No energy bill anymore for the home of the Stuart Family in Glasgow

For many people a heat pump is a bit of a difficult thing to understand. That’s because many people confuse the concepts of heat and temperature. However, everyone has one in the house, namely a heat pump on the refrigerator. An air conditioner is a heat pump as well, that works the other way around.

(image credits: Photographer Stirling)

What does a heat pump do exactly?

In short: it moves heat (= energy) from a lower layer (not usable) to a higher temperature level (usable) temperature level. In the case of the Stuart family, a air/water heat pump was used in the summer of 2016.
Air is used as a heat source (outside air) and the heat is issued to water used for the underfloor heating system.
For example, even if the outdoor temperature os -5 °c, heat from the outside air can still be extracted by, for example, to cool the air down to -10°c. Reverse this process, then air from -10 °should be heated up to -5 ° so a certain amount of heat (energy) needs to be added. Vice versa you can extract the same amount of energy from the air when it is cooled from -5 ° to -10 ° c.

The beauty of a heat pump is, that when you put 1 kW of power in, and the outside air has thermal energy of 2kW, then there is  a quantity of thermal capacity of 2 + 1 = 3 kW available in the house. In other words, you use 1 kW of electric energy and you get (in this example) 3 kW of heat in return! By comparison, normal electric heating systems (e.g. electric heating devices) takes e.g.. 1 kW of electricity on and also give you back 1 kW of heat, if it is a really efficient system.

The ratio between the quantity of useful heat delivered and the electrical energy that is pur in, is called the COP (Coefficient Of Performance). So in the above example the COP so 3. In our House we took an average over the period May 20 2015/May 2016 (including warm water supply) of 3.51. We measure the heat with a Kamstrup Multical 601 heat meter in combination with an Ultra flow part 54. The maximum water temperature in the underfloor heating (with am outside temperature of -12° c outside) is about 35°c. And our tap water temperature is 52°c. That could be set lower for showering, but then it’s too cold for kitchen use.

Our heat pump can to-15 ° c rated capacity. Remark: In the UK central heating installations are designed for outdoor temperatures of -12 ° c.

Energy efficiency with underfloor heating

The COP of a heat pump becomes higher as the temperature of the heat source and the temperature at which the heat is issued are closer together. So the heat source should be as warm as possible and the heating system must be a low temperature heating system to be able to work efficiently with a Heat Pump. 

Examples for low temperature heating systems are underfloor heating Glasgow as well as wal heating or special convectors with a large surface area (and/or support by making a few small fans).
So it does not make sense for an existing conventional central heating system, originally designed for the standard water temperatures of 90 ° c and 70 ° c supply return (at the design point of -12 °c outside temperature), to be used in combination with a heat pump.

There are also heat pumps using soil as heat source (by means of a water source or by hundreds of metres of hose in the garden like you see in the image above. These systems are more expensive to purchase, but cheaper in use (higher COP), because the heat source (ground) in the heating season gives a higher temperature (ca 11 ° c) than the outdoor air.