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Effects of a Clogged HVAC Air Filter

What happens when your filter clogs up with dust and debris? The effects of a clogged HVAC air filter in your air conditioner or heat pump can cause failure of the system operation that will result in a call for an air conditioner repair service.

An AC, heat pump, gas furnace, electric furnace, or even an oil furnace needs the proper amount air flow across the coil or heat exchanger to operate properly. When the filter clogs the air flow is minimized the blower works harder and harder to pull the proper amount of air across the coil or heat exchanger. What are the effects of a clogged filter in your HVAC system?

Serious Harmful HVAC Issues Caused by a Dirty Clogged Air Filter

  1. Cause your air conditioner to stop cooling
  2. Cause your air conditioner to freeze up and when it thaws the water from the melting ice can overwhelm your condensate drain and cause water damage
  3. Cause a serious mechanical issue with your compressor in your air conditioner or heat pump technically referred to as liquid slugging
  4. Cause your furnace and/or heat to stop heating

Effects of a Clogged HVAC Air Filter

  • As the system airflow is reduced from a restricted and clogged filter your air conditioner or heat pump (in cooling mode) evaporator coil will begin freezing up resulting in less and less air flow from the supply vents. The refrigerant is still flowing through the coils but with no air-flow, there is no heat absorbed into the coils and refrigerant. The refrigerant falls below the freezing point and any moisture in the air around the coil begins freezing. It effectively turns the air conditioner or heat pump evaporator coil into a freezer. If the air filter is changed then you will still have problems until the evaporator coil is defrosted. With an air-conditioner, you turn it off for several hours to allow it to thaw. With a heat pump, you can simply turn it to heat mode and it will automatically defrost the evaporator coil. This problem can cause serious water damage to your home or business.
  • Furnaces with clogged air filters will stop heating. As the air flow is restricted the heat in the heat exchanger gets higher and higher. Designed into every furnace are limit switches that will open when the temperature exceeds a designed temperature. When the high limits open the burners turn off and the heat stops. Many times these are automatic reset switches and will automatically reset when the temperature of the heat exchanger cools but if the clogged air filter is not corrected then the problem will recur and the furnace will not provide the proper amount of heat. Sometimes the limit switch will get stuck in the open position resulting in a service call from a contractor to replace the part.

Pressure in the System and Effects of a Clogged HVAC Air Filter

The pressure in the HVAC duct system and air handler are very important. If your system was designed correctly the designer designed the system for a specific amount of pressure as recommended by the manufacturer of the equipment. When your filter gets dirty you change the pressure inside the system and by doing so you change the amount of airflow flowing through the air handler and ductwork. There are other ways to change this pressure inside the ductwork such as closing off too many supply vents, a bad blower motor, or collapsed/restricted ductwork, but here we are focusing on dirty air filters. We have covered those other things in other articles if you want to browse our other articles for that information. Here is an example to illustrate the pressure in the system:

dirty air filters and system pressure

So you can see from the chart above the manufacturer recommends a design pressure of .05″ water column. If this was your system then it should be designed and balanced to achieve that pressure. At that pressure, we get the other variables listed and highlighted on the chart. CFM’s, and temperature rise. This is also the design pressure needed for the evaporator coil to function properly. When you have a dirty clogged air filter, you change all that including the pressure, the CFM’s, the temperature rise, and the proper functioning of the evaporator coil. That leads to problems that will affect your comfort.

Effects of a Clogged HVAC Air Filter

The effects of a clogged air filter in your HVAC system can end up causing damage to the air conditioner, heat pump, or furnace so make sure you keep the air filter clean and free of debris. One other big mistake that will have the same effect of closed off supply vents. Anything that reduces the air flow in your HVAC system is not good for it. Keep the supply vents open and filters clean and avoid costly repairs.

High Performance HVAC

Effects of a Clogged HVAC Air Filter - HVAC

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