Air Handler Components 3 - Hot water coils inside of air handler units combine a boiler system or a special capacity hot water heater and forced air to provide forced-air heating. The concept is simple and used a lot in commercial and some in residential.
Commercial systems almost exclusively use a boiler to provide hot water to the hot water coil inside the air handler unit while the residential systems use mainly special capacity hot water heaters to provide hot water to the hot water coil inside the air handler unit.
On a call for heat, a pump is energized to begin the circulation of hot water from the heating source to the coil. The air handler unit blower motor energizes and the air begins to move across the coil. Heat is exchanged from the hot water coil to the air and the air is delivered to its appropriate destination through the ductwork.
Air Handler Components 3 - Heat Pump AHUs
Heat pump air handler units are similar to straight air conditioning air handler units except heat pump air handler units have a backup source of heat inside them. Most heat pump air handler units utilize electric resistance heating for backup heat but there are some that utilize gas for backup heating.
See the Air handler unit Components section for detailed specifics on the electric and gas heating in air handler units. The heat pump air handler unit has a refrigeration coil located inside of the air handler unit or very close to the air handler unit connected with a plenum transition.
On a call for heat, the condenser and the air handler unit blower motor energize. The heat pump condenser pulls heat from the air outside even when it is cold and through the process of refrigeration it moves this heat to the indoor coil. This indoor coil becomes very hot and as air passes over it the air absorbs the heat and is delivered to its destination through the ductwork. This process continues until the condenser unit kicks into the defrost cycle which is necessary (except for geothermal heat pumps).
When the condenser changes to defrost the indoor coil becomes cold like it would in the air conditioning cycle. The condenser, when it begins the defrost cycle, sends a signal to the air handler unit signaling it to energize the backup heat source to counterbalance the cool coil. This process continues until the defrost cycle ends and the condenser begins sending hot refrigerant gas back to the coil in the air handler unit.
Air Handler Components 3: The Air Conditioner Evaporator Coil
The air conditioner evaporator is normally located in the AHU but it can also be located in the ductwork a short distance away from the air handler unit. A plenum transition is used to separate the AHU and the evaporator if the evaporator coil is not an integral part of the air handling unit. Either way it is configured the evaporator coil is a heat exchanger and it generally operates below the dew point. For comfort reasons, it needs to operate below the dew point. This coil dehumidifies as it cools.
The air in the conditioned spaces contains humidity and the higher the humidity in the air the warmer a person feels. Therefore, by design, the evaporator operated just below the point but stays above 32° F. to prevent this moisture from freezing to the evaporator coil.
Because the AHU evaporator coil operates below the dew point it sweats. This moisture must be drained away from the AHU otherwise the area around the AHU will become wet and possibly flood the area surrounding the AHU. A drain pan is installed by the manufacturer of the air handler unit to drain away this condensation.
Air Handler Components 3
Older model air handler units have drain pans constructed of metal and this metal will eventually rust and start leaking the condensation around the air handler unit. Newer model air handler units have plastic pans which are durable and never rust through. They are subject to cracking if impacted hard enough.
It is important that the air handler unit and the evaporator drain pan inside the air handler unit are slightly tilted towards the drain of the evaporator drain pan and to the drain pipe. It is also important that this drain pipe have a negative slope so that the condensation drains properly.
It is also important that a periodic inspection be made of the evaporator coils to ensure they are not clogged with dust, dirt, or debris. If you have a heat pump the same coil used for heating is also used for cooling.
Air Handler Unit Components 3: The Air Handler Unit Blower Motor
The AHU blower motor is generally mounted to a squirrel cage with a heavy-duty mounting bracket. Attached to the shaft of the blower motor is a blower wheel. This blower wheel and the RPMs of the blower motor have been engineered to deliver a specific quantity of air through the air handler unit and the ductwork.
It is important that if either the blower motor or the blower are replaced that the same exact RPMs of the blower motor and the same diameter of the blower wheel be used for the replacement part if an exact replacement can not be found.
Air Handler Components 3 | Commercial Air Handler Blowers
Large commercial air handler units use indirect drive motors where they need a belt(s) to connect a blower motor pulley to a shaft pulley. The blower wheel is attached to the shaft which is driven by the belts and the blower motor.
Some of these pulleys are adjustable and set by testing and balancing professional who uses sensitive instruments to make CFM calculations and set the pulley adjustment to engineering specifications. Belts on large commercial systems get changed on a regular maintenance schedule along with the blower motor and shaft bearings being lubricated.
Air Handler Unit Components 3: The Air Handler Unit Filter
The air handler unit filter is necessary for the proper functioning of the equipment. In some air handler units, the air handler unit filter is not located in the air handler unit but in a remote location in the return duct where is easier to access. It is necessary to maintain a good air filter maintenance schedule for all air handler units otherwise problems begin occurring and the heating and cooling systems could fail.
An HVAC technician can always tell if a good filter maintenance plan has been implemented when the tech looks in the air handler unit. If the tech sees dust and dirt build-up then the filters have not been maintained properly. This creates more problems and leads to more breakdowns of equipment including motors and clogged and inefficient coils.
Air Handler Components 3
Technical Resource: Refrigeration and Air Conditioning Technology