Radiators or Air Conditioning in an Extension: the Real Numbers

This is the question we get asked most about extensions, and it deserves a straight answer rather than a sales one. Yes, heating an extension with an air-to-air system usually costs a little more per hour of heating than running radiators off a gas boiler. The interesting part is how small that gap is, and what it buys you.

Start with the honest disadvantage

In the UK, a unit of electricity costs roughly four times what a unit of gas costs. Check your own tariff for the exact figures, because they move, but that ratio has been broadly stable for years. Taken at face value it looks like an argument that ends the discussion.

It does not, because the two systems do very different things with the energy they buy.

What each one gives you for a unit of energy

A gas boiler burns fuel to make heat. A good modern condensing boiler is around ninety per cent efficient at the boiler itself, and you lose a little more getting the hot water to a distant radiator. Put one unit in, get a bit less than one unit of heat out.

A heat pump does not make heat, it moves it. For every unit of electricity it uses it delivers roughly three to four units of heat into the room in mild conditions. That falls when it is genuinely cold outside, because there is less heat in the air to move, but across a British heating season it averages well.

Multiply those through and the two land remarkably close together. The heat pump tends to cost slightly more per hour in the depths of January, and can be level or cheaper in the shoulder months — which, in an English autumn and spring, is most of the heating season.

The three things the per-unit comparison misses

Zoning. This is the big one. Heating an extension with radiators usually means firing the boiler and warming the circuit. A split system heats that one room and nothing else. For an extension used in the evenings and at weekends, that difference comfortably outweighs the unit price gap, because most of the time you are heating one room rather than a system.

Response. A split unit brings a room up to temperature in minutes. Underfloor heating takes hours, which means running it on a schedule whether or not you end up using the room. You pay for heat you did not need.

Capital you do not spend. No drain-down, no pipework through finished rooms, no boiler upgrade if yours turns out to be at its limit. That is money saved on day one, and it is frequently larger than several years of the running cost difference.

And the four months nobody counts

The radiator option leaves you with a glass-heavy room that is unusable on hot days unless you buy something else to deal with it. The split system is both machines in one. If the alternative plan involves radiators now and portable air conditioners later, the comparison is not close.

When radiators are the right answer

There is a case where this flips, and it is worth being clear about it. If you are already laying underfloor heating as part of the build, the pipework is going down anyway, and your boiler has comfortable spare capacity, then the marginal cost of heating that room through the wet system is genuinely low.

In that situation the honest framing is that air conditioning is a cooling decision rather than a heating one — you are buying it for the summer, and the winter performance is a bonus. That is still a decision plenty of people make after their first August in a new extension. But it is a different argument, and we would rather set it out properly than pretend otherwise.