Immersion On but Still No Hot Water

Is the immersion really getting power?

Most immersion heaters are wired to a 20 amp double-pole switch on the wall, often in the airing cupboard or on the landing, with a small red neon that lights when it is on. The neon only proves power is reaching the switch. It says nothing about the element or the thermostat beyond it.

Work through the controls between you and the cylinder before you think about the heater itself:

• The double-pole switch is on and its neon is lit
• Any timer or programmer is set to on or boost, and its clock has not reset after a power cut
• On an Economy 7 setup, you are inside the off-peak window, or you have pressed the boost button
• A dual switch marked "bath" and "sink" is set to bath, which heats the whole tank from the lower element
• The immersion circuit breaker on the consumer unit is on, and the RCD has not tripped

If the breaker or RCD trips again as soon as you switch the immersion on, leave it off. That usually means water has got into the element and there is an earth fault. Resetting it repeatedly is not safe.

Have you given it long enough to heat?

An immersion heater is slow compared with a boiler. A typical domestic element is 3 kW. Heating a 120 litre cylinder from cold mains water to about 60°C takes roughly 7 kWh of energy, so a 3 kW element needs a little over two hours to do it, and a 150 or 180 litre cylinder takes longer.

If the immersion only came on half an hour ago, feel the pipe coming out of the top of the cylinder. If it has started to warm, the heater is working and you are just waiting. If the timer only gives it an hour a day, it may never catch up after a few showers.

The other slow-heating trap is the "sink" setting on a dual switch. That powers a short top element that heats only the top part of the tank. You get a small amount of hot water, then lukewarm, then cold. For the whole cylinder you need the bottom element.

Has the thermostat cut-out tripped?

A modern immersion thermostat combines two parts: the adjustable thermostat you set to around 60°C, and a separate safety cut-out that trips if the water gets far too hot, typically somewhere in the 80 to 90°C range. When the cut-out trips, the element gets no power even though the switch and neon look normal. It has to be reset by hand.

The cut-out sits under the round plastic cap over the immersion boss, where the cable goes in. It is a small button, often red, sometimes marked "reset"; on some makes it is a separate unit on the element head. If it has tripped you will feel or hear a click when you press it.

If the cut-out trips once and then stays set, the thermostat may simply have been knocked or the water overheated once. If it trips again within days, the normal thermostat has stuck on and is letting the water climb until the safety device steps in. That thermostat needs replacing, not resetting every week. Our guide on how to reset an immersion heater covers the button in more detail.

Switch the immersion off at the wall switch and at its breaker before you take the cap off. The terminals under it are at mains voltage. If there is any sign of scorching, melted plastic or water inside the cap, put it back and call someone.

How to tell if the element has burnt out

If power reaches the heater, the cut-out is set, the thermostat is turned up and the cylinder is still stone cold after three hours, the element itself has almost certainly failed. Elements burn out through age, hard-water scale building up on the sheath until it overheats, or being switched on in an empty cylinder.

An electrician or plumber with a multimeter can confirm it in a minute. With the power isolated and the wires off, a working 3 kW element reads roughly 17 to 20 ohms across its terminals. An open circuit means the element has broken inside. A low reading between a terminal and the metal boss means it is leaking to earth, which is what trips an RCD. See how to test an immersion heater if you want the full procedure.

A thermostat can also fail open, cutting power while the element is fine. The engineer tests both before replacing anything, because the thermostat is the cheaper part.

When the water heats but still does not reach the taps

Sometimes the top of the cylinder is hot to the touch but the hot taps run cold or trickle. That is not an immersion fault, and the same pattern of a heater that works while the taps stay cold shows up on combis too. On a traditional vented system the cylinder is fed from a cold water tank in the loft. If that tank has emptied because its float valve has stuck, or the gate valve on the cold feed into the bottom of the cylinder has been closed, the hot water stops coming out even though it is hot.

An airlock after work on the pipes has the same effect: spluttering, then nothing. If you have hot water in the cylinder but not at the taps, the no hot water coming out of taps guide follows that route.

What the engineer does and what it typically costs

On the visit the engineer isolates the immersion, tests the thermostat and element, and checks the wiring and the switch for heat damage. If the element has failed, they turn off the cold feed to the cylinder, drain it down to below the immersion boss and unscrew the old element with an immersion spanner, the large box spanner that fits the standard 2¼ inch boss. A new element, thermostat and fibre washer go in, the cylinder is refilled and checked for leaks, and only then is the power switched back on.

Typical UK market ranges: an element costs about £20 to £45 as a part, and a replacement fitted by a plumber is commonly £130 to £280 depending on access and how stubborn the old one is. A thermostat on its own is cheaper still. If the cylinder is badly corroded around the boss, the engineer may advise against disturbing it, because a seized element can pull the boss and leave you needing a new cylinder. Immersion heater replacement cost has the full breakdown.

Our immersion heater repair and replacement visit has the £60 call-out at any hour, and the repair price is agreed with you on site before any work starts.

Never switch an immersion on in an empty or part-drained cylinder. An element running in air overheats and can burn out very quickly. Refill the cylinder and run a hot tap until it flows steadily before the power goes back on.