High Performance HVAC Air Handler Components

HVAC Air Handler Components Quickpage jump links:

Air Handler Components Brief

Air Handler Cabinet

Air Handler Plenum Transitions

Air Handler Control Panel

Air Handler Control Panel: Fan Relay Control

Air Handler Control Panel: Heat Relays

Air Handler Related Links

Air Handler Components

This article will take different types of air handlers and disassemble them part by part to give you a good description of each part and hopefully a better understand of the air handler in general. Most of the components associated with the air handler are in the air handler. However, there are a few components which are not in the air handler but associated with the air handler. There are some hot water and cold water coils which are not fixed into the air handler but downstream in the ductwork. There are also some gas and electric duct heaters which are mounted in the ductwork and completely reliant on the air handler for air flow. These systems will also be covered and hopefully this article will offer you a comprehensive look at the air handler, all its parts, and how it functions.

The Air Handler Cabinet

The air handler cabinet is comprised of sheet metal. Manufacturers use a heavy gage sheet metal for durability and usually give the cabinet a good finish to prevent rust especially in moist environments where air handlers usually operate. The heavy gage sheet metal for durability is generally required to meet code requirements which many manufacturers try to exceed for compliance reasons. Some air handler cabinets can be disassembled and reassembled at the job site so they can fit in tight places. Another reason for disassembly is for a multi-use air handler that can be used for up flow, down flow, or for horizontal flow left to right or horizontal flow right to left. Whatever the job calls for the multi-use air handler can be quickly converted in the field without special ordering from the manufacturer.

Air Handler Plenum Transitions

When the air handler is installed the HVAC contractor has to fit the ductwork up to the new air handler. Depending on the type of ductwork and the type of air handler can depend on the type of material selected to make this transition to the ductwork. It is recommended that sheet metal be used for this transition especially if the heating system is in the air handler. If sheet metal is not used in an air handler with the heating system inside the air handler then the installation could fail a code inspection. Always read the manufacturer’s instructions on this as the temperatures inside the supply air side of the air handler can ignite combustible materials. The HVAC contractor will be fully aware of this and most contractors use sheet metal to make these transitions to other types of ductwork. If the ductwork is round then a square to round plenum transition will usually be custom made by the contractor or by a sheet metal shop which the contractor utilizes. If the ductwork is square then a plenum transition will be made to fit the air handler and the duct work together. This usually requires a highly skilled sheet metal mechanic to custom make these plenum transitions. After the plenum transitions are installed then an insulation blanket is added to the plenum transitions to ensure there is no heat loss or heat gain and also to provide a vapor barrier so that when the air in the duct work is less than the dew point temperature the duct work and the plenum transition does not sweat. It is very important to maintain this vapor barrier throughout the ductwork. If one piece of ductwork, whether this is in the plenum transition or ductwork, loses the vapor barrier it can have a domino effect throughout the whole ductwork system as moisture fills the ductwork. As the ductwork insulation gets wetter the R-value of it decreases and it begins to fall off the ductwork. Eventually all of the ductwork insulation is penetrated by moisture and the insulation needs to be replaced. If it is not replaced moisture problems occur with mold, mildew and other problems associated with moist environments.

The Air Handler Control Panel

Most air handler control panels contain high voltage and low voltage wiring, relays, and or a control board. The transformer is usually located in the air handler control panel along with either control wire splices coming from the thermostat and the condenser unit. The air handler control panel is generally the central location where the control wiring meet and are distributed to the proper locations for control. Usually, all control voltage for the thermostat, condenser, and air handler originates from the air handler control transformer. This transformer is a step down transformer which turns high voltage into low voltage. Many un-fused transformers have been burned up by homeowners who change the thermostat without first turning the power off to air handler first. Subsequently, this results in a service call to an HVAC contractor who replaces the transformer and finishes the thermostat installation. There are many different types of controls and ways to control an air conditioning and heating system and we will do our best to describe as many as possible. If you have any questions or would like to suggest a control method which you are familiar with please don’t hesitate to use our forum or the High Performance HVAC contact page to make this suggestion.

The Air Handler Control Panel: Fan Relay Control

Other components in the air handler is a fan control relay or fan control board. This gets its signal from the thermostat on a call for cooling, heating, or if the fan selector switch on the thermostat is set to the on position. Depending on the type of heating system you have the blower may turn on right away or it may not turn on right away. This small time delay for heating is to allow the heat exchanger to heat up before the fan turns on there is also a small time delay for both heating and cooling to allow for the conditioned air, whether it be heated or cooled, to be distributed throughout the ductwork and in to the spaces where conditioning is necessary. This time delay allows for the heat exchanger to cool off and for usable heat to be used in the spaces instead of letting it dissipate in the air handler or up the flue. For cooling this time delay takes advantage of a cool evaporator coil so that it can absorb a little more heat out of the air for conditioning. Some gas and oil furnaces have the heating fan control circuit run outside of the control and to a different control which is mounted near the heat exchanger and has a temperature probe which goes into the heat exchanger. This control is called a fan-limit switch and you will find it in gas furnace air handlers and oil fired furnace air handlers. This temperature probe is for heating fan control and completely bases the control of the fan on the temperature of the heat exchanger. If the temperature reaches a certain manually set temperature the fan energizes. When the temperature of the heat exchanger cools the fan de-energizes. On some of these fan-limit controllers the white wire from the thermostat also runs through these controls. This white wire controls the burner and if the temperature of the exchanger exceeds a safe limit this fan-limit switch controller will turn the burner off and keep the fan energized to dissipate the heat in the system.

The Air Handler Control Panel: Heat Relays

Depending on the type of heat (if you have a hot water boiler utilized for heating this does not apply) the air handler control panel will also have some heat relays or a heating control board to energize the heating system in the air handler. This can be a very complex circuit as it should run through several safety switches before it allows the heat relay to energize. For electric heat you should have some Klixon safety switches and a thermal safety switch which actually melts if it reaches a set temperature which is dangerous. If there is a problem with any of these switches they will prevent the air handler heating relays or control board from energizing the heat in the air handler. This is for your safety and the safety of the structure to prevent a fire or fire hazard. Some of these switches, especially in gas furnaces, are manual reset. These particular manual reset switches are generally referred to as roll-out switches as they prevent a roll-out fire fro occurring. On occasion, these switches will give you nuisance trips and its important to note the exact switch that tripped before resetting and to not reset this switch more than once. The best thing to do is to call an HVAC heating and air conditioning contractor if you have this problem or a problem with any of the safety switches. A dangerous condition may exist and if you continue to manually reset these switches you could make the problem worse.

Air Handler Components Continued on Page 2

Air Handler Related Links

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Air Handler Reviews

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