Air Handler Components

There are some hot water and cold water coils which are not fixed in the air handler. They are downstream in the ductwork. There are also some gas and electric duct heaters which are mounted in the ductwork. They are completely reliant on the air handler for air flow. These systems will also be covered. Hopefully, this article will offer you a comprehensive look at the air handler, all its parts, and how it functions.

Air Handler Components - The Air Handler Cabinet

The air handler cabinet is comprised of sheet metal. Manufacturers use a heavy gauge sheet metal for durability and usually give the cabinet a good finish. This prevents rust especially in moist environments where air handlers usually operate. The heavy gauge sheet metal for durability is generally required to meet code requirements. 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-position air handler. This type of air handler can be used for up flow, downflow, or for horizontal flow left to right or horizontal flow right to left. Whatever the job calls for the multi-position air handler can be quickly converted in the field. All without special ordering from the manufacturer.

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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 ductwork is less than the dew point temperature the ductwork 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.

Air Handler Components - AHU 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. Usually when they attempt to change the thermostat without first turning the power off to air handler first. Subsequently, this results in a service call to an HVAC contractor. This requires replacing the transformer to make the system work again.

There are many different types of controls and ways to control an air conditioning and heating system. 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.

Air Handler Components - The AHU 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 or heating. Additionally, 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. It may be on a time delay. 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 into the spaces where conditioning is necessary.

Time Delays

This time delay allows for the heat exchanger to cool off. It also allows for usable heat to be used in the spaces. That is 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. This is so that it can absorb a little more heat. Some gas and oil furnaces have the heating fan control circuit run outside of the control. It runs to a different control which is mounted near the heat exchanger. It has a temperature probe which goes into the heat exchanger. This control is called a fan-limit switch. You will find it in gas furnace air handlers and oil fired furnace air handlers.

This temperature probe is for heating fan control. It 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. If the temperature of the exchanger exceeds a safe limit this fan-limit switch controller will turn the burner off. It will also keep the fan energized to dissipate the heat in the system.

Air Handler Components - The AHU 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. These controls are used 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 from occurring.

On occasion, these switches will give you nuisance trips. It is 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. Especially 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.

Your Resource for HVAC Air Handler Components Information

Technical Resource: Refrigeration and Air Conditioning Technology