What a Thermostat Setback Actually Saves

A thermostat setback — lowering the heat when you are asleep or out, and reversing it in summer — is the only home energy measure with no purchase attached. There is nothing to buy, nothing to install, and the saving begins immediately.

It is also surrounded by a persistent myth: that reheating the house afterwards consumes whatever was saved. That claim is wrong for most systems, and understanding why explains the cases where it is not.

Why the reheating myth is wrong

Heat loss from a building is proportional to the temperature difference between inside and outside. A house at 20°C loses heat faster than the same house at 17°C, in exactly the same weather.

During a setback period, the interior is cooler, so it loses heat more slowly the whole time. That reduced loss is the saving, and it accumulates for every hour of the setback.

Reheating does require energy — you are replacing heat that left. But you are replacing less than would have left had the house been held at the higher temperature throughout. The reheat cost is real and smaller than the accumulated saving.

The saving therefore scales with duration, not with the number of adjustments. A long setback overnight saves considerably; a brief one saves little. That is the practical rule.

The thermostat setback calculator estimates it against your own setback depth and hours, and the degree days guide covers how climate scales the result.

Where a thermostat setback does not work

The myth is wrong in general and right in specific cases, which is why it persists.

Heat pumps with resistance backup. This is the important exception. A heat pump recovering from a deep setback may trigger auxiliary electric resistance heat, which is far more expensive per unit than the heat pump itself. A large setback can then cost more than it saved.

The fix is not to abandon setback but to moderate it: shallower setbacks, and a thermostat designed for heat pumps that ramps recovery gradually rather than calling for backup. The heat pump guide covers how the running cost behaves generally.

Radiant floor and high-mass systems. These respond over hours, not minutes. A deep setback may not recover before you need it, and the comfort cost is real. Shallow setbacks only.

Very short absences. A setback of an hour saves almost nothing, because the temperature barely moves.

Homes at freezing risk. Never set back far enough to risk plumbing. A floor of around 13°C is a common recommendation for unoccupied periods, and lower in a house with pipes in exterior walls is a false economy.

Humidity-sensitive situations. In cooling season, raising the setpoint too far can allow indoor humidity to rise, which is uncomfortable and, at the extreme, a problem for the building.

The cooling case works differently

In summer the same principle applies in reverse — a warmer house gains heat more slowly — but two things change.

The available temperature difference is smaller. Outdoor temperatures exceed indoor ones by less than winter differentials in most climates, so the proportional saving is smaller.

Humidity matters. Air conditioning removes moisture as well as heat. A system that runs less removes less, and a house that has been allowed to warm and humidify takes longer to feel comfortable than the temperature alone suggests.

Practical version: setbacks in cooling season work, are worth doing, and should be shallower than heating setbacks. Raising the setpoint a few degrees while out is sensible; turning the system off entirely in a humid climate is usually not.

Getting the schedule right

Overnight is the highest-value setback — long duration, and you are under bedding.

During predictable absences, if they are long enough to matter.

Recovery timing should be set so the house reaches temperature slightly before you need it. Programmable and learning thermostats handle this by starting recovery early based on how quickly your house actually responds, which is the main advantage they have over manual adjustment.

Two habits that undo the saving:

Overriding constantly. A schedule that gets manually overridden most days is not producing the modelled saving.

Cranking the setpoint to force faster recovery. Setting the thermostat to 27°C to warm the house quickly does nothing — most systems have one output level. It simply overshoots and wastes energy. The house warms at the rate it warms.

Where it sits against everything else

Setback is free, which gives it an infinite return and a modest absolute size. It is worth doing first because it costs nothing, and it is not a substitute for the measures that reduce the loss itself.

A poorly insulated house loses heat quickly, which limits how much a setback can save and makes recovery slower. Improving the envelope raises the value of everything else — insulation and air sealing is the highest-return work in most houses, and the energy audit guide covers finding where your specific losses are.

After that, the remaining large loads are water heating — the water heater operating cost calculator covers it — and the heating system itself. The energy cost calculator is where the units get valued at your actual rate, which is the input every one of these estimates depends on and the one most often taken from a national average instead.