How long should I heat my hot water cylinder for?

A stored hot water system works completely differently from a combi, and people coming from a combi almost always set the schedule wrong. There is no on-demand hot water: there is a tank of water that gets heated at fixed times and then slowly cools. The whole art is having it hot when you need it and not paying to keep it hot when you do not.

Step by step

1. Time your own reheat once.Run all the hot water off in the evening, then time how long the boiler runs for hot water the next morning before it satisfies. That figure — commonly somewhere between forty minutes and an hour and a half on a boiler-fed cylinder, longer on an immersion — is the number your whole schedule is built around.
2. Set periods to END when you need water, not start.If everyone showers at seven, the heating period runs up to seven, not from seven. This single change fixes most complaints about tepid morning showers on a cylinder system.
3. Use two shorter periods rather than one long one.A morning period ending before the first shower and an evening one ending before washing up and baths. Between them the cylinder coasts on what is left, which is enough for hand washing.
4. Match the length to the reheat time, not to the demand.The cylinder thermostat stops the boiler once it is up to temperature, so a period longer than the reheat time costs nothing extra in heating — but it does mean the cylinder sits fully hot for longer, losing heat through the jacket. That is the cost of an over-long period.
5. Do not leave hot water on constantly to be safe.A well-insulated modern cylinder loses heat slowly, but a continuous schedule means the boiler tops it up all day and all night for water nobody uses. Constant is only sensible in a household with genuinely unpredictable all-day demand.
6. Use the boost for one-off demand.Almost every programmer has a one-hour hot water boost or override. That is the right tool for an unexpected bath, rather than permanently extending the schedule for something that happens once a fortnight.
7. Check the cylinder insulation before you tune anything.A modern factory-foamed cylinder holds heat for a day. An old copper cylinder with a thin loose jacket loses it in hours, and no schedule will fix that — a proper jacket or a new cylinder is the real answer.
8. Adjust for the seasons.Incoming mains water is much colder in winter, so the same cylinder takes longer to reheat. A schedule that works in July can leave you short in January. Extend the periods rather than raising the temperature.

Tips

• If the water is hot at bedtime and still warm at breakfast, your cylinder is well insulated and you can afford shorter periods. If it is cold by morning, insulate before you re-time anything.
• A cylinder that runs out mid-shower is usually about size or reheat time rather than temperature. Check whether a second short afternoon period fixes it before considering a bigger cylinder.
• Households with an economy electricity tariff and an immersion should heat overnight on the cheap rate and use the boost sparingly during the day.
• Do not push the cylinder thermostat higher to make the water last longer without understanding the scald risk — the blending arrangement at the taps matters.

Questions people ask