Air source heat pumps have the efficiency rate that is conventionally named as COP. This rate varies from 1 to 4 and that means how many units of heat it delivers per 1 used unit of electricity taken to operate.
Indicator 1 is when we talk about its lower COP that is at winter season at the temperature lower that -5 °C. Above that point, COP increases and it makes 2 in about +5 °C, while during +10 °C it makes from 2.5 to 3. Indicator 4 is when the temperature is higher than +20 °C, and it is the maximal rate that can be produced by an air pump. In other words, the higher the indicator is the lower your monthly electricity bill is.
With indicator equal to 4, you drop up to 75% of this amount. With indicator 1, you have 0% dropdown, as in this case, your heat pump is the same effective as the air conditioner or air warmer fed by electricity, gas, or fossilized fuel. Although, in the case when they cost more than electricity, you win in the difference of such cost.
But you have to be aware that air source heat pump energy efficiency () is usually lost when the outside air temperature drops down lower than -5 °C. It is true for the conventional systems with ammonia filling of their pipes. There are other systems working on carbon dioxide that lowers the allowable temperature to -30 °C. But you also have to know that they are less efficient than ones having ammonia in pipes during cooling and may require additional helper in the hot days to cool down the premise.
Air source pump is usually used in several further modifications:
· Air-to-air (when a pump takes heat from the outside air and delivers it to your premise through the air module that is installed on the wall, similar to air conditioner),
· Air-to-water, that are further divided onto these:
o Air-to-warm floor – it traverses the heat to the water system of pipes warming up your concrete or wooden floor,
o Air-to-boiler – the heat goes to your boiler that delivers you the hot water to wash or to drink,
o Air-to-heat sinks – the heat is delivered to your heat sinks on the walls.
Air-to-air source heat pump is most efficient one (see here https://www.huskyheatpumps.co.uk/products/air-heat-pumps ), and because of that, the cheapest in installation and maintenance, and it gives up to COP 4. Other kinds of it are less efficient due to energy loss in the transitions between air and water environments and are in between 2.2 and 3.3 in COP in the average temperatures.
If to compare air pump with others, it is not the most efficient, but the cheapest. The ground pump produces up to 7.2 COP indicator, while other pumps mostly used in industrial objects have the theoretical limits of COP up to 8.8 (Carnot cycle device) and 10.1 (Lorentzen cycle device), with +35 °C temperature when air pump gives out 3.8 indicator. Taking the temperature of +55 °C, air pumps produce 2.2, 3.7 for ground pumps, 6.0 for Carnot cycle devices, and 7.9 for Lorentzen cycle devices.
The lower the outside temperature is the smaller the COP indicator is either. In addition, working in winter, it needs to defrost time to time. Also, an air pump is the only of all types of pumps that emanates noise when working that can reach up to 55 dB, which is forbidden in some countries. Air source heat pump winter efficiency drops down to 1 if you temperature is below -5 °C (-30 °C for carbon dioxide models) and is around 3 with +10 °C. The most high efficiency air source heat pump has in summer with mild weather, if it is filled with ammonia that is considered the best fluid for such purposes.
To increase its efficiency at any time of year, you may combine it with solar panels on the roof deriving energy from the sun, and make the house more heat-efficient itself.