Air-Conditioning nowadays is a must have item in many hot countries, but did you know most units are useful in cold countries, as they can be reversible, to heat your homes.

Basically an Air-Conditioning unit is a Heat Pump.

Heat Pumps work by removing heat from one area and moving (Expelling) it to another area.

Most of us have a Heat Pump device in our homes already: The humble Fridge or Freezer! These use heat pump technology, by removing heat out of the Food compartment (Cooling), to outside of the fridge, where it is expelled (Heating the surrounding air).

To understand how Heat Pumps work, we can dissect the inner workings of a Fridge and analyse what makes it tick.

All Heat Pumps use a medium to transport the heat energy from one area to another. The medium used is normally a gas which at high pressure will condense into a liquid. The changing in state allows this medium to absorb heat energy and expel heat energy.

From the slide below, it can be seen that by changing the pressure (by compressing/expanding), of the gas/vapor/liquid it can be used to absorb and expel heat energy, from one region to another.

From general observation, we know that heat will always flow from a hot area or surface to a cold area or surface, for example if you open your house door during winter, the nice warm air will disappear outside. In our fridge the warm air will be attracted to the cold evaporator coils in the food compartment, and the heat from the radiator (Condenser), on the outside of the fridge will be transferred to the cooler air there.

Different mediums will change state at differing pressures and temperatures. We all know that when you boil water, it turns into steam, so at 100 degrees at normal atmospheric pressure it turns from a liquid to a vapor (Gas). Water would not be a good medium to use in a Fridge Heat Pump, as temperature & pressures could not be controlled low enough to be of any use. However Freon Gas’s temperature & pressure can be controlled adequately to expel heat energy from inside our fridge. This relationship between temperature & pressure is better explained below.

Now we can see how Heat Pumps are useful for transferring heat from a relatively small space (Fridge Food Compartment) to a relatively larger space (Room housing Fridge). If we scale this process up we can see how Air Conditioning units work, transferring large volumes of heat from inside, to outside our houses. Plus, as mentioned previously, if we reverse the Heat Pump we can effectively transfer heat into our house to heat it in Winter.

In Heating mode most modern Units will have an average COP of between 3 to 5, meaning they will be 3 to 5 times more efficient than an equivalent electric bar fire.