Should I get a heat pump?

You have probably heard two things about heat pumps, and they contradict each other. One is that they are the obvious replacement for a gas boiler. The other is that they cost a fortune to run and leave you cold. Both are true of some installations, which is not very helpful when you are trying to decide about yours.

Which of the two you end up with depends far more on your house than on the heat pump, and mostly on one number: how hot the water has to be.

What a heat pump is

A boiler burns gas and gives you the heat that was in the gas, minus a bit up the flue. It can never give out more than it took in.

A heat pump does something that sounds like a trick and is not. It collects warmth that already exists in the outside air and moves it into your house. The electricity drives the moving, and ends up as heat itself, but it is the smaller part of what you get. So one unit of electricity in can put three or four units of heat out, because most of the heat came from your garden.

A fridge does exactly this, with the cold end inside the cabinet and the warm end at the back. A heat pump is a fridge arranged so the useful end is the warm one.

This is also why they still work in winter. There is heat in air at minus five; there is heat in air right down to minus two hundred and seventy. There is just less of it, so the machine works harder and gives back less for what it uses.

How hot the water has to be

How hard a heat pump works depends on how hot it has to make the water going round your radiators.

Producing water at 40 degrees is easy. Producing water at 55 is hard. Roughly speaking, every degree hotter costs you two to three per cent on the running cost, so the gap between a system designed at 40 and one designed at 55 is something like a third of your heating bill, every winter, for as long as you live there.

The part that catches people out is that the temperature is not chosen by you or by the heat pump, but by your radiators.

Why the radiators decide it

A radiator gives out heat because it is hotter than the room. The bigger the difference, the more heat comes off it.

Your radiators were almost certainly sized for a boiler, which cheerfully sends them water at 70 degrees. At 45 degrees, the same radiator gives out roughly a third of what it did, not two thirds.

So a heat pump on unchanged radiators has two options. Either it runs the water hot, which works and costs money, or the rooms do not get warm. That is most of the "heat pumps do not work in British houses" story, and it is a radiator problem, not a heat pump one.

The fix is usually not dramatic. Radiators come in the same length and height but deeper, with more fins inside. Swapping a single panel for a double convector can nearly double the output without taking any more wall. In a typical house a few radiators get changed and the rest stay.

Is your house a good candidate?

This is not a survey, but the two lists below will tell you which way your house leans.

Signs it will go well: your radiators feel oversized for their rooms, or the house is comfortable with the boiler thermostat turned down; you have loft insulation and filled cavities; you have somewhere sensible outside for a unit about the size of a washing machine on its side; you have room for a hot water cylinder, or already have one; and there is space around the radiators, with no sofas pushed against them.

Signs it needs thought first: solid walls with no insulation; single glazing; rooms that never quite get warm now; microbore pipework, which is the very thin stuff, often about 8 mm; no obvious place for a cylinder; a boiler that runs flat out in January.

None of the second list rules it out. All of it means the honest answer is "yes, and here is what else needs doing", which is a different conversation from "yes, sign here".

Will it be cheaper to run?

Against oil, LPG, coal or electric heaters, almost certainly. Those fuels are dear per unit of heat, and a heat pump beats them comfortably.

Against mains gas, it depends on one comparison and only one.

Gas is cheap per unit. Electricity is about four times the price. A boiler loses some of what it burns, so a heat pump has to deliver about three and a half units of heat for every unit of electricity just to break even. A well-designed system running on cool water does comfortably better than that, and a system running at boiler temperatures may not manage it at all.

Which brings you back to the radiators, as most of these questions do.

What this means practically: if somebody quotes you a heat pump without telling you what water temperature it is designed for, they have left out the number that decides whether you will be pleased or furious in two winters' time.

What it is like to live with

Different from a boiler, and the difference surprises people more than the technology does.

A boiler is powerful and comes on in bursts. A heat pump is gentle and runs most of the time. On a January day it will tick over quietly all day, producing water at 38 degrees, and the house simply stays where you put it. The radiators will feel lukewarm, not hot. This is correct, and it is the system working properly, but it does not feel like it at first.

Because of that, the old habit of turning the heating off when you go out mostly costs money instead of saving it. Getting a cooled house back up needs hotter water than steady running ever did, so the better habit is to set a temperature and leave it.

Hot water comes from a cylinder, not on demand, so there is a tank of it and not an endless supply. For most households that is a non-event; for a house with four teenagers and one bathroom it is worth thinking about.

Noise: a modern unit is roughly a fridge, and quieter still a few metres away. What people notice is not the loudness but the character — a low hum, and a change of note when it defrosts on a cold morning. Where it is put matters more than what it is.

What to do next

Two things, in this order, before you talk to anybody.

Find out how much heat your house needs. A year of gas bills tells you, and it is a much better guide than anything a salesperson can eyeball. If you can also find a summer month, when the heating is off and the gas is all hot water, better still. The heat pump size calculator will turn those into a rough kilowatt figure, which is enough to tell you whether the 12 kW machine somebody later quotes you is proportionate.

Deal with the fabric first if it needs it. Insulation is cheaper than heat, it makes the heat pump smaller and cheaper, and it makes the flow temperature lower, which makes the running cost lower. Doing it afterwards means you have paid for a bigger machine than you needed. There is more on that in what to sort out before a heat pump.

Then get quotes, and read what a good quote looks like before you read the prices.