Air Source Heat Pump

What is an Air Source Heat Pump? and what does an Air to Air heat pump mean?

We have talked about how the heat pump works and by now you understand  (much like an air conditioner) it transfers the heat from one place to another. The only difference between an air conditioner and a heat pump is that the it can reverse the cycle of the refrigerant using the reversing valve. This means that the it can cool or heat your house.

But where is that heat that it moves? The heat is in the AIR of your home.

So basically an Air To Air heat pump or Air source heat pump means it absorbs the heat from the air inside your home and rejects it the air outside your home. In the summer time it moves heat from inside- that’s why when you put your hand to feel the air coming out of your outdoor unit it feels hot.

And in the winter the it reverses the cycle and absorbs the heat from the outside air and moves (or rejects) this heat to the inside air of your home. Again a picture is worth a thousand words:

Air Source Heat pump Cooling
Air Source Heat pump Cooling

Got the picture? The Air to Air Heat Pump or Air Source Heat Pump transfers the heat back and forth between the home air and the outside air…

Having said that, I bet you have some questions so I will try to answer them:

First Question - In the winter time the air is cold outside so how can we absorb heat from a very cold air already?

In order to understand how this works fully please review the page on the theory behind the heat pump and the air conditioner (this page explains everything, I promise) . But a quick general answer is that even though the air is cold it still has some heat so if the Air to Air heat pump absorb even a little bit of heat from a huge amount of air, it adds up so it can heat up your home.

But in order to do that the refrigerant going to the outdoor coil has to be colder than the outside air for it to absorb any heat. That requirement creates a limit on how low the outdoor temperature can be for the heat pump to work. As a rule of thumb if the outdoor temperature is any colder than 400 F, the Air to Air heat pump will be very inefficient and will fail to give enough heat for comfort.

The reason is that the refrigerant going to the outdoor is colder than the outside air by about 20 degrees.  That’s means the coil temperature will go below 32 degrees- the freezing point of water.  In a humid climate or rainy weather the outdoor coil will freeze over.

Frozen heat pump outdoor coil

Yup that’s ICE, frozen over an outdoor Air source heat pump coil, and that ice prevents the coil from working.  That’s why all Air to Air heat pumps have what we call it a de-ice or defrost cycle to melt iced up coil.  So if you live in a very cold climate like Canada an Air to Air Heat Pump or Air Source Heat Pump will have a hard time meeting your heating requirements.   

Most heat pumps have an electric coil heater to heat up your home when it is too cold for the normal Air to Air heat pump cycle.  We call this the Emergency Heater.  Some units also have a similar Auxiliary Heater.  These units will still provide enough heat for comfort when it is below 40 degrees outside.  However these electric resistance heaters are very inefficient and expensive to run (if they were cheaper you wouldn’t have a heat pump in the first place).  So you have effectively lost the energy saving benefits of a heat pump if the weather is below 40F for extended periods of time.

Please view the electric coil heater diagram:

Heat Pump auxiliary emerency heater

Second Question- How does can a or Air source heat pump (or an air conditioner for that matter) reject heat from the inside of the house to the outdoors when it is already so hot outside?

Well the answer is almost the same. You still need the refrigerant to have higher temperature than the outside air, for it to be able to move or reject heat. So there is still a practical limit for the outdoor temperature to be. But this limit is about 120F and it is very unusual to find 120F outdoor temperature anywhere.

Don’t get me wrong the hotter the outside temperature the harder your heat pump or your air-conditioner will have to work to keep up and the more hurt you will feel in your wallet when you pay your electric bill. It helps when you choose the location of the outdoor equipment to make sure there is no additional heat load on it- for example, don’t put it on a flat black tar roof. Put it in the shade with sufficient ventilation if you live in a very hot area and make sure the temperature where you locate the equipment is not more than the ambient outdoor temperature.


· Air to Air Heat Pumps can be very efficient and cost effective systems. Since they are by far the most efficient electric HVAC systems they are especially cost effective if you live in area where the price of natural gas is more expensive than the electricity.

· They also make sense if you live in a rural area where it is difficult to get natural gas or propane.

· In a cold climate areas Air to Air Heat Pump or Air source Heat Pump will have a very hard time & sometimes may fail when the outdoor temperature falls below 35o F.

· The Heat Pump thermostat is completely different from and more complex than the air conditioner thermostat, because the Heat Pump thermostat controls the heat pump itself as well as the Auxiliary Heater, the Emergency Heater and the Reversing Valve.

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