Why Heat Pumps Perform Poorly — Causes & Prevention

Why some heat pumps perform poorly (and how to avoid it).

Heat pumps occasionally get a bad reputation for high bills and cold homes. Almost always, the technology is not at fault. The building was not ready, the system was oversized, the emitters were too small, or it was set up to run hot like a boiler. Each of these is avoidable — and nearly all of them are decided before the heat pump is switched on.

The real reasons heat pumps disappoint

High fabric heat loss, so the heat pump must run at a high flow temperature and the efficiency collapses
Oversizing, which causes short-cycling, higher wear and lower seasonal efficiency
Running at boiler-like high flow temperatures (55–60°C) instead of 35–45°C
Undersized emitters that force a high flow temperature to keep rooms warm
Air leakage and cold surfaces that undermine comfort regardless of the heating
Poor system design — no weather compensation, badly set controls, unnecessary buffer tanks

The flow-temperature trap

The most common cause of high running costs is a flow temperature that is too high. Every degree higher than necessary costs efficiency. This happens for two reasons: the emitters are too small to deliver enough heat at a low temperature, or the installer has set a high, 'safe' flow temperature to avoid call-backs. Weather compensation — where the flow temperature is automatically lowered in milder weather — is essential to keeping the average flow temperature low across the season, yet it is sometimes left switched off.

Why short-cycling wastes energy

An oversized heat pump delivers more heat than the house needs at part-load, so it switches on and off repeatedly instead of modulating smoothly. Each start is inefficient and increases wear. A correctly sized unit runs long, gentle cycles at a steady low flow temperature — exactly where heat pumps are most efficient. This is why accurate, measured sizing matters so much.

How to avoid a poorly performing heat pump

Measure heat loss and improve the fabric first — fabric first, plant last
Air-seal the home and add controlled ventilation
Size the heat pump and emitters for low-temperature operation, with no excessive safety margin
Insist on weather compensation and a low design flow temperature
Commission the system properly and verify the result with measured data

Common questions from clients.

Are heat pumps just unsuitable for old UK homes?

No. Older homes simply need fabric improvements first so the heat pump can run efficiently at a low flow temperature. Many Victorian, Edwardian and 1930s homes run heat pumps very successfully after readiness works.

My neighbour's heat pump bills are high — will mine be?

Not if your home is prepared properly. High bills usually trace to a leaky, under-insulated home, an oversized unit, or a system run too hot — all avoidable with a readiness assessment and good design.

Can you check a heat pump that's already underperforming?

Yes. We can diagnose whether the cause is the fabric, air leakage, emitter sizing, flow temperature or controls, and recommend the fix — whether that is fabric improvement, larger emitters, or correcting the set-up.

What is weather compensation and why does it matter?

Weather compensation automatically lowers the flow temperature as the outdoor temperature rises, so the heat pump only ever runs as hot as it needs to. Keeping the average flow temperature low across the season is one of the biggest factors in real-world efficiency.

Does a buffer tank help or hurt?

A buffer tank can help with defrost cycles and minimum flow, but an unnecessary or poorly configured one adds heat losses and can lower the flow temperature reaching your emitters. Good design uses one only where genuinely needed.