Heat pumps in older homes: separating experience from folklore

Few pieces of domestic technology carry as much folklore as the heat pump. Depending on which neighbour you ask, they do not work in old houses, cannot cope with real winters, and require ripping the house back to brick. Meanwhile the same devices heat most new homes in Norway, a country not renowned for mild Januarys. The gap between reputation and physics deserves untangling, because millions of UK households will face the decision within a boiler's lifetime.

The physics is not in dispute. A heat pump moves heat rather than generating it, and delivers roughly three units of warmth per unit of electricity, an efficiency of around 300 percent against the low nineties for the best gas boilers. Large field trials in ordinary British homes, including Victorian terraces and 1930s semis, have confirmed those efficiencies outside the laboratory, and monitoring in cold snaps has shown systems maintaining comfort at temperatures well below freezing. The technology question, in short, is settled.

The installation question is where the folklore has roots. Heat pumps supply water to radiators at lower temperatures than boilers, so the same radiators emit less heat. A successful retrofit therefore usually involves upsizing some radiators, occasionally pipework, and always a proper room-by-room heat-loss calculation. The failures that built the bad reputation trace overwhelmingly to skipped design work: undersized emitters, wrong flow temperatures, systems bolted onto houses nobody surveyed. An older home raises the odds that some upgrading is needed. It does not disqualify the house, and full wall insulation, though always worthwhile, is not the precondition it is commonly believed to be.

The honest remaining problems

Two difficulties are real. The first is cost and disruption: installation runs well beyond a boiler swap, though government grants currently cover a substantial slice, and a proper install takes days rather than hours. The second is running cost, which depends on the price ratio between electricity and gas. Electricity in the UK carries policy levies gas does not, and while a well-designed system undercuts a boiler at current ratios for many homes, the margin is thinner than the efficiency figures alone would suggest. Rebalancing those levies is the single policy decision that would change the arithmetic fastest.

The practical advice converges from every trial and every credible installer body. Get a full heat-loss survey, not a quote from a photograph. Ask what flow temperature the design assumes, since lower means cheaper to run. And judge the technology by designed installations, not by the anecdote in which a device sized by guesswork struggled in a house nobody measured. Old houses are not the obstacle. Old habits of installation are.