Tip & How-To about Heating & Cooling

How Heat Pumps Work

How on earth can you get hot air or hot water from very cold outside air? How does a heat pump make this heat out of cold air?
When certain gases change their state from a liquid to a gas or from a gas to a liquid the magic happens.

A heat pump can do some pretty amazing things, when it comes to making heat. To understand the way they work, you must first understand what happens when certain gases change from a liquid state to a gas and back to a liquid again. To simplify it a bit we need to look at a basic air conditioning system. The refrigerant in an air conditioning system is changing to a liquid in the outdoor condenser. The compressor compresses the gas forming a hot gas. As this gas cools under high pressure it changes into a liquid form.

Inside the indoor coil the liquid is pushed through a small hole or orifice. When it comes out the other side it rapidly changes it’s state to a gas as the pressure suddenly drops. The rapid pressure drop changes the refrigerant to a very cold gas. Air flowing over the cold gas inside the pipes makes the air cool and provides air conditioning.


Now for a heat pump the process is much the same, only coils are just switched by used a reversing valve in the outdoor condenser unit. Instead of the heat being purged to the outside from the condenser unit, the indoor unit then becomes the condenser coil. The outdoor coil becomes the cold coil and cools the outside. This is why if it is very cold outside the unit will have to run in air conditioning mode for a short time to defrost the outside cold coil. When this happens the system will usually have electric heat inside to run and keep the indoor air warm.

The reality is that a heat pump is really no more than an air conditioner running in reverse. Through the magic of using refrigerants, a heat pump can then run many times more efficiently than straight electric heat. A careful heat loss/gain should always be performed by a qualified service person before any sizing of a unit. Too large or too small of a heat pump system can make it inefficient and possibly cause it to be very uncomfortable. The longevity of the unit can also be affected by sizing and installation. Be patient and look carefully for the best contractor to install your system.

A properly designed and installed heat pump system can give you many years of comfort and efficiency. If you look for and find the contractor that can do the job right you will get many trouble free comfortable years from your heat pump unit.

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I have a Lennex heat pump unit which was installed in 2008. Our area (Savannah, GA) has experienced some abnormal cold temps lately. Yesterday, I noticed the outside of the unit had frost on the unit. The frost didn't cover the entire unit, yet yet in perfectly shaped lines (if you will), that encompassed the shell of the unit.. I also noticed frozen water at the base of the unit. The system is heating okay. The temp was in the low 20s when I noticed it. Is this something that needs attention or is it "normal"? I have a picture of it as well.


Heat pumps are the same as an A/C unit only it works in reverse. Instead of blowing cold air inside and hot air outside it blows hot air inside and cold air outside. The heat being blown inside is extracted form the cold air outside. Its called Latent heat Latin for hidden. Believe it or not even though its 20 degrees outside there is still heat in the air. Example if you want to drop the temperature of a freezer from 20 to 10 degrees you have to remove heat .I know your getting unusal weather and that is the problem. Heat pumps can only work no lower then around 20 degrees outside.Heat pumps are never used above the freeze line. With your unusal temperatures thats basically where you are now. Ride it out. When your temps rise a little your be fine. Im in Chicago.

Jan 15, 2011 | Heating & Cooling

1 Answer

All water lines are not hot


Do you mean refrigeration lines? In general the bigger line of the two should be cold and the smaller line should be warm to touch. Depends on what the ambient temperature is outside. If it is cold outside the little line or liquid line will be cold. And if both lines are Hot you either have a heat Pump or the Valves are out of the Compressor. Will need more information on your problem.

Dec 19, 2010 | Heating & Cooling

3 Answers

uotside unit blowing cool air but hot on the inside


It sounds like you have a unit called a heat pump. When a heat pump unit is in cooling mode the outside unit blows hot air and the inside unit blows cold air. If the outside unit is blowing cold then the indoor unit is most likely blowing room temperature or warm air. It sounds like the thermostat could be controlling incorrectly or you could have a failed reversing valve which is the device that switches the unit from heating to cooling mode.

May 31, 2009 | Ruud UBHC Air Conditioner

1 Answer

1995 Nissan pathfinder over heating blowing cold air.


Sounds like either your thermostat, which is the least expensive and easiest thing to change, or your water pump, or clogged radiator, which is a whole new world! The reason it wont blow hot, is because you are not getting any water to the heater core. Hope this helps.

Jan 15, 2009 | 1995 Nissan Pathfinder

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