Hot Water Not Working? Is It the Cylinder?

Why would I suddenly have no hot water?

Hot water stops for a small number of reasons, and which one applies depends on how your home makes it. A sudden loss usually means something switched off or tripped: a power cut that scrambled a timer, a breaker, a blown fuse, a cut-out that reset itself to off, or a control that lost its signal. A slow fade over days or weeks points to something wearing out, such as a scaled element or a sticking valve.

The useful first move is to stop thinking of the fault as a tap problem. Taps only deliver what the cylinder holds. If the cylinder is cold, the water is cold everywhere. If only one tap runs cold, you have a local issue such as a mixer cartridge. This page covers the first case, a whole-house loss, and stays with cylinders. It does not cover boiler faults.

What kind of hot water system do I have?

Four set-ups cover almost every home with a cylinder. Knowing which is yours decides which guide you need, so spend a minute on this before you test anything.

• Boiler-fed indirect cylinder: a tall tank, usually in an airing cupboard, with two thick pipes from the boiler running into it. Heated by the boiler through a coil, often with an immersion heater as a back-up.
• Immersion-only cylinder: the same shape but no boiler pipes in the lower half. Heated only by an electric element, controlled from a switch and a thermostat.
• Vented cylinder: fed by a cold water tank in the loft, with a small vent pipe that rises above that tank. Lower pressure at the taps.
• Unvented cylinder: fed straight from the mains, with a valve group, an expansion vessel and a discharge pipe ending in a visible tundish. Strong pressure at the taps, and safety devices that must never be tampered with.

Three questions that point to the right fault

Answer these in order. Each answer closes off a branch, and the last one leads to a specific article on this site.

Question one: is the heating working? If the radiators are hot and the taps are cold, the fault is on the hot water circuit rather than the boiler, and the right page is no hot water but the heating works. If neither the heating nor the hot water is working, check the boiler has power and pressure before you assume the cylinder is the fault.

Question two: is the cylinder warm anywhere? Put the back of your hand on the side of the cylinder, high up, then low down. Cold everywhere means no heat is arriving. Warm at the top and cold at the bottom is normal for a recent short heat-up on a stratified tank, but if it stays that way all day, the lower element or coil is not delivering. Hot to touch with lukewarm taps points the other way, towards a mixing or flow fault rather than heat input.

Question three: does it change with the heat source? If your cylinder has an immersion heater, switch it on and check again after a few hours. If hot water returns, the boiler side or its controls are at fault. If nothing changes, the element, its thermostat or its supply is the next suspect, and our immersion heater test order walks through it.

Which cylinder guide should I read next?

Match your answers to the symptom. Heating works, hot water does not: read the two-port valve guide. The cylinder never gets above lukewarm: read why a cylinder is not heating up. Water at the taps is scalding or never hot enough: read the guide on hot water too hot or too cold. Water is dripping from the cylinder or its pipes: read how to find where a cylinder leak starts.

If none of those fits, or you cannot tell which of the four set-ups you have, the sensible step is a proper hot water cylinder diagnosis, where each part is tested in order until the fault is identified by name.

When should I stop checking and get the cylinder looked at?

Stop at anything that involves opening a cover. The thermostat, the immersion element terminals, the valve head wiring and every valve on an unvented cylinder are engineer territory. You can check switches, timers, whether a breaker has tripped, and what you can feel with your hand. You cannot safely test continuity, replace a valve or keep resetting a tripped safety device.

Also stop if you find water on the floor, a smell of burning at the cylinder, or a breaker or RCD that trips again as soon as it is reset. Each of those means a component has failed in a way that a reset will not cure. Switch the heat source off at the isolator and have it looked at before it is switched on again.

Repair or replacement only enters the picture once the fault has a name. A failed thermostat, a stuck valve or a tripped fuse is a repair. A leaking shell, or a coil that has failed on a cylinder near the end of its life, is a replacement conversation. Neither answer is available until the fault is found.