Hot Water Cylinder Not Heating Up
Check the steps below first — if you're not confident, get it fixed safely today.
Post a job — we'll find you an engineer →Do not attempt to replace an immersion heater element yourself unless you are a qualified electrician. It involves working with 240V mains electricity.
A cold shower is never welcome, especially on a winter morning. If your hot water cylinder has stopped heating properly, you're not alone — it's one of the most common complaints we hear from homeowners with tank-based systems. The good news is that the fault usually lies with one of just three components: the immersion heater element, the thermostat, or the boiler coil. Most issues can be traced and resolved without major expense, though some do require a qualified engineer. This guide walks you through the checks you can safely do yourself, explains what's likely gone wrong, and tells you when it's time to call in a professional to avoid costly damage or safety risks.
Not sure if this matches your problem?
Use our interactive tool — answer a few questions and get a personalised diagnosis.
Most likely cause & what to check
Check whether the cylinder is heated by an immersion heater (electric), a boiler coil (indirect cylinder), or both. This determines where to look for the fault.
For immersion heater systems: check the timer or programmer is set correctly and the circuit breaker for the immersion has not tripped. Press the reset button on the immersion thermostat (usually a small red button visible through a plastic inspection cap at the side of the cylinder).
If the immersion heater has power but still isn't heating, the element has likely failed. This is the most common fault — immersion heater elements last 5–15 years. A qualified electrician or plumber can replace it for £100–200.
For indirect cylinders (heated by the boiler): check whether the boiler is working and reaching temperature. If the boiler is working but the cylinder isn't heating, the motorised valve controlling hot water may have failed. This is a common fault — typically £80–150 to replace.
Check the cylinder thermostat (usually a strap-on thermostat around the side of the cylinder). Set it to 60°C — if it has slipped off or is faulty it won't signal the boiler to heat the cylinder.
Check your timer or programmer — hot water and heating are often on separate timers. If hot water is not enabled during the expected time, adjust the programmer settings.
If the water is warm but never fully hot, the cylinder thermostat may be set too low, or scale has built up on the immersion element reducing its efficiency.
Not confident doing this yourself?
Post the job and we'll match you with vetted local engineers. Free, no obligation.
Was this guide helpful?
Frequently asked questions
Why has my hot water cylinder suddenly stopped heating?
The most common cause is a failed immersion heater element, which typically lasts 5–15 years before burning out. Other frequent culprits include a tripped circuit breaker, a faulty thermostat that's lost contact with the cylinder, or (in boiler-heated systems) a failed motorised valve. Start by checking your electrics and thermostat before assuming the element has failed.
How do I know if my immersion heater has actually failed?
If the timer is set correctly, the circuit breaker hasn't tripped, and the thermostat reset button doesn't restore heat, the element has almost certainly failed. You can also feel the cylinder — it should be warm or hot to the touch if working; if it's cold, the heating isn't happening. A qualified electrician can test it with a multimeter to confirm before replacement.
Can I replace the immersion heater element myself to save money?
No — immersion heater elements operate on 240V mains electricity, and replacing one requires specialist knowledge and certification. Attempting this yourself risks serious electric shock or fire. A Gas Safe or electrically qualified plumber will replace it safely for £100–200, which is money well spent for safety.
My water is warm but never fully hot — is the cylinder broken?
Not necessarily. If your water reaches 40–50°C but won't get hotter, the thermostat is likely set too low or mineral scale has built up on the immersion element, reducing efficiency. Check the thermostat dial (usually on the cylinder's side) and ensure it's set to 60°C; if that doesn't help, the element may need descaling or replacing.
How much will it cost to fix a cold water cylinder?
A simple reset or timer adjustment costs nothing. Replacing an immersion heater element typically costs £100–200, whilst a motorised valve replacement runs £80–150. If the problem is the boiler itself or a more complex fault, costs can be higher — always get a quote before work starts.
Do I need a boiler engineer or a plumber to fix this?
For immersion heater element replacement, you'll need a qualified electrician or a plumber with electrical certification. If the fault is in a boiler coil or motorised valve, a Gas Safe registered engineer is essential. Many plumbers hold both qualifications, so it's worth checking before you book.