Heat pump or boiler? How to actually compare them
Every comparison you read online uses somebody else's house and somebody else's tariff. Here is the method, so you can do it with yours.
Why published comparisons are usually useless
A running cost comparison depends on four things: how much heat your house needs, how efficiently each system makes it, what you pay per unit of gas and electricity, and how you actually run the heating. Change any one and the answer changes.
So an article that tells you a heat pump costs £X a year is describing a house that is not yours. What is worth understanding is the shape of the comparison.
Efficiency: the one number that matters
A modern condensing gas boiler turns most of the energy in the gas into heat, but it can never exceed the energy it burns.
A heat pump does not make heat, it moves it, so it is not bound by that limit. A well designed system delivers three to four units of heat for every unit of electricity it uses. That ratio is the whole argument.
But electricity costs more than gas
It does, per unit, and that is the counterweight. The comparison is therefore between the ratio of heat produced and the ratio of the two unit prices. Take your own unit rates off a recent bill; do not take them off a website.
The number to ask any installer for is the design flow temperature. A heat pump designed to run at a lower flow temperature is more efficient. A system designed at a high flow temperature to avoid changing radiators is cheaper to install and more expensive forever.
Where insulation changes the answer
A poorly insulated house needs more heat, which means a bigger heat pump, running harder, at a higher flow temperature. Every part of that makes the comparison worse.
Insulate first and the heat demand drops. The unit gets smaller, fewer radiators need changing, and the efficiency in practice gets closer to the efficiency on the datasheet. The grant itself no longer has an insulation condition — that went in 2024 — but the physics has not changed.
The costs that are not the fuel bill
| Gas boiler | Air source heat pump | |
|---|---|---|
| Grant | None | £7,500 through the Boiler Upgrade Scheme |
| Hot water | Combi needs no cylinder | Needs a cylinder |
| Radiators | Usually unchanged | Some may need to be larger |
| Time on site | Usually one day | Two to four days |
| Servicing | Annual | Annual |
What we would actually tell you
On mains gas, in a house with little insulation, a correctly sized boiler is usually the sensible answer today — and insulating puts a heat pump within reach later.
Off the gas grid, on oil, LPG or electric heating, the heat pump case is much stronger, because the fuel you are comparing against is expensive.
In between it is close enough that the answer comes from the house. Price both. We install both.
Sources. The three-to-four ratio is the performance range of a well designed domestic air source heat pump system. Your own figure comes from the MCS calculation for your property. Unit prices change; take them from your own bill.