Caution is always advised around mechanical and electrical equipment including the air handler. Turn the power off before removing any panels.

Air HandlersAir Handlers - An air handler is usually located in the garage, a closet, or in the attic (unless you have a package unit then the air handler is incorporated into one system). It is essential to any AC unit and heating forced air system. All AC units need an air handler or way to move air. It can be a part of your furnace and houses the evaporator coils, the blower motor, and some HVAC controls. It can be an up-flow, downflow, or horizontal flow AHU (air handling unit).

Air Handlers | Heating and Cooling

To determine which one you have to follow the return duct. The return duct should originate where you put the HVAC filter and/or is the biggest register grill in the house. If the return ends at the bottom of the unit it is an up-flow. If the return duct ends in the top of the unit it is a downflow. If the unit looks as if it is lying on its side with the return duct coming in one side the supply ducts going out the other it is a horizontal flow.

Filters for Air Handlers

Knowing this information can help you find the filter if you have never changed the filter in this air handling unit. Some air handling units (AHU’s) have the filter inside.

If there is no filter in the return register or grill then the filter is either in the return duct somewhere or in the unit itself. Filtering the air is not only important for the indoor air quality in your home but it is essential for the proper operation of the air handling unit itself. The air must be filtered before it reaches the evaporator coils or heat exchanger.

What Happens When No Filter

If it is not then there will be a build-up over time of dust and debris that get sucked into the return. This build-up clogs off the evaporator coil and causes the heat exchanger to operate at higher than normal temperatures. The air handling unit becomes less and less efficient and will eventually fail to cool or heat the home. HVAC preventive maintenance on your air handler includes maintaining good filter maintenance schedules. Carrier air handling units are no different that York Air Handlers or Trane Air Handler Units and they need a basic preventive maintenance schedule to prevent premature breakdowns and to keep the air handling unit operating at peak efficiency.

Air Handlers - Spring Maintenance Checks: Cleaning the Evaporator Coils

Spring maintenance checks to the air handler can help you avoid costly AC repairs (heating repairs in winter) when the heat of summer arrives. An air handling unit is essential to any AC unit and requires some preventive maintenance attention. A word of caution is advised here before you open the panel. Air Handling Units have high voltage running into them and there is a shock hazard.

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Before you remove the panel, make sure the power is turned off to the air handler. There are times when a heating and air conditioning technician has to operate the air handling unit with the air handler panels off. Only an HVAC professional should operate the air handler unit with the panels off. Even with the thermostat in the off position the air handling unit has high voltage running into it.

Turn the circuit breaker off before opening any panel on any HVAC equipment. Inside you will find the evaporator, metering device (on most units), the blower motor, and some electro-mechanical controls for HVAC control. The metering device and the electro-mechanical controls should be checked by an HVAC professional. These components are highly technical and beyond the scope of this site to explain in detail. The evaporator and blower motor can be maintained by the homeowner as long as safety and common sense are applied.

Checking the Evaporator Coil

The evaporator coil, inside or near the air handler, carries refrigerant inside it. This coil and refrigerant, through the heat exchange process, absorbs heat from the air passing through the coils inside the air handler. The heat causes the refrigerant inside the evaporator coils to boil and change state. The refrigerant, where it enters the coil, is mostly a liquid. By the time it reaches the end of the coils it should have absorbed enough heat to change it from a liquid to a vapor*.

On the outside of the unit, there are two copper lines. One large and insulated line, and one small and un-insulated line. The large line is the suction line. This line carries the vapor (refrigerant) back to the compressor in the condensing unit. The small line is called a liquid line. This line carries liquid (refrigerant) from the condensing unit coils to the evaporator coil in the air handler. When the unit is running the liquid line should be hot and the suction line should be cold. *(Unless it is a heat pump in the heating mode).

Varying Temperatures

The temperatures of these lines will vary depending on how hot the house is inside and the ambient temperature outside the home. A big problem most people encounter with the evaporator coils is blocked or dirty coils. The coils are plugged with dust, dirt, and other debris, there is a duct collapsed somewhere, or there are too many supply vents closed off in the home. For the evaporator to work properly and efficiently the coils must be clean and have a measured amount of airflow (the amount of airflow required for your evaporator coil inside the air handling unit depends on the tonnage of the system. More precise, 400 CFM’s equal one ton or 12,000 BTU’s).

Thus, the necessity of a good air filter. A good filter to filter all the particles from the air before it reaches the coils. Coils operate below the dew point when the air conditioner or heat pump cooling cycle is on. This will make the evaporator coil wet. When the dust makes contact with the evaporator coil it will often stick to the evaporator coil. Over time this will cause a buildup and eventually the HVAC system will stop cooling.


With improper flow across the evaporator coils, there is no heat exchange process. The coils will freeze and ice will form on them. Another cause of ice forming on the evaporator coil is a low refrigerant charge. If the evaporator coils are clean and they are icing up, you need to call an HVAC professional. They need to check the air handling unit and the condenser.

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