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IAQ - Indoor Air Quality

Part Glossary


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  • Musty smell
  • System not cooling properly
  • Is my system ready for the cooling season?
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Musty smell


Some cooling systems develop a musty, unpleasant smell when they run in cooling mode for the first time after a season or sometimes after installation.

This is commonly known as "dirty sock syndrome" in most cases and it is related to microbial activity in you evaporator coil, in most of the cases you cannot see growth on the surface of the evaporator coil but your nose can definitely tell.

This happens most of the times in heat pump systems rather than furnaces, since temperatures in gas fired furnaces exceed 160 degrees which kills micoorganisms. Temperatures at the evaporator coil in heat pumps are between 105 to 130 degrees, making this pleasant for microorganism activity.


So what can you do to alleviate this?


Ice Around Your Heat pump

During the heating season you might see ice around the coil of your outdoor system, this happens usually when the temperature outside drops below 42-45 degrees and there is enough moisture in the air to produce ice. While your heat pump is running in heat mode, the outside air is used as part of the refrigerant cycle since it helps convert liquid refrigerant into a gas as it goes through the outdoor coil, that's the purpose of the top condenser fan motor, to move air across the coil. In this case, the air coming from the top is at least 10 degrees colder than the air going in through the sides.

So if we are at 42 degrees outside the temperature of the air going through the outside coil is about 32 degrees or lower and that is when ice starts to build around your system.


















The Defrost Cycle

Your heat pump comes with a built in defrost cycle that is triggered by an outdoor sensor, runtime and a coil temperature sensor, if the system detects that a defrost cycle is needed you will see this happen:




















  • The outdoor system will turn off.
  • You will hear a switchover valve (sounds like something releasing pressure).
  • The outdoor system will come back on with exception of the top fan, it will run anywhere from 1 minute up to approx. 10 minutes. The compressor will sound a little louder than usual, that is completely normal because the compressor is building up heat as it runs without the top fan to defrost the outdoor coil. At this point, you might see condensation around the system.
  • During the defrost cycle you might see EM heat or Aux Heat displayed on your thermostat, that is an indication that your "electric back up heater" is on to keep your home warm during the defrost cycle.
  • Once the defrost cycle is done, the condenser will turn off, the valve will switch over again and the system will come back on, this time with the top fan on. At this point you might see vapor coming out from the top of the system, that is completely normal.
  • Once the defrost cycle is done, the system will resume normal operation.


Other brands:

  • The same steps above apply with the exception of the system turning off while the valve switches over. Trane incorporated that feature to make the defrost cycle quieter. Other brands are adding that as an option that can be turned on by a certified technician.
frozen heat pump, heatpump, defrost cycle
Frozen heat pump, frozen coil, trane coil, defrost cycle

Before the defrost cycle

During the defrost cycle

During the defrost cycle

Things To Look For

One of the most common things to look for during a cold rainy day is ice building on the top of your heat pump, this happens when water drips from the edge of your roof right over your heat pump. This causes that loud banging noise that sounds like your outdoor system is falling apart or a piece of metal fell off and started bouncing in there. Most of the times it is small pieces of ice breaking down and hitting the fan blades, some systems accumulate enough ice that it builds a hard layer around the fan blade causing them to get stuck and cause one of the following problems:

  • Fan motor failure
  • Unbalanced or broken fan blades
  • Compressor failure


If this happens, switch your system to Em (emergency or auxiliary heat) mode on your thermostat, this will turn off the outdoor system during the heating cycle and will start using only the electric heat (located at your indoor system) to heat your home.

You will need to talk to your roofer or a handyman to install a rain guard or drip guard (unless you want to install rain gutters), it is a piece of sheet metal that will prevent water from falling over your condenser and causing damage.

frozen condenser, condenser top, ice on condenser
condenser top iced, frozen top, frozen condenser
frozen condenser top, ice on condenser, frozen heatpump

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