Scenario

When Should You Turn the Heating On?

When should I turn the heating on?

Use a trigger, not a date: turn it on when indoor temperature drops below 18 °C (64 °F), or when outdoor lows sit below about 12 °C (54 °F) for several nights running. Holding out past that point saves little and starts costing you in condensation and damp.

Why the date is the wrong question

Every autumn the same argument happens in the same houses, and it is conducted in dates: not before October, never before the clocks change, my parents waited until November.

A date is a poor instrument for this because the thing it is standing in for — a sustained drop in outdoor temperature — arrives at a different time every year and in a different week in different places. Some Octobers are mild throughout. Some late Septembers are not.

A trigger settles it, because it is checkable and it does not need re-arguing:

Indoor temperature below 18 °C (64 °F) in the rooms you actually use, or
Outdoor lows below about 12 °C (54 °F) for three or four nights running.

Either one means the house is no longer holding its own, and the heating is doing a job rather than being a luxury.

What holding out actually costs

Delaying is usually framed as free money. It is not, and the costs are worth naming.

Condensation and damp. This is the real one. Cold glass and cold walls plus ordinary household moisture produces water on surfaces, and sustained damp produces mould on frames, reveals and behind furniture. It appears within weeks, it damages decoration, and it is more expensive to fix than the heating was to run. The mechanism is in window condensation.

Reheating the fabric. A house that has gone properly cold has cold walls, floors and furniture, all of which have to be warmed before the rooms feel comfortable. That is why the first day of heating after a cold spell feels like it achieves nothing.

Comfort you compensate for anyway. A cold living room usually means an electric heater in one corner, which is often the most expensive heat in the house.

The building at very low temperatures. Not a September concern, but the reason there is a floor at all: at some point you are not choosing between warm and chilly but risking the plumbing, and that is covered in what temperature pipes freeze at.

The trigger table

What to do at each stage of autumn, keyed on outdoor lows rather than the calendar.
Outdoor lows	Action
Above 15 °C (59 °F)	Nothing. Open windows during the day
12–15 °C (54–59 °F)	Service the system, bleed radiators, test it runs
8–12 °C (46–54 °F)	Short morning and evening periods
4–8 °C (39–46 °F)	Normal winter schedule with an overnight setback
0–4 °C (32–39 °F)	Full schedule; watch for condensation
Below −6 °C (20 °F)	Drop the overnight setback — plumbing outranks the saving

Do the free things first

Everything here is worth doing before the first radiator comes on, and each one lets you hold a lower setting at the same comfort:

Bleed the radiators. Air at the top of a radiator means the top third is not working. Ten minutes for the whole house.
Check nothing is blocking them. A sofa across a radiator or a long curtain over it sends the heat into the furniture.
Draught-proof the obvious gaps. Letterbox, keyholes, gaps under external doors, unused fireplaces. Moving air strips warmth off skin the same way wind does outdoors — the wind chill mechanism, indoors and smaller.
Close curtains at dusk, but leave a gap at the bottom so room air still reaches the glass. Tight-drawn curtains make the glass colder and condensation worse.
Set the timer to warm the house before you need it, rather than heating hard once you are already cold.
Test the system in September, not on the first freezing morning when every heating engineer is booked for a fortnight.

The one thing worth spending on first

Draught-proofing, before insulation and before any control upgrade. It is the cheapest intervention per degree of perceived comfort, it takes an afternoon, and it means the trigger temperature above arrives later in the year.

Once it is on

The question changes from whether to what setting, which is the ideal indoor temperature — roughly 20 °C while you are up, 17 °C asleep, 13 °C minimum for an empty house.

One caveat carries over from this page. On the coldest nights of the year, give up the overnight setback. Most of winter it is free money; on a night forecast below about −6 °C the margin it removes is the margin protecting your pipes, and the few pence saved is not worth the trade.

Common questions

Is there a date when most people turn the heating on?

In the UK the common answer is late October, and in much of the northern US it is earlier, but the date is a symptom rather than a rule. What actually drives it is a run of cool nights, which arrives at different times each year and in different weeks in different parts of the same country. A trigger travels; a date does not.

Does it cost more to heat a house from cold than to keep it warm?

For a short absence, keeping a low background temperature is usually cheaper overall, because you are not reheating the building fabric from scratch. For a long absence, letting it drop and reheating wins. The crossover is roughly a day, and it moves with how well insulated the building is.

What is the lowest temperature I should let the house get to?

For an occupied home, 18 °C (64 °F) is the usual guidance floor and 16 °C (61 °F) the point below which a living space is too cold to sustain. For an empty house in winter, 13 °C (55 °F) is the standard minimum, and that figure is about protecting the plumbing rather than the occupants.

Why do my windows stream with water before I put the heating on?

Because cold glass plus normal household humidity is exactly the condition for condensation. Cooking, showering and drying laundry put moisture into the air, and the colder the glass the sooner that moisture settles on it. Heating raises the glass temperature and moves it out of range, which is one of the real costs of holding out.

Is it worth heating rooms nobody uses?

Not to full temperature, but do not let them go cold and closed either. An unheated room with the door shut gets cold surfaces and no air movement, which is where condensation and mould appear first. A low background setting and an open door is usually the better trade than a sealed cold room.

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