be sure to clean the drain pan of any dirt and debris and to ensure nothing is blocking the drain piping leaving the drain pan.

Circuit Breaker has Tripped

It may be possible that the system has tripped your circuit breaker. This would stop all electricity from reaching your air conditioner. The indoor and outdoor units should be indicated on the electrical panel. See if the breaker for either of them has been tripped. The position of the breaker will be slightly different than the rest if the breaker has been tripped.

Disconnect Switch

Located near the outdoor unit will be a little box that has a means of shutting off the power to the unit for service. When any service is being done on the outdoor unit the technician will make sure this is shutoff to prevent an electrical shock. See if the disconnect switch is in the on position, if not then the unit is not getting electrical power.

Leaking Air Ducts

When replacing air conditioners, it would be a good time to check the condition of the ductwork that carries the air throughout the building. Leaks in ductwork should be fixed, as any leak is a loss of cooling capacity and a waste of energy.

Fix Leaking Ductwork. Make Sure all Joints and Seams are sealed.

It’s like carry a bag of quarters from the bank and having a hole in the bag, where quarters fall out unnoticed. Seal any leaks to ensure that the energy you are using is only used for cooling the space and not the attic or outdoors. Fixing ducts that leak is one of the greatest energy saving strategies.

Air Flow Obstructions

Make sure that the outdoor condenser section doesn’t have its airflow restricted by walls, bushes, or other objects. The outdoor unit needs to move air across the condenser coil to reject the heat absorbed in the building. If the condenser is not getting proper airflow, there will be a loss in cooling capacity.

Bad Indoor Fan Motor

The indoor fan moves the air over the evaporator coil. If the fan is dirty or has malfunctioned then little to no air will be coming out of the supply grilles. You can turn the thermostat setting to fan only to see if the fan is blowing at its normal rate. If nothing comes out when set to fan only there is a good chance something is wrong with your fan motor or a capacitor or relay.

Thermostat

Make sure that the stat is on and has power, and that the proper settings have been entered. Check to see that the setting for cooling is lower than the current room temperature. If it’s a new thermostat, then re-read the instructions to ensure proper installation and setup.

The Obvious

Make sure windows are closed and air vents are open. Make sure nothing is blocking the return air opening.

When done of the above helps improve the cooling, then it’s definitely time to call a professional air conditioning technician who can properly diagnose the problem. Here are a few things they may discover.

Leaking Refrigerant System

If there is a refrigerant leak in the system, the cooling capacity will be diminished. If a leak is suspected the technician will be able to tell by checking the temperature and pressures of the system, much like the doctor listening to your heart.

The refrigerant is the medium by which heat is moved from one location to the other, such as absorbing the heat indoors and rejecting it at the outdoor unit. If low on refrigerant the system compressor could burn out.

Compressor Malfunction

This is definitely one of the most expensive problems to occur. The compressor is the heart of the system and is responsible for circulating the refrigerant from the indoor coil to the outdoor coil. If the compressor burns out, it may be time to look at an updated air conditioner with a better energy rating if your system is 10 years old or older.

Capacitor or Relay

Hopefully the compressor is all good and it’s just a capacitor, that small shiny object in the outdoor unit that holds an electrical charge for assisting in the starting of the compressor. Or maybe it’s a relay contactor, those electric devices that allow electricity to certain parts of the unit when requested. A malfunctioning relay or contactor is an inexpensive fix compared to other repairs.

Capacitor and Contractor - Less Expensive Fixes — Capacitor and Contractor – Less Expensive Fixes

Frozen Indoor Evaporator Coil

The indoor evaporator coil is located above the furnace or in a sheet metal housing the indoor heat pump section. A frozen coil can occur when the refrigerant runs low or airflow to the indoor evaporator coil is restricted, like when a filter is clogged. Can you see any frost on the copper tubing leaving the evaporator coil, or is there excessive water dripping at the indoor coil?

Undersized Air Conditioner

Hopefully the engineer or installing contractor perform a load calculation to determine the correct size for your space. Rules of thumb should never be used for the proper sizing of air conditioners. If it is determined that the system was undersized look at supplementing the existing system with a split system for a particular area with different usage patterns and re-duct the air from that space to serve the rest of the spaces.

Over-sized air conditioners are also a problem as they frequently cycle on and off, without the proper run time to remove moisture from the air, leaving the space cool but clammy.

Minimum Maintenance you should provide for your Air Conditioner.

Replace filters seasonally or sooner based on usage.
Inspect and clean if required Condenser and Evaporator Coils seasonally.
Inspect and lubricate fan motors seasonally.
Flush and blowout condensate piping

Why is my Air Conditioner not working

Construction of a Furnace

Furnace Heat Exchangers

Combustion Air

Condensate from Condensing Furnace

Electrical

Filters

Smart Thermostat

Upflow Furnaces

Downflow Furnaces

Horizontal Furnaces

Natural Gas Regulators

Combustion Intake and Exhaust

Gas Venting

Carbon Monoxide Detectors

Gas Ignition System

Furnace Safety Switches

Furnace Rating AFUE Rating

Maintenance of your Furnace

Retrofitting Existing Furnaces

How Heat Moves

Refrigerant

Compressor

Evaporator Coil and Supply Fan

Condenser Heat Exchanger and Fan

Expansion Valve

Thermostat

Cooling Capacity

Load Calculation

Energy Efficiency of Air Conditioners

Active Chilled Beam

Passive Chilled Beam

Two-Pipe or Four-Pipe Chilled Beams

Air Stratification

DOAS or Central Air Handling Unit

Chilled Water Supply Temperature & Humidity Levels

VAV System vs Chilled Beam

Dewpoint Transmitters and Moisture Sensors

Benefits of using Active Chilled Beams

Installed Cost of a Chilled Beam

Dirty Filters

Dirty Coils

Circuit Breaker has Tripped

Disconnect Switch

Leaking Air Ducts

Air Flow Obstructions

Bad Indoor Fan Motor

Thermostat

The Obvious

Leaking Refrigerant System

Compressor Malfunction

Capacitor or Relay

Frozen Indoor Evaporator Coil

Undersized Air Conditioner

Minimum Maintenance you should provide for your Air Conditioner.

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