In a reasonably insulated home, a heat pump runs cheapest when it runs continuously at the lowest flow temperature that keeps rooms comfortable — every 1 °C cut in flow temperature improves efficiency by roughly 2–3%. Deep overnight setbacks force a high-temperature morning recovery that usually cancels the savings. A shallow setback of 1–2 °C is the sensible compromise; only leaky, low-mass buildings gain from switching off.
Why is low-and-slow more efficient for a heat pump?
Because heat pump efficiency is ruled by the temperature lift between outdoor air and the water it produces. Running continuously lets the unit meet the building's load with the lowest possible flow temperature — and each degree shaved off the flow temperature raises efficiency by roughly 2–3%. Intermittent operation works the other way: to reheat a cooled house quickly, the heat pump must produce hotter water at a worse COP, exactly when outdoor temperatures are lowest. This inverts the logic homeowners learned from gas boilers, whose efficiency barely changes with output temperature; a boiler rewards switching off, a heat pump rewards running at the lowest workable flow temperature. Continuous running also means fewer compressor starts — the wear events that shorten unit life — and steadier room temperatures.
Doesn't running 24/7 use more energy than switching off?
A building loses heat in proportion to the temperature difference between inside and outside, so letting the house cool overnight genuinely reduces heat loss — the setback saving is real. The catch is the recovery cost: every kWh of heat the building shed must be pumped back in the morning, at a higher flow temperature and therefore lower COP, often during the coldest hours of the day. In a well-insulated, high-mass building the overnight temperature barely drops, so the setback saves little heat while the recovery still costs peak-effort running: continuous operation wins. In a poorly insulated, lightweight building the overnight drop is large and fast, the saved losses outweigh the recovery penalty, and a setback (or off period) wins. The physics does not pick a universal answer — the building does.
| Building type | Overnight cooling | Best strategy |
|---|---|---|
| Well insulated, underfloor heating (high mass) | ~1 °C | Run continuously on weather compensation |
| Well insulated, radiators | 1–2 °C | Continuous or shallow 1 °C setback |
| Average insulation, radiators | 2–3 °C | Shallow 1–2 °C setback overnight |
| Poorly insulated, lightweight | 3 °C + | Deeper setback or scheduled off periods |
When does a setback clearly make sense?
Three cases. Long absences: for a working household gone 10 hours or a holiday, the cumulative loss saving beats any recovery penalty — use a modest setback rather than full off, so recovery stays within the unit's low-temperature comfort zone. Leaky buildings: where a heat pump serves an older, uninsulated house, heat dumped in is lost quickly, and holding temperature around the clock is expensive. Time-of-use tariffs: shifting run hours away from peak-priced electricity can outweigh a small COP penalty — pre-heat the house on cheap rate and coast through the expensive window, a strategy that pairs naturally with choosing the right electricity tariff.
How does defrost change the picture?
Against deep setbacks. A morning recovery concentrates maximum output into the coldest, most humid hours — precisely the conditions in which an air-source unit must interrupt itself to run its defrost cycle. More output means more frost accumulation and more frequent defrosts, each of which borrows heat back from the heating circuit. Spreading the same daily energy over 24 gentle hours keeps output low during the frost-risk window and defrost interruptions rarer.
What settings should I actually use?
Run the unit on weather compensation with the curve set as low as comfort allows — the tuning method is covered in how to set a weather compensation curve. Program a shallow setback of 1–2 °C overnight if your building cools noticeably, and resist deep setbacks unless the house is genuinely leaky. Leave scheduling experiments running for at least a week each and compare metered consumption in similar weather — measured kWh, not intuition, settles the low-and-slow debate for your specific house.
Frequently asked questions
Should I turn my heat pump off at night?
Usually not. A shallow 1–2 °C setback captures most of the overnight saving without forcing a hard morning recovery at poor efficiency. Fully switching off pays only in poorly insulated, lightweight buildings or during longer absences.
Does running a heat pump continuously wear it out faster?
No — the opposite. Compressor wear is dominated by starts and stops, not by hours of gentle modulated running. Long, steady cycles at partial load are the operating pattern heat pumps are designed for and the one associated with the longest service life.
Is it cheaper to keep the house at the same temperature 24/7?
In a well-insulated home, effectively yes: the extra standing loss of holding temperature is small, and avoiding the high-flow-temperature recovery preserves the heat pump's efficiency. In a leaky home the balance flips, and a setback saves money despite the recovery penalty.
Why is my heat pump more expensive since I started using timer blocks?
Timer blocks force the unit to reheat a cooled house in short, hard bursts at high flow temperature — the least efficient operating mode — and often into the coldest hours with frequent defrosts. Reverting to continuous weather-compensated running with a shallow setback typically lowers consumption.