What if you turned the heating down a little?

On the wall there is a little dial or screen. It is the thermostat. It tells the heater how warm to make the house.

What if you turned it down just a little? Would it make any difference?

A thermostat is the control that decides how warm your home should be. When the house gets cooler than the setting, it turns the heater on; when the house is warm enough, it turns the heater off.

People often say that turning the thermostat down a little saves energy. But two degrees sounds tiny. In this lab, a house on a 0 °C day is kept at 20 °C. What happens to the heat it needs if the setting goes down to 18 °C?

A thermostat holds an indoor setpoint by switching a heater on and off. In steady winter conditions, the heater must supply exactly the heat lost through the envelope, which depends on the envelope's U-value and area and on the indoor-outdoor temperature difference.

This lab lowers the setpoint from 20 °C to 18 °C, with the outdoor temperature fixed at 0 °C, and asks how large the resulting saving is, and whether it depends on the weather.

The thermostat is set to 20 degrees. Outside it is freezing.

Warmth leaks out all day. The heater keeps putting it back.

With the thermostat at 20 °C and 0 °C outside, our insulated model house loses heat at about 3.6 kilowatts. The 10-kilowatt heater runs about 36% of the time to keep up.

Over a whole day, that adds up to about 86 kilowatt-hours of heat. The important number is the gap between inside and outside: 20 degrees.

The model house has a 300 m² shell with U = 0.6 W/m²·K, a conductance of 180 W/K. At ΔT = 20 K it loses Q = 3.6 kW, so the 10 kW heater runs 36% of the time and delivers 86.4 kWh per day.

The thermostat switches the heater fully on below 19.75 °C and off above 20.25 °C, so the house hovers within a quarter of a degree of the setting.

Now Olly turns the thermostat down to 18 degrees.

The house is a little cooler. Warmth leaks out a little more slowly. The heater runs a little less.

Set the thermostat to 18 °C. The gap between inside and outside shrinks from 20 degrees to 18, a tenth smaller, and the heat loss shrinks by a tenth too: about 3.24 kilowatts. The heater runs about 32% of the time and the house needs about 78 kilowatt-hours a day instead of 86.

Now try a milder day. At 10 °C outside, the gap is only 10 degrees, so taking 2 away saves a fifth of the heat. On a very cold −10 °C day, it saves only about 7%.

At 18 °C, ΔT = 18 K, so Q = 3.24 kW, the duty cycle is 32.4% and daily heat falls from 86.4 to 77.76 kWh: a 10% saving, equal to ΔT_saved ÷ ΔT = 2 ÷ 20.

The fractional saving depends on the outdoor temperature. At 10 °C outside, 2 ÷ 10 = 20% (43.2 → 34.56 kWh a day); at 12 °C, 2 ÷ 8 = 25%; at −10 °C, 2 ÷ 30 ≈ 7% (129.6 → 120.96 kWh). The kilowatt-hours saved per degree are the same in every case, 4.32 a day per kelvin, but they are a larger share of a smaller demand.

Heat leaks out faster when the house is much warmer than outside.

A slightly cooler house has a smaller difference. So less warmth leaks out.

A heater does not keep the house warm by 'filling it up' once. It has to keep replacing the heat that leaks out, all the time. And heat leaks out faster when the inside is much warmer than the outside.

So a slightly cooler house leaks a little less heat every hour, all day and all night. Our model is simple: it holds the outside temperature steady, leaves out drafts and sunshine, and keeps the setting the same all day, while real homes might turn the heating down only at night or when nobody is home.

Since Q = U·A·ΔT, the heat saved by lowering the setpoint by δ is U·A·δ, independent of the outdoor temperature, while total demand is U·A·ΔT. The fractional saving δ/ΔT is therefore largest in mild weather.

Model limits: a constant outdoor temperature, no air leakage or ventilation, no solar or internal gains, and a fixed setpoint. Real savings also depend on schedules (for example, lower settings overnight), on how quickly a house reheats, and on how the heating equipment's efficiency varies, none of which this model includes.

Experts say turning the heat down a little can save money and energy.

It is also important to stay warm enough. Health experts say the rooms you use most should be at least 18 degrees, if possible.

University of Illinois Extension energy experts estimate that turning the thermostat down by 1 °F, a little more than half a degree Celsius, can save about 3% on the bill in winter. When a house is empty in cold weather, they suggest setting it to 55 °F (about 13 °C), which they say can save 10 to 30% of heating costs.

But comfort and health matter too. England's National Health Service advises heating the rooms you use most, like the living room and bedroom, to at least 18 °C if you can, especially for people with a health condition.

University of Illinois Extension estimates that a 1 °F setpoint reduction in winter can save 3% on the bill, and that setting the thermostat to 55 °F while away in cold weather saves 10–30% of heating costs.

There is a floor to sensible savings. The NHS advises keeping regularly used rooms at a comfortable temperature and, where possible, at least 18 °C, particularly for people with health conditions. The model can show the energy side of the trade-off; the comfort and health side needs the people living in the house.