Heat Pump Technical Guidance

Heat Pump Technical Guidance

Types of Heat Pump

There are three main types of heat pump:

• Water source - requires a stream or large lake.  Will deliver all the heat needed, at the best efficiency, at the lowest operating cost.
• Ground Source - about the same capital cost as water source, but needs a lot of space if using a trenching system.
• Air Source - low capital cost, but less efficient and therefore higher running costs. Air-to-air is more efficient than air-to-water, but will need a warm air heating system.

They all work in a similar way - by extracting the energy from heat and transporting it elsewhere.  This is essentially exactly how a fridge works.  

Coefficient of Performance

To move the heat requires a pump (hence the name!) and the key information is the "coefficient of performance" or COP which measure how much energy is usefully extracted from the system compared with how much energy has been input.  Generally, heat pump COPs range from 2.5-5; a COP of 4 means that for every 1kW of energy input into the system, 4 kW of useful energy is extracted.
The closer the source and delivered temperatures are together, the better the COP.

Uses

Heat pumps can be used for both space heating and domestic hot water (DHW).  They  work especially well with "wet" underfloor heating which is normally around 35 degrees C (and thus reduce the difference between source and delivered temperatures).  Providing DHW at 50 degrees is less efficient as the temperature difference is greater.

Running costs

A 200m2 house complying with Part L will consume around 11 MWh of space heating annually.  This equates to £495 of gas @ 4.5p/kWh or £550 of oil @42p/litre.

If a 10kW heat pump is installed, and assuming electricity costs are 12.5p/kWh, the annual running costs will be:

Air source (COP 3.1) - £443
Ground Source (COP4) -£344
Water source (COP5.2) £264

Many people are concerned by the excavation required for ground source heat pumps.   In simple terms, the trench required for "slinky" or coiled pipe works well for outputs of 20-30 kW.  Beyond this it is normally necessary to use boreholes to a depth of 100m.  The output can be increased by using more boreholes, although the spacing must be well designed to prevent the ground freezing.
Some of the latest technology uses ground collectors that are pushed in rather than lowered into an excavated bore hole.  These have the advantage of being quicker, quieter and less messy, although they are more expensive and can only reach 40-50m depth.