Air Source Heat Pumps come in two basic models: a low temperature monobloc unit or high temperature split units.

Low temperature heat pumps consist of an outdoor unit that can comfortably heat water to 50C with outdoor temperatures down to -5C without losing efficiency or heat output so ideal for our climate. High temperature heat pumps consist of an outdoor unit and an indoor unit which heat the water to 65C. Low temperature units are more efficient with a SCOP (Seasonal Coefficient Of Performance) of above 4.0 for a well-designed system. This basically means their efficiency rating so for 1 Kilowatt of electricity they produce 4 Kilowatts of heat.

With a new build property which is well insulated with under floor heating the flow temperature can be designed for 40C greatly increasing the efficiency/ SCOP.

When retro fitting an air source heat pump into an existing property which has cavity wall insulation, double glazing and 200mm of loft insulation we can design the system to run at 50C but sometimes need to increase the size of some radiators (three on average) and swap the hot water cylinder as the lower water temperature (compared with 65C of a traditional gas boiler) needs a larger surface area to create the same amount of heat.

High Temperature heat pumps are not so popular despite the fact they get the water flow temperature to 65C because they are not so efficient with a SCOP of around 2.8. and obviously we need space for the indoor unit to be sited. Government grants and incentives do not generally apply to high temperature units.