Constant Hot Water: Should Your Hot Water Be On All the Time?

What the constant setting actually does

Most boiler programmers give hot water the same choices: off, once a day, twice a day, or constant, sometimes labelled "on" or "24 hours". Constant does not mean the boiler burns gas all day. It means the programmer is permanently asking for hot water, and the cylinder thermostat decides when heat is actually needed.

When the stored water drops below the thermostat setting, usually around 60 degrees, the thermostat opens the motorised valve to the cylinder and the boiler fires. When the water is back up to temperature, the valve closes and the boiler stops. On constant, that happens whenever you draw off a lot of hot water and occasionally in between, as the tank cools. On a timer, it only happens inside the windows you set.

A combi boiler works differently. It has no stored water, so it heats water only while a tap is running, and it gives you constant hot water by design. The only related setting on a combi is a preheat or "keep warm" mode, which keeps the heat exchanger warm for faster hot water at the tap.

Does constant hot water cost more than a timer?

A little, and how much depends almost entirely on the cylinder. Every cylinder loses heat through its walls and through the pipes leaving it. On a timer the water cools between heating periods, and the lower average temperature means slightly less heat lost. On constant it is held hot, so the loss continues around the clock.

The heat you use for washing and showers costs the same either way. What differs is only the standing loss. On a modern factory insulated cylinder, the kind with thick foam bonded to the outside, that loss is small and the difference between constant and timed is modest. On an old copper cylinder with a thin jacket or none, it is much larger, and a timer or better insulation pays back quickly. Our guide to a hot water cylinder jacket shows how to tell which you have.

There is a second cost that people rarely think about. On constant, a boiler with a poorly insulated cylinder may fire for a few minutes every hour or so just to top up lost heat. Short bursts like that are the least efficient way a boiler runs. If you can hear the boiler kick in regularly overnight when nobody is using water, your cylinder is losing heat faster than it should.

When constant is the right setting

Constant makes sense for a lot of households, and in the heating season the case for it is stronger, because the boiler is running for the radiators anyway.

• A busy household where people shower at unpredictable times, shift workers, or anyone tired of finding a cold tank at the wrong moment.
• A cylinder that is on the small side for the household, where keeping it topped up is the only way to avoid running out.
• A modern insulated cylinder, where the standing loss is small enough not to matter much.
• Homes where the stored water would otherwise sit lukewarm for long periods. Water held below about 60 degrees for long spells gives Legionella bacteria the conditions they like, which our guide on what temperature should a hot water cylinder be set at explains.

When a timer is the better choice

A timer does not save much if the windows you set are so long that the cylinder is effectively on constant anyway. The saving comes from letting the tank cool when nobody needs it.

• An old cylinder with poor insulation, where holding it hot all day wastes a noticeable amount of heat.
• A cylinder heated only by an electric immersion. Leaving an immersion on constant is expensive, and a timer set to the cheap overnight hours is the right answer. Our immersion heater timer guide covers the options, and our page on immersion heater running cost shows the sums.
• Homes that are empty all day, where the water can be heated an hour or so before people get up and again before they come home.
• Summer, when the heating is off, and the boiler would otherwise be firing just to keep the cylinder hot.

A sensible timer setting for the heating season

If you do use a timer, set it around when hot water is actually used, not around the clock. A typical pattern is one period finishing just before the morning showers and a second finishing before the evening, lined up with the heating periods so the boiler is not firing twice.

Allow enough time for the cylinder to recover. A boiler heated cylinder will usually heat from cold in well under an hour, while one heated by an immersion alone can take a couple of hours; how long does hot water take to heat up gives realistic timings by cylinder size. If you regularly run out before the second period starts, lengthen the first or switch to constant rather than turning the thermostat up.

Check the programmer clock after every power cut and when the clocks change in October. A programmer an hour out, or reset to off, is one of the commonest reasons for cold showers on a winter morning. Our guide on how to set a heating timer walks through dial and digital programmers, including the separate hot water channel.

If you are on constant and still run out of hot water

That is the point where the setting is no longer the question. On constant, a cylinder heated by a working boiler should recover within the hour after a bath or a couple of showers. If it does not, something else is going on.

• The cylinder is too small for the household. Our page on why hot water runs out quickly explains how to tell.
• The hot water only works when heating is on, which points to a motorised valve or wiring fault rather than the programmer.
• The boiler fires but the cylinder never gets hot, which suggests a scaled or blocked coil, or air in the primary circuit.
• The cylinder thermostat has failed or been set low, so the boiler stops before the water is properly hot.

Systems that give you constant hot water by design

If what you actually want is hot water that never runs out, that is a question of system rather than setting. A combi gives endless hot water at one outlet at a time. An unvented cylinder gives mains pressure hot water to several outlets at once and recovers quickly from a boiler. A thermal store such as a Pulsacoil heats mains water on the way through, which is explained in how a pulsacoil works. Which suits you depends on your mains supply, the number of bathrooms and how many people use them at the same time, and our guide to combi or system boiler with a cylinder sets out the trade-offs.

If you think the hot water is not behaving the way it should, whether it is taking too long, running out on constant or firing the boiler all night, we can find out why. We are 20 engineers across London working on hot water systems of every type, and our rate is set out on the pricing page before anyone attends. Call 020 4652 5111 or send an enquiry.