How a heat pump works, without the jargon

The fridge, in reverse

Your fridge takes heat out of the food and dumps it out the back. A heat pump does the same job the other way round: it takes heat from outdoors and dumps it inside your home.

A refrigerant liquid boils at very low temperatures. Outside air warms it into a gas, a compressor squeezes that gas so it gets hot, and the heat passes into the water in your heating system. The refrigerant cools, turns back to liquid and goes round again.

What the numbers mean

• COP (coefficient of performance): heat out ÷ electricity in at one moment. 3.5 means 3.5 kWh of heat for 1 kWh of electricity.
• SCOP: the same measure averaged over a whole Irish heating season. This is the number to compare.
• Flow temperature: how hot the water going to your radiators is. Heat pumps are most efficient at 35 to 50°C, compared with 70°C+ from a boiler.

Why it suits Ireland

Our winters are mild and damp rather than freezing. Outdoor temperatures in Dublin rarely drop far below zero, which is exactly the range where air source heat pumps work best.

The trade off is that a heat pump heats gently and steadily. It is designed to keep the house at an even temperature all day, not to blast it warm for an hour in the evening.