How to Turn On Hot Water: Combi, Cylinder or Immersion

First, which system do you have?

Look in the airing cupboard or wherever the hot water is stored. If there is a large copper or white insulated cylinder, you have stored hot water, heated by a boiler, an immersion heater or both. If there is no cylinder anywhere and the boiler is a single box on a kitchen wall with pipes going into the bottom of it, it is almost certainly a combi.

• Combi boiler, no cylinder: hot water is made on demand. Go to the combi section.
• Boiler plus a cylinder: hot water is heated in batches and stored. Go to the programmer section.
• Cylinder with no boiler, often in a flat: hot water is heated electrically by an immersion heater, sometimes on a cheap overnight tariff. Go to the immersion section.
• Small heater under the sink or a gas multipoint over it: go to the last section on water heaters.

How to turn on hot water on a combi boiler

A combi does not store hot water, so there is nothing to switch on in advance. When you open a hot tap, a flow sensor inside the boiler notices water moving and the boiler fires. If that is not happening, check these in order.

• Power. The display should be lit. If it is blank, check the fused switch next to the boiler and the consumer unit.
• The mode. Many combis have a mode dial or button with a tap symbol for hot water and a radiator symbol for heating. Some call the hot water only setting summer mode and the heating and hot water setting winter mode. If it is set to heating only, or to off or standby, you will get no hot water.
• The hot water temperature. There is usually a separate dial or button for the hot water temperature. If it has been turned right down, the water will come out barely warm. Around 50 to 55 degrees is a sensible setting for most homes.
• Pre-heat or eco. Some combis have a pre-heat setting that keeps a little water warm so it reaches the tap faster. Turning it off does not stop hot water, it just takes a few seconds longer to arrive.
• Faults. A code or flashing light means the boiler has locked out. One reset is reasonable; our guide on how to reset a boiler explains how.

How to turn on hot water with a boiler and cylinder

With a system or regular boiler, the hot water has its own channel on the programmer, separate from the heating. It is common for one to be on and the other off, which is why people so often have heating and no hot water, or the other way round.

On a digital programmer, find the button marked hot water, HW or with a tap symbol, and set it to on, constant, or auto for your usual times. On a mechanical dial timer, the hot water usually has a small slide switch with positions for off, timed and constant. For timed use, push the pins or tappets for the hours you want hot water. Our guide on how to set a heating timer walks through both kinds in detail.

Once the hot water is on, the programmer tells the boiler to heat the cylinder. A motorised valve opens, the boiler fires, and hot water from the boiler runs through a coil inside the cylinder. A typical cylinder takes somewhere between 30 minutes and an hour to heat from cold. Our guide on how long does hot water take to heat up gives rough figures by cylinder size.

If the hot water channel is on and the boiler fires but the cylinder stays cold, the fault is usually the motorised valve, the cylinder thermostat or a wiring centre. That is a job for an engineer, not a setting you have missed.

How to turn on an immersion heater

The immersion heater is an electric element inside the cylinder. Its switch is usually a large white switch, often marked water heater, in the airing cupboard, on the landing or in the kitchen. Some have two switches, or one switch with a choice of bath and sink, for dual element heaters. Others run through a timer, or through a separate off peak meter on an Economy 7 tariff.

Switch it on, and the red neon light on the switch should come on. The water then takes somewhere between one and a half and two and a half hours to heat from cold, depending on the size of the cylinder. Our guide on how to turn on an immersion heater explains the different switches and what each light means.

How to boost the hot water

If you have used the hot water and want more before the next timed slot, most programmers have a boost or advance button for the hot water channel. Boost usually runs the hot water for an extra hour, or sometimes 30 minutes or two hours depending on the programmer, then returns to normal. Advance moves the channel forward to its next setting, so if it is off, it switches it on until the next programmed off time.

Smart controls such as Hive have a boost option in the app as well as on the receiver. If the boost button seems to do nothing, check that the hot water channel is not permanently set to off, because some programmers ignore boost when the channel is disabled. If you have Hive and the hot water never responds, our guide on Hive hot water not working goes through the usual reasons.

An immersion heater can also act as a boost. Switching it on alongside the boiler shortens the heat up time a little, but the boiler coil does most of the work, so it is rarely worth doing unless the boiler is off.

Should the hot water be on all the time?

With a combi, the question does not arise. With a cylinder, you can leave the hot water on constant and let the cylinder thermostat decide when to heat, or set times so it only heats when you need it. Constant is convenient and in a well insulated cylinder the difference in running cost is smaller than many people think. Our guide on constant hot water compares the two.

Whatever you choose, the stored water should reach at least 60 degrees at some point each day. That is the temperature that controls legionella bacteria in a cylinder. Our guide on what temperature should a hot water cylinder be explains the figures.

Under sink heaters and gas multipoints

An electric under sink heater is switched on at a fused spur or plug near the heater, and usually has its own temperature knob on the front. A gas multipoint water heater over a sink lights automatically when you open the hot tap, as long as its pilot light or ignition is working and the gas is on. If yours does not, see our guide on a gas multipoint water heater not working.

Hot water turned on, but still nothing?

If the controls are right and the water is still cold, the problem is a fault rather than a setting. With a combi the usual suspects are the diverter valve, the flow sensor or the plate heat exchanger, and our guide on no hot water from a combi boiler explains how each one shows itself. With a cylinder, it is usually the motorised valve, the cylinder thermostat or the immersion element. Our no hot water checklist before you call covers the quick things to rule out first.

If you have been through those and still have no hot water, call us on 020 4652 5111 or send an enquiry with the make of boiler, whether you have a cylinder, and what the display shows. We have 20 engineers across London, and our rate is on the pricing page and agreed before we attend.