How Steam Traps Work. We’ll explain the important job of steam traps and how the three most common steam traps work. We’ll also show you examples of where they’re used in the HVAC industry.
if you prefer to watch the video of this presentation scroll to the bottom or click on this link. How Steam Traps Work.
When steam gives up its heat it turns into condensate, which must be removed from the system. The rate of heat transfer through the heat exchanger is highest when the heat exchanger is full of steam. If condensate wasn’t drained out of the system, and it collected in the heat exchanger, then the rate of heat transfer would drop.
Remember that a pound (0.45 kg) of water or condensate holds 1 Btu (1.5 kj) as it gives up its heat one degree (0.55 K), and that steam gives out approximately 1,000 Btus (1,055 kj) for every pound (0.45 kg).
IThe Heat Content of Steam vs Condensate
if condensate is allowed to sit in the heat exchanger the system will lose capacity. Therefore, it’s important to make sure that the condensate is quickly removed while not allowing the steam to escape without first transferring its heat.
Condensate if left to collect in the pipes could cause problems such as water hammer noises, where a high velocity slug of condensate water is slammed against fittings or equipment. This is like a tidal wave in the pipes. The energy is dissipated onto the piping components and is noticeable by the noise it makes or the rattling of the pipes.
Steam Condensate Water Hammer caused by Slugs of Condensate
When the steam boiler starts the system will be full of air and non-condensable gases that need to be vented out of the system. If a steam trap fails to remove air from the system, the air and non-condensable gasses will reduce the heat transfer rate of the heat exchanger. Non-condensable gases behave like an insulator, thereby reducing the heat transfer rate.
The steam traps job is to remove condensate, air, and non-condensable gases, while preventing steam from leaking past it in a steam system. For the basic understanding of a steam system see our video on Steam Heating System Basics.
There are three main types of steam traps, but they all require a differential pressure between the inlet and the outlet of the steam trap. We’ll cover how each of the three steam traps categories work.
Mechanical Steam Traps
The density difference between the steam and condensate is how a Mechanical steam trap works. Steam in the form of vapor is less dense than that of liquid condensate which makes the use of a float as an effective means of control. The float and thermostatic (F&T) trap and the inverted bucket trap are the two most common mechanical steam traps. These traps use floats that rise, and fall based on the amount of condensate, and either open or close the trap.
Float and Thermostatic Steam Trap – Mechanical Steam Trap
When the condensate level is high in a float & thermostatic trap, the float rises and opens the trap to allow condensate to flow out. When the condensate level drops, so does the float which causes the trap to close.
With an inverted bucket trap, steam will push the bucket upward closing the trap, while condensate will pull the bucket down opening the trap.
These steam traps open and close based on the density of the steam and condensate being different. With the F&T trap the heavier condensate lifts the float, while with the Inverted Bucket the heavier condensate sinks the float.
Thermostatic Steam Traps
The temperature difference between steam and condensate is how a thermostatic steam trap work. It’s based-on temperature. After the condensate forms its temperature drops below that of the steam, and after a certain drop in condensate temperature the trap opens. When steam first condenses its temperature is the same as the condensate. This will require that the condensate cool further before the trap opens.
Thermostatic Steam Trap
There are various substances used as the controlling element in thermostatic steam traps, such as fluids and metals.
The bimetal type thermostatic steam traps uses the differing coefficients of expansion of various metals. When two metals are attached together, and one expands at a different rate then the other based on temperature, this can be used to open and close a steam trap. As more and more steam enters the trap, the bimetal heats up and expands while pulling up on the stem of the valve that closes the steam trap, and when it cools down it opens the trap. The presence of steam will cause the bimetal to bend in a certain direction and close the trap, while the cooler condensate will contract the bimetal to open the trap.
When the boiler starts up after being off, the air, condensable gases, and the condensate in the system will be pushed through the open steam trap.
Thermodynamic Steam Traps
The Velocity difference between steam and condensate is how a thermodynamic steam trap works. Steam moves at a greater velocity than condensate. The only moving part in this thermodynamic steam traps is the disc, which moves up and down, opening and closing the port for condensate to flow through.
Thermodynamic Steam Trap
When the steam system is started, the cool condensate and air is pushed by pressure into the trap, causing the disc to move upward, allowing the condensate and air to pass through.
As hot condensate moves through the steam trap some of it flashes into steam, causing the pressure to drop below the disk, which causes the disk to drop.
As the condensate heats up and flows through the trap it cause steam to flash under the disc, which drops the pressure causing the disc to drop lower towards the seat or opening. As this is occurring the flash steam on top of the disc is pushing the disc closed. This traps the flash steam in the upper chamber, which causes an equalization of pressure on both sides of the disk, preventing any more condensate from passing through.
Flash steam in the upper chamber condenses, causing the disc to be forced up, while allowing condensate to enter. This process repeats itself over and over again.
Steam Heating System Basics. Learn how steam systems work and where they’re used in the HVAC Industry. We’ll show you three systems that use steam for heating.
If you prefer to watch the Video of this presentation, than scroll to the bottom or click this link. Steam Heating System Basics.
Steam heating systems are found in Commercial, Residential, and Industrial Facilities. They can be found in central plants of large University Campuses and in District Steam Distribution systems.
Steam is used in laboratories, breweries, food processing, industrial plants, in hospitals for sterilization, and in the production of electricity through steam driven turbines. There are two basic system types, direct and indirect steam systems.
We’ll discuss the basic indirect steam heating system.
Steam is moved through the system of pipes by the pressure created when water is vaporized into steam. There are no pumps required, as the higher pressure within a steam system causes it to move through the pipe, valves, and equipment.
We can witness this on any stovetop when a tea kettle whistles. As the water turns to steam it seeks to escape the tea kettle through the small opening and whistles as it escapes.
Condensate from the steam condensing needs to be brought back to the system for reuse unless the steam is used for a process. The condensate can be brought back by gravity or with the use of a condensate return pump.
When heat is added to water its temperature will rise until it hits the point of evaporation, at this point it changes from a liquid into a vapor or steam. This occurs at 212 Fahrenheit or 100 degrees Celsius at normal atmospheric pressure.
Steam Boiler feeding a Steam Radiator
If we enclose the water within a steam boiler with no opening to the atmosphere, the steam will increase the pressure within the system as the molecules move faster and faster, colliding into each other and the walls of the boiler looking for a lower pressure escape route like the hole in the tea kettle.
If we connect piping to an opening in the steam boiler the steam will exit the boiler into the piping without the need of a pump, but just by the pressure created by the steam.
Here we use a steam boiler to feed the primary side of a heat exchanger. The steam will transfer heat to the secondary loop which is feeding the heating hot water coils in Air Handlers. The steam gives up its latent heat and condenses as the heat is transferred to secondary side of the heat exchanger.
Steam Heat Exchanger serving a Heating Hot Water Coil
To make sure that we are not letting steam into the condensate return we install a steam trap. The steam trap prevents steam from passing through and only lets condensate through. See our video on How Steam Traps Work for a explanation of steam trap types.
On the secondary side of the heat exchanger is a separate loop of water that serves to heating the air in the building air handlers.
If we look inside the heat exchanger, we can see that the primary and secondary piping never mix. The steam enters the heat exchanger as steam and leaves as condensate, while the secondary loop heating hot water is moved by a pump through the system.
Steam Heat Exchanger
The secondary loop heating hot water return enters the bottom of the heat exchanger and picks up heat from the steam and leaves through the top to the air handlers heating hot water coil.
The steam boiler doesn’t need a pump as the molecules are moving fast and exerts pressure on the walls of the boiler and when the piping is connected to the boiler than pressure pushes the steam out of the boiler and into the pipes just like a tea pot that is boiling, and the steam is escaping out of the spout whistling as it exits.
Now we’ll add another piece of equipment requiring a supply of steam.
Steam Heating System Basics
We’ll have to redo the steam piping to make connections to the domestic system. We’ll install a new steam main and make a connection to the Heat Exchanger making sure to come off the top of the steam main, which we’ll explain why in a minute. Next, we’ll connect the domestic hot water heater and storage tank.
We’ll need a steam trap at the end of any main steam run to allow only condensate to pass through. A main condensate pipe needs to collect all the condensate from any steam systems, here we show a simple system.
Next, we’ll connect the condensate from the heat exchanger to the main condensate line and be sure to install a steam trap to prevent steam from escaping. Allowing steam to escape would be a waste of energy. We’ll also connect the condensate from the heat exchanger that feeds the domestic hot water system and include a steam trap.
We need to connect the main steam line through the steam trap to the main condensate piping going back to the boiler.
Now we can feed the building with domestic hot water, including a return line, and makeup water. This completes a simple layout of a steam heating system. Of course, there are many other components to a steam system which we’ll cover in a more advanced video on steam heating, but this gives you a simple view of what it might look like.
Why Tap Steam off the Top
It’s important that any steam branch line be taken off the top of the main steam pipe in order to prevent pulling condensate into the branch. As you can see the steam will rise to the top because of its lighter density and higher temperature.
Steam off the top of the Main and Condensate off the bottom.
For similar reasons you want to make any condensate branch connections off the bottom of any steam line or system to ensure that you are only pulling condensate. And all condensate branches should contain some type of steam trap to ensure that only condensate passes through to the condensate system.
Why use Steam?
Steam has a higher heat content then hot water, which means it can carry a lot more heat in a smaller volume. This heat is in the form of latent heat, which is the change in state from liquid to vapor, and from vapor to liquid. This helps keep the pipes smaller. Steam is water based, so its non-toxic nor flammable, although it’s very hot and will need insulation on the distribution piping to avoid heat loss and prevent someone for getting burned if touched.
The main components of a steam system are the steam boiler, the distribution piping, the steam traps, and the heat exchanger or equipment that requires the use of steam. There will need to be a source of fuel, such as natural gas or fuel oil. A means for makeup water supply, and for boiler blow down to remove unwanted dissolved solids.
This is the basics of a steam system. We’ll get into the details of how each of these components work in other videos.
How Solenoid Valve Work. We’ll discuss how Solenoid Valves are constructed and how they work in a typical mechanical system. We’ll explain where they’re commonly used in refrigeration and air conditioning systems, and why.
If you prefer to watch the video of this presentation, scroll to the bottom or click this link How Solenoid Valves Work
The main purpose of a solenoid valve is to automatically control the flow of fluids and gases. Solenoid valves use electromagnetism to electrically control the opening and closing of valves. They serve an important function in the automatic control of refrigerants, water, natural gas and other liquids in the HVACR industry.
Solenoid valves are in many systems besides the HVAC industry, but we’ll keep our discussion to the Mechanical Construction Trades. Here are some examples of where you might find solenoid valves.
Water-Source Heat Pumps and AC Units
Various energy codes require that flow to a water-cooled heat pump or AC unit be shut off when not operating during normal business hours. This prevents additional pressure drop on the pump when the water-cooled Air Conditioner isn’t operating. The condenser water flow to the non-operating compressor is stop by the controls sending a signal to the solenoid valve to close.
Water-Source Heat Pump System with ASHRAE 90.1 Shutoff Requirement using a Solenoid Valve
ASHRAE 90.1 2019, section 6.5.4.5.1 states “Each hydronic heat pump and water-cooled unitary air conditioner shall have a two-position automatic valve interlocked to shut off water flow when the compressor is off”.
ASHRAE Standard 90.1-2019 The use of a Solenoid Valve for Compliance
Refrigerant Pump Down
Solenoid valves are used for the automatic pump down of refrigerant systems. This is done by placing a solenoid valve in the liquid line before the expansion valve. See our other video for explanation on How Electronic Thermal Expansion Valves Work.
Refrigerant Pump Down Cycle using a Solenoid Valve
When the thermostat is closed upon a rise in temperature the solenoid is energized causing it to open and allow refrigerant to flow through the normally closed solenoid valve to the expansion valve. The refrigerant entering the evaporator causes a rise in pressure that triggers the controller to turn the compressor on. When the thermostat is satisfied and cooling is no longer required, it stops the flow of electricity to the solenoid valve, causing it to close.
The compressor keeps running, pulling the refrigerant out of the low side and into the high side liquid receiver.
When the suction pressure gets too low, before it reaches a vacuum, the low-pressure safety switch will shut off the compressor to prevent it from burning out. The pressure-sensitive controller will turn on and off the compressor based on suction pressure.
When choosing solenoid valves for refrigerant systems, the engineer will use the refrigerant type, the tonnage, and the Maximum Operating Pressure Differential as the selection criteria. Manufacturers provide tables for selection of their solenoid valves.
Hot Gas Bypass
To prevent the evaporator coil from freezing during low load conditions, a bypass is routed from the hot gas discharge of the compressor to the low side of the system right before entering the evaporator. Bypass is controlled by a Hot Gas Bypass Valve that maintains a minimum evaporator pressure to prevent freezing of the coil.
Hot Gas Bypass using a Solenoid Valve
During a pump down cycle, you don’t want this valve to interfere with lowering the evaporator pressure, so a solenoid valve is installed ahead of the Hot Gas Bypass Valve. When the pump down cycle is initiated, the solenoid stops the flow to the Hot Gas Bypass Valve preventing it from bypassing hot gas into the evaporator.
The use of Hot gas Bypass is considered old technology, as it’s more energy efficient to use multiple compressors or variable capacity compressors.
How do Solenoid Valves Work?
An electrical coil within the solenoid valve housing creates an electromagnetic field when energized. The magnetic field induces the opposite polarity in the metal plunger which can be made of iron or steel. Because opposite magnetic poles attract, the plunger will be pulled up into the coil within housing causing the valve to open.
If electrical power remains supplied to the coil, the valve will remain open. This is how a normally closed valve works. When the coil is de-energized the magnetic field disappears and the plunger falls by gravity, or a spring pushes the plunger to close the valve. The coil can use line or low voltage power to be energized.
So, it’s the magnetic field that operates the valves and creates the valve to open.
Solenoid valves are available in the normally open or normally closed position. The designation of normally open or normally closed applies to the position of the valve with no power applied, hence its normal position.
Normally Closed Solenoid Valves
The normally closed solenoid valve is the more commonly used form. The normally closed solenoid valve opens when power is applied to the solenoids coil windings. When no electrical power is applied the valve will remain closed.
Normally Closed Solenoid Valve – Valve will open when energized
With a normally closed valve the spring keeps the valve closed when no electricity is applied to the coil. When the coil receives power, the magnetic field induces an opposing magnetic charge to the plunger causing it to be pulled upward into the heart of the magnetic field, opening the valve.
When the power is shutoff the magnetic field disappears allowing the spring to push the plunger downward closing the valve.
Solenoid Valve – Normally Open
The normally open solenoid is always open until power is applied to its coil windings. This is the type used for refrigeration pump down, as you want the flow of refrigerant to be unhindered until its time to activate the solenoid valve and force closed the valve.
Normally Open Solenoid Valve. Valve closes when energized.
When power is applied to the coil it pushes the plunger downward instead of pulling upward. This causes the plunger to close the valve. When power is shutoff to the coil, the spring will force the plunger upward opening the valve.
VRF Heat Recovery vs VRF Heat Pump. VRF stands for Variable Refrigerant Flow, a method of varying the volume of refrigerant circulated in the system in relation to demand. As demand drops so does the volume of refrigerant circulated by the compressors. The compressor can slow down and pump less refrigerant to meet the current demand of the system. We’ll explain the differences between a VRF Heat Recovery System and a VRF Heat Pump.
If you prefer to watch the Video of this presentation than scroll to the bottom or click on this link. VRF Heat Recovery vs VRF Heat Pump.
One of the big differences between a VRF Heat Pump and Heat Recovery system is that with a VRF Heat Pump system all the units must be in the same mode. Either cooling or heating.
VRF Heat Pump with Individual Zones
As seen here this one outdoor unit can feed many indoor units, but all the indoor units need to be in the same mode, such as heating as shown here, or all in cooling. With a VRF Heat Pump system if each zone or office wanted to control whether to heat or cool their space, then each would require their own outdoor unit.
Here we have an Executive in Office #1 wanting heating, while the second office wants cooling, each indoor fan coil will require a separate outdoor unit, and the third office is also calling for cooling and will have their own outdoor unit.
To reduce the number of outdoor units, the engineer will group together those zones that have similar load profiles. So, indoor Fan Coil #2 and #3 have similar load profiles, so we can get rid of one of the extra outdoor units and its piping and connect Fan Coil #3 together onto the outdoor unit #2, this saves on outdoor units.
VRF Heat Pumps with Consolidated Zoning
Now whenever Fan Coil #2 and #3 switch modes of operation they will always be in the same mode as they’re on the same outdoor heat pump system, as shown here now in heating mode together, while the outdoor unit #1 can be in any mode separate from the other system.
The benefit of using a VRF Heat Pump system over the traditional split system heat pump is that you have inverter compressor and multiple indoor fan coils all working together to achieve a lower energy consumption.
VRF Heat Recovery vs VRF Heat Pump (Heat Recovery System with 3-Port Branch Controller)
The disadvantage when we look at the VRF Heat Pump versus the VRF Heat Recovery as shown here is that if we install a 3 port Branch Controller Box and connect the refrigerant piping to the outdoor unit, we can then connect fan coil #1 to the branch controller allowing that office to choose to be in heating or cooling mode, in this case heating.
Then without having to install another outdoor unit we can add fan coil #2 to the branch controller and that office can be in cooling mode. The same for fan coil #3, we just pipe the fan coil to the branch controller box and that office can choose any mode of operation, in this case heating.
This can all be done using one outdoor unit with a 3-port branch controller, which is one of the advantages of using a heat recovery system as opposed to a VRF Heat Pump System. The heat Recovery system also makes great use of the heat being rejected. The heat rejected from fan coil #2 can be used to heat the spaces serving fan coil #1 and #3.
To determine which system is more expensive an analysis will have to be done. If we have two small identical office buildings as shown here, with the first building using a VRF heat pump system. The heat pump system would require at least two outdoor units, one for the South exposure and the other for the North exposure.
The Heat Recovery system is perfect for this type of balanced zoning, and will require only one outdoor unit, but will need a Branch Controller box to route heating or cooling to the indoor units.
VRF Heat Pump versus VRF Heat Recovery (Cost Factors)
The total amount of piping, fittings and insulation cost will need to be determined between the two designs.
All of the outdoor units and the Branch controller will need electrical power. All of the indoor fan coils will have their own controller, so there is no difference in cost between the two systems here.
The VRF Heat Recovery systems will most likely require the greatest initial investment when compared to VRF Heat Pump or traditional air conditioning systems. The additional cost for the VRF Heat recovery system comes from having more components and a higher level of installation and commissioning cost.
The initial cost of a VRF system should be analyzed based on life cycle cost which will be impacted by the energy cost and savings factors.
If you are an HVAC contractor or Engineer that tracks the cost of VRF Systems, then the VRF Wizard has a VRF System Cost Spreadsheet for keeping track of these types of projects. To get this spreadsheet click on this link >> VRF System Cost Tracking Spreadsheet.
Mode of Operation
All air conditioning systems including VRF systems are made to move heat from one location to another depending on the mode of operation. Heat is either brought into the building or rejected out of the building.
Using a VRF Heat Pump in cooling mode, heat is absorbed into the refrigerant from the warm indoor air blowing over the coil. The heat is then circulated by the compressor to the outdoor unit where the heat is rejected through the outdoor coil into the outdoor air. For a complete understanding of the refrigeration cycle see our video on “Refrigerant Cycle 101”.
The heat pump has a reversing valve which changes the direction of flow to each of the two coils. In Heating mode, the indoor coil now becomes the outdoor coil. So during the cold winter, the outdoor coil acts as the evaporator and absorbs heat from the cold outdoor air. You’re properly wondering how there is any heat in the cold winter air.
Well, this is the magic of refrigerants, as R410A is commonly used in VRF systems, and this refrigerant boils at -55.3F (-48.5C). That means this refrigerant circulating in the VRF System can still absorb heat from the cold outdoors and send it to the space to be heated. See our video on “How Heat Pumps Work” for a better explanation.
So it’s easy to remember, one coil absorbs heat (takes it in) and the other rejects heat (throws it out) all with the magic of refrigerants.
A much better and more efficient use of the heat being removed is to use it for some purpose, such as for a room needing heat while another zone is in cooling mode, this is what a Heat Recovery system is good at.
With a VRF Heat Recovery System, the Branch Controller will either send the heat absorbing or heat rejecting refrigerant to any space depending on whether the space wants heating or cooling, based on the demand of the controller.
So, if heat is absorbed into the refrigerant from a space needing cooling, it can be rejected by the refrigerant into a space needing heating. This is the basic principle of a VRF Heat Recovery System. We’ll cover these systems in more depth in other videos.
When should you use a VRF Heat Pump versus a VRF Heat Recovery System?
The decision will be based on several factors, one of which is what do we want to do with the heat that is recovered from a space. Do we want to throw it away, like we do in a traditional air conditioner, or do we want to use it somewhere else?
The main difference between the VRF Heat Recovery and Heat Pump system is that the VRF Heat Recovery uses the heat which allows for simultaneous heating and cooling.
VRF System Comparison
The VRF Heat pump system allows for only one mode of operation at a time. This means that you have the choice to have all the zones in heating mode or all in cooling mode. If you have one space calling for heating and another for cooling, the one with the controller will determine the mode of operation. The heat pump uses a reversing valve to change the direction of the refrigerant flow. See our video on “How Heat Pumps Work”.
Therefore, VRF Heat Pump systems need to be designed with zones having similar load profiles, such as all spaces have exterior South facing offices.
The VRF Heat Recovery System uses a Branch Controller to send cooling or heating to any of the zones connected to the system. The branch controller acts like a Flight Controller, directing traffic. This allows for the heat rejected from one space to heat another space on a different zone.
Heat is removed from a space requiring cooling and rejected through the indoor coil into another space requiring heating. This is the efficient use of energy as its utilized instead of with traditional air conditioners where this heat gets rejected to the outdoors.
In an all-air system, heat is moved from one coil to another. This means in a heat pump system there is an indoor and outdoor coil, so heat is either entering the outdoor coil or exiting the coil. With a VRF Heat Recovery system this same heat can be entering or exiting any of the indoor coils, allowing for either heating or cooling at the same time.
The designer will need to review the owners’ requirements, how the building is oriented, the need for simultaneous heating and cooling, zoning layout, ventilation requirements, and accessibility for routing of piping and or ductwork including available space for new equipment.
Simultaneous Heating and Cooling with VRF Heat Recovery System
The decision on which system to use comes down to whether the zone can take advantage of simultaneous heating and cooling. This requires that the cooling and heating loads are balanced. Another consideration is the cost, as the initial cost of the VRF Heat Recovery System is higher.
Its possible to provide VRF Heat Pumps for each zone, but if you have a lot of zones that are balanced then it probably makes more sense to use a VRF Heat Recovery system, as this can save energy and reduce the number of outdoor units required for the total tonnage.
Depending on the configuration of the spaces and the orientation of the building, the use of a VRF heat pump system can reduce cost and installation and commissioning time by simplifying the install and commissioning process.
Here is a simple example of the differences with an analysis of a building that has the same north and south exposure areas. There are two options.
First, we can install a VRF Heat Pump Systems, one for the North Zone and another for the South Zone. That way when the North calls for heating all the North zones can be in heating mode, while at the same time the South Zones is in cooling mode. Two separate systems operating in two different modes. This requires a minimum of two outdoor units and at least two indoor units.
VRF Heat Pump System
Outdoor Units required = 2
Indoor Units required = 2 minimum
With the VRF Heat Recovery System this can be accomplished with just one Outdoor Unit and two indoor fan coils or more depending on the zone size.
VRF Heat Recovery System
Outdoor Units required = 1
Indoor Units required = 2 minimum
Controller Box = 1 minimum
As our example shows, we’ll need two outdoor units when designing for a Heat Pump system, and only one for a Heat Recovery System.
The engineer needs to determine what the owners project requirements (OPR’s) are and how to best accomplish those. How the building is zoned, the type of spaces and the operating schedule of the various spaces will need to be considered. Do the spaces share the same orientation to the sun?
You can see in the image below that with a Heat Pump system you’ll have all zones in Heating or Cooling, there is no simultaneous heating and cooling. You can switch to all cooling or all heating.
If you have a long southern exposure in the northern hemisphere, during the summer the South facing spaces may require cooling, while the North exposure may want heating during various hours of the day.
This could be accomplished with two Heat Pump systems or just one Heat Recovery System depending on the size of the zone. Interior zones are usually in cooling mode to satisfy the internal heat gains from people, lights and plug loads, since they have no exterior exposure.
A Heat Recovery System could be a good option if the load profile is balanced allowing for simultaneous heating and cooling. This allows for the heat that is recovered from the space being cooled to be used in spaces requiring heating, hence the terminology Heat Recovery. For locations that have very cold winters, the engineer must ensure that the VRF system can meet the design load at these extreme temperatures, otherwise supplemental heating maybe required.
Heat Recovery Branch Selector Control Boxes
The VRF Heat Recovery system will require an additional component when compared to the VRF heat pump system. With a heat recovery system there is the need to install a box controller between the outdoor and indoor units. This box controller can handle dozens of indoor units, not just one.
The branch controller box acts like a traffic controller, sending either hot gas to zones requiring heating, or sending cold low pressure liquid refrigerant to those zones needing cooling. You will find that the different VRF manufacturers have different names for this controller box.
These boxes require an electrical connection which adds to the cost of these systems, and depending on which manufacture you use, there might also be the need for a condensate drain.
VRF Indoor Unit Sizing and Style
The heating and cooling load will determine the size of the indoor units. For locations where the winters are extreme the selection may be based on the heating load to make sure that the VRF system can handle the extreme winters. The engineer will size the unit based on the meeting the maximum heating or cooling capacity.
Indoor units come in various styles and configurations. They can be ducted or un-ducted. There are un-ducted wall and ceiling mounted units and concealed ducted indoor units. You can serve more than one space when using the ducted units. With the concealed ducted units there is no visible component expect for the air distribution grille requires for the supply and return air paths from the space. With a ceiling or wall mounted unit, the VRF equipment is visible to the occupants and must be taken into consideration for aesthetic reasons.
Space Allocation Requirements
The use of a VRF system is less invasive for renovation projects when compared to an all-air system. The routing of small copper ACR piping is a lot easier than routing larger air ducts to each space when working in an existing building. The capacity of piping to carry the cooling or heating load is much greater than that of air ducts on an area basis. Looking at the chart we can see that the same carrying capacity in tonnage between refrigerant piping and air ducts requires a much smaller piping than air ducts.
Comparison of Air Duct vs ACR Piping (20–Tons of Capacity)
If a Heat Recovery system is being installed, then additional space will be needed for the branch controller box.
VRF Electrical Requirements
VRF Heat Recovery systems require three electrical connections, as an additional electrical connection is required at the branch controller box. Traditional split system air conditioners and heat pumps typically have two connections, one at the outdoor unit and another at the indoor unit. If using a rooftop packaged unit, then only one electrical connection is required. On small split systems the outdoor unit may come with cabling that powers the indoor unit.
Packaged AC Unit – 1 Electrical Connection
VRF Heat Pump – 2 Electrical Connections
VRF Heat Recovery – 3 Electrical Connections
Limitations on VRF Piping
The use of a VRF system requires that attention be paid to the distances between all the components. This is to ensure that the system operates correctly. The traditional split system air conditioner also has limitation on the distance piping can be ran, and if that distance is exceeded there could be a drop in capacity and or failure of the system to return any oil required by the compressor.
If we look at one of the VRF manufacturers product data sheets, we can see that there are piping limitations for various distances between components.
The engineer has the option of choosing which systems to use for the different areas of a building. Some areas or floors of the building maybe suitable for a VRF Heat Recovery System, a Heat Pump System, Traditional Air Conditioners, or a central plant of some type. The engineer can mix and match these systems by using more than one type of system within the building.
Energy Efficiency of VRF Systems
The use of VRF systems can save on energy compared to the traditional air conditioners. This is one of the benefits when using VRF. There is one or more compressor in an outdoor unit of a VRF system. These compressors can be digital scrolls or similar that can save energy by matching the compressor output to the cooling or heating demand for the system.
The compressor ramps up and down to match capacity, saving energy by only pumping the volume of refrigerant required to meet the current load. The fans on some of there outdoor units can speed up or slow down to match capacity also. The traditional air conditioner just cycles on and off to meet demand, which is not as efficient as the modulation of these inverter and/or digital scroll compressors.
ASHRAE 62.1 Ventilation Requirements
All occupied spaces require ventilation, that is fresh outside air for the occupants of the building. This is usually done in accordance with ASHRAE standard 62.1, which defines the quantity of ventilation air required based on occupancy levels and area of the space. ASHRAE stands for the American Society of Heating, Refrigeration and Air Conditioning Engineers. They work on establishing standards for the industry and have branches all around the world.
There are several methods for providing outdoor or ventilation air to the occupied space when using a VRF system. Bringing in outside air is energy intensive, especially in hot or humid climates where levels of moisture are high. VRF indoor coils are not efficient at removing a lot of latent loads, as they have high sensible heat ratios.
Sensible heat ratios define the amount of sensible heat to total heat that the unit can provide. The difference between the total and the sensible is the latent load. This means that the higher the sensible heat ratio, the lower its ability to remove latent or moisture from the air.
This means that areas that have high moisture levels will need a way to precondition the air before its delivered to a VRF system. This can be done with a Dedicated Outside Air System (DOAS) unit. The DOAS unit will remove moisture from the outside air and deliver this condition air either directly or indirectly to the VRF indoor unit. See our video on “How Dedicated Outside Air Systems Work”.
Schedule of Occupancy
The time when spaces are occupied is an important factor when engineering a system. Zones with similar occupancy schedules can be combined on the same system which allows for optimal tonnage selection. Spaces with varying occupancy schedules, those spaces that might be used intermittently and not on a regular schedule will require additional analysis and likely a separate system for optimal energy usage.
For instance, small IT closets or areas that require 24 hours a day, 7 days a week operation would not make sense on a larger house system. You don’t want to run a large system off hours to feed a small load. These loads are best off with their own system.
Other spaces like schools and offices usually have a fixed schedule that can be relied upon for accurate programming of a schedule. Spaces that have varying occupancies levels like meeting rooms, conference rooms, and assembly halls can benefit from a dedicated system to meet the unpredictable schedule. Any spaces that produce high levels of moisture will need to be evaluated for their latent load component as the VRF fan coil unit has a high sensible heat ratio (SHR) and doesn’t handle lots of latent load.
When designing a VRF system or any system, occupancy schedule is an important consideration. VRF Heat Recovery systems require similar occupancy schedules to be energy efficient. Its important to balance the heat rejection from the cooling needs with the heating requirements of the system. This allows for the use of the rejected heat to be used for heating simultaneously while cooling is demanded.
Summary
The use of a VRF system can save energy but comes with a higher initial cost. A payback analysis will need to be run to determine the payback period based on the energy savings and the customer required ROI. The use of the VRF Heat Recovery system allows for greater savings if the system has a good balance between the heat rejected for cooling zones being utilized to those zones needing heating. A balanced heat recovery system allows for simultaneous heating and cooling, while heat pumps only allow one mode of operation for the complete coverage area. VRF systems are a good option for renovations because they are less invasive to the existing structure.
The engineer might consider the following when deciding which VRF system to use.
Does the zone layout benefit from simultaneous heating and cooling by allowing the heat rejected from one space to be used by another?
Is initial cost the determining factor, as VRF Heat Recovery often has the highest initial investment?
Does the area have high moisture or relative humidity? This will require the use of additional equipment such as a DOAS unit to remove the moisture or latent heat.
How will code required ventilation air be provided to the space?
Are the occupancy schedules of the various spaces similar?
Is there space for the branch Selector Box if using a Heat Recovery System?
Where can the outdoor units be located in relationship to the indoor units?
Is refrigerant monitoring required and do the spaces meet the minimum volume for the ASHRAE 15 requirements?
