How it works in reverse
An air conditioner doesn't make cold. It moves heat from inside your house to outside it. Run the cycle backwards and it moves heat from outside to in, even when it's cold out, because there's still heat energy in cold air.
Because it's moving heat instead of generating it, you get several units of warmth for every unit of electricity. That's why the comparison against an electric heater isn't close.
When it beats the central heating
In spring and autumn, comfortably. On a chilly April morning with one room uncomfortable, your options are firing the boiler and heating the whole house, or heating that one room directly. The second is usually cheaper.
In the depths of January, with the whole house cold, a gas boiler and a full set of radiators is still doing the heavy lifting. One split system is not a central heating replacement and we won't pretend otherwise.
What to watch for
- Output drops as the outside temperature falls. Manufacturers quote heating capacity at a stated outdoor temperature, so check which one
- The outdoor unit defrosts itself now and then in cold damp weather. That's normal, it's brief, and it isn't a fault
- Warm air arrives at head height instead of rising off a radiator. Most people take to it quickly, a few find it odd for the first week