Question 1

How does a heat pump work?

Accepted Answer

A heat pump works on the same refrigeration cycle principle as a fridge or air conditioner, but in reverse. A refrigerant fluid is evaporated by absorbing heat from a low-temperature source (outdoor air for an ASHP, or ground/water for a GSHP). The refrigerant vapour is then compressed by an electrically driven compressor, which raises its temperature. The hot compressed vapour passes through a condenser coil where it releases its heat to the building's heating and hot water circuit. The refrigerant then expands through an expansion valve, dropping in temperature, and the cycle repeats. The key characteristic is that heat pumps move heat rather than generate it - they deliver 2.5-4 units of heat for every unit of electricity consumed.

Question 2

What is COP and SCOP for a heat pump?

Accepted Answer

COP (Coefficient of Performance) is the ratio of heat output to electrical input at a specific operating condition (e.g., at -7 degrees C outdoor temperature and 35 degrees C flow temperature). A COP of 3.0 means 3 kWh of heat is delivered for every 1 kWh of electricity consumed. SCOP (Seasonal COP) is the average COP over the entire heating season, accounting for varying outdoor temperatures and is a better indicator of annual efficiency. UK air source heat pumps typically achieve SCOP of 2.5-3.5. Heat pump efficiency falls as outdoor temperature drops (less heat available to extract) and as flow temperature rises (harder to compress to a higher temperature) - this is why well-insulated buildings with underfloor heating (low flow temperature) are ideal for heat pumps.

Question 3

What is the difference between an air source and ground source heat pump?

Accepted Answer

An air source heat pump (ASHP) extracts heat from outdoor air. The outdoor unit (similar in appearance to an air conditioning condenser) houses the evaporator coil and fan. ASHPs are easier and cheaper to install than GSHPs as they do not require groundworks. A ground source heat pump (GSHP) extracts heat from the ground via a network of buried pipework (ground loops) filled with antifreeze solution, or via a vertical borehole. The ground temperature is more stable than air temperature through the year (typically 10-12 degrees C at 1m depth in the UK), so GSHPs achieve higher and more consistent SCOPs (typically 3.5-4.5). The ground loops require significant land area (typically 200-400m2 for an average house) or expensive boreholes, which limits GSHP suitability.

Question 4

Does a heat pump need a well-insulated house?

Accepted Answer

A heat pump works best in a well-insulated, airtight house with a low-temperature heat distribution system (underfloor heating or oversized radiators). This is because: a better-insulated house has a lower peak heat demand (kW), so a smaller and cheaper heat pump meets the load; and a low-temperature distribution system (underfloor heating at 35-45 degrees C flow temperature rather than radiators at 60-70 degrees C) allows the heat pump to achieve a higher COP. Heat pumps can be retrofitted into older, less well-insulated houses, but the heat pump will need to be sized for the higher heat demand, may need to run at higher flow temperatures (reducing efficiency), and may need supplementary electric immersion backup in very cold weather. Improving the building fabric before installing a heat pump is always advisable.

Heat Pump

Just so you know...