What is a heat pump? How it works in an Irish home

What it is

There's no flame and no fuel. The outdoor unit pulls air across a cold surface, takes the heat out of it, raises that heat to a useful temperature and passes it into the water in your pipes. The same water goes to the radiators and to the hot water cylinder. SEAI put the size of it like this: a heat pump uses less than a third of the energy an oil or gas boiler uses for the same heat.

Three things change in the house. A unit about the size of a large suitcase goes outside, ideally on the gable and away from bedroom windows. A hot water cylinder goes in, if there isn't one already. And some radiators get bigger, because the water going round is cooler than a boiler's.

That cooler water is the part people notice. A boiler blasts a room hot and then stops. A heat pump runs long and low, so the house sits at one temperature instead of swinging. It is a different way of heating a house, and it takes a week or two to get used to.

Compared with the boiler you have now

	Oil or gas boiler	Heat pump
What it uses	kerosene, mains gas or bottled gas	electricity only
What a unit of heat costs	oil 18.0c, gas 14.4c, bottled gas 25.2c	9.8c
How it runs	short hot bursts, on a timer	steady and cool, most of the day
Hot water	as hot as you like, on demand	at least 80% of it, and usually all
What the house needs first	nothing	to hold heat well enough for SEAI's test
What changes outside	a flue, or an oil tank at the gable	a unit the size of a large suitcase
While the work is on	a day	SEAI say to expect a few days with no heat

Useful c/kWh, SEAI Domestic Fuel Cost Comparison, 2026-07-01 edition: heat pump 9.8c at SPF 3.5 · gas 14.4c at 90% · oil 18.0c at 70% · bulk LPG 25.2c · direct electric 34.4c. Air-to-water assumed throughout; air-to-air gives no hot water. Grant for the unit €4,500 to €6,500, plus €1,000 to €2,000 for pipework and radiators and €4,000 for leaving fossil heating behind — up to €12,500 for a house, €9,500 for an apartment, read on seai.ie 8 September 2026.

Where the saving comes from

A boiler makes heat. Burn a unit of oil and you get most of a unit of heat back — never more, because you can't get more out than you put in. A heat pump doesn't make heat, it carries it: a unit of electricity goes in and about three and a half units of heat come out, because most of the heat was already in the air outside and only had to be moved. That's the whole trick, and it's why the running cost falls even though electricity costs more per unit than oil does.

The saving is therefore biggest where the heat is dearest now — bottled gas, then electric heating, then oil. It's smallest on mains gas, and on a house that loses heat as fast as you can make it.

detached|1990-2005|C|oil: cost p50 €16,500, grant €12,500, net p25–p75 €4,000–€8,930, saving €820/yr, payback 4.3–13.4 yrs, n=47,829, confidence 0.9. Cheapest grant-eligible route to a B (≤150 kWh/m²/yr) for that cell, DEAP standard occupancy; the model, not observed invoices.

The test the house has to pass first

SEAI won't fund a heat pump for a house that leaks heat. There's a number for it — how much heat the house loses for its size — and the limit is 2.3.

If the house was built from 2007 on, SEAI take it as passing and ask for nothing. If it was built before 2007, you either have a rating certificate that already shows the house is under the limit, or you pay a technical advisor to measure it. That assessment has its own €200 grant, and it has to be done before you apply.

Most older houses fail on the first go and pass after the attic and the walls are done. That's the honest order of work: roof, walls, then the heating.

Heat Loss Indicator ≤ 2.3 W/(K·m²), from a valid BER or a technical assessment (grant €200, individual route). Self-declaration accepted for dwellings built 2007 onwards. Read on seai.ie 8 September 2026.

Common questions

Do heat pumps work in old Irish houses?

Yes, and SEAI say so plainly — there are plenty of them in older homes. The catch is not the age of the house, it's how fast the house loses heat. A draughty house with a cold attic will need a big heat pump running hard, and the bills follow. Insulate first and the same house runs on a smaller machine for less money.

Will my electricity bill go up?

Yes. Electricity ends up as 85 to 90% of the energy in the house, so that bill rises. The oil or gas bill goes to zero. A unit of heat from a heat pump costs about 9.8c against 14.4c on gas and 18.0c on oil, so the two bills added together come down.

Does it work in the cold?

It works when it is cold outside — there is still heat in air at 2°C, and SEAI say many of the machines on sale here run down to minus 15°C. It works harder on a bad night, so it uses more electricity in January than in April. Ireland does not get the cold that troubles a heat pump; it gets the damp.

What a heat pump is, and how it heats a house

What it is

Compared with the boiler you have now

Where the saving comes from

Is it right for a house like yours?

A 1990s detached house on oil, rated CEnergy rating C

Common questions

Where to next

Where this comes from