The number of heat pump installations in the UK has increased dramatically in recent years; when correctly sized and installed with a properly designed heat emitter they deliver savings in two key areas compared with a conventional heating system. Firstly in the quantity of carbon used per kW of heat output to a building, and secondly by achieving a marked reduction in running costs compared with conventional boiler driven heating systems.
For these two factors to be realised the proposed installation needs to be carefully considered: this includes the proper sizing of the heat pump to suit the demands of the property, the correct sizing of the heat emitter to maximise the efficiency of the heat pump, and the integration of appropriate controls to satisfy the demands of the end user and provide a successful integration with the heat pump.
Heat pump size
The sizing of the heat pump is usually as specified by the heat pump supplier and tends to be smaller then that of the equivalent boiler - most boilers in the UK are 'over sized', usually by about 20% to allow for heat up etc. Heat pumps are usually designed for steady state conditions and are therefore sized more tightly.
The heat emitter also has a major impact on the efficiency of the system. European studies have proven that underfloor heating used in conjunction with air source heat pumps are 20% more efficient than when used with radiators, and underfloor heating used with ground source heat pumps are a stunning 30% more efficient. The underfloor heating system design should also be maximised to give greatest advantage and the trick here is to aim to design the underfloor heating system so the required flow temperature of the underfloor heating is as close as possible to the flow temperature for the heat pump: at Continal we specifically design our underfloor heating layouts for use with heat pumps to maximise these efficiencies.
Underfloor heating controls
Integrating the controls between the underfloor heating system and the heat pump can cause issues for the uninitiated, which is why it is preferable to pick a bespoke system tailored specifically for the proposed project that takes into account the customer and heat pump requirements.
There are many heat pump manufacturers with many variants on their design, which require different control solutions. The key revolves around the type of compressor within the heat pump. Most ground source and many air source heat pumps use scroll compressors, which are either on or off, and there is virtually no modulation with them.
Most do have some form of soft start meaning there is a reduced starting current, however it is not good practice for these heat pumps to be 'hunting on and off' because:
- It can send power spikes up and down the power lines and interfere with the neighbouring dwellings' power supplies
- It can reduce life expectancy of the heat pump
- It reduces the efficiency of the heat pump
Therefore the controls and system need to be designed to reduce these effects. The way of doing this is ensure that a heat pump always 'sees' a minimal water quantity.
Some manufacturers do not advocate the use of buffer vessels since they make the system marginally less efficient and therefore recommend the use of open zones to act as the 'buffer' water supply for the heat pump. Open zones do have a potential clash with building regulations, which needs to be clarified. In brief, building regulations require temperature controls for each room. This is achieved with a radiator system by using thermostatic radiator valves (TRV), and for underfloor heating systems by a thermostat located in each room with a corresponding actuator. An open zone can potentially go against this basic principle.
However there are some manufacturers who use reciprocating compressors. The reciprocating compressor is ideally suited for use with an inverter drive, which gives a very low starting current and allows the heat pump to match the demand of the heating system, meaning full control can be used.
