Vented system — boiler on but no hot water
Check the steps below first — if you're not confident, get it fixed safely today.
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A boiler that's running but not heating your hot water is one of the most common complaints we hear, especially in older properties with vented systems and separate cylinders. It's frustrating because the heating works fine, yet you're left with cold water from the taps. The good news is that nine times out of ten, it's a simple fix—usually the programmer has been switched to heating-only mode, the cylinder thermostat has been accidentally turned down, or a motorised valve needs a bit of attention. This guide walks you through the checks in the right order, starting with the easiest and cheapest solutions first. If you've worked through these steps and nothing's resolved the issue, or if you're not confident working around your boiler, it's time to call a Gas Safe registered engineer.
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Most likely cause & what to check
Check the programmer or timer first — go to your programmer (usually on the wall near the boiler) and make sure hot water is scheduled to come on. It's very easy to accidentally switch it to heating-only.
Check the cylinder thermostat — this is the dial on the side of your hot water cylinder in the airing cupboard. It should be set to 60°C. If it's been turned down to a very low setting, the boiler won't fire for hot water.
Listen at the airing cupboard when hot water should be coming on — you should hear a motorised valve click and a faint whirring sound. If you hear the boiler fire but the water doesn't heat up, the motorised (zone) valve may be stuck in the closed position.
Try the immersion heater as a temporary backup — there should be a red-switched socket near the cylinder. Switch it on and leave for 45–60 minutes. If the water heats up, the problem is confirmed to be the boiler circuit rather than the cylinder itself.
Check the boiler display for any fault codes and look them up in our Boiler Fault Code tool.
Check the cold water storage tank in the loft is not empty — if it is, the cylinder won't refill and there will be no hot water. The ball valve in the tank may be stuck.
If the motorised valve is not clicking at all when hot water is called, it has likely failed and needs replacing — this is a job for a Gas Safe engineer.
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Frequently asked questions
Why is my boiler running but not heating hot water?
In a vented system, the boiler needs to receive a call for hot water via the programmer and thermostat before it fires for that circuit. If the programmer is set to heating-only, the thermostat is too low, or the motorised zone valve is stuck, the boiler simply won't know to heat your cylinder. Start by checking your programmer—it's the most common culprit.
How do I know if my motorised valve has failed?
When hot water is scheduled to come on, listen carefully in your airing cupboard for a distinct 'click' sound followed by a faint whirring. If you hear nothing at all, the valve has likely failed and will need replacing by a Gas Safe engineer. If you hear the boiler fire but the water stays cold, the valve may be stuck in the closed position.
Is it safe to use an immersion heater if my boiler won't heat water?
Yes, using your immersion heater is a safe temporary solution and actually helps you diagnose the problem. If the immersion heats water successfully but the boiler circuit doesn't, you've confirmed the fault lies with the boiler system rather than the cylinder itself. Never use an immersion heater as a permanent replacement—it's expensive to run.
What should I check in my loft to fix no hot water?
Check your cold water storage tank (usually a large plastic or metal tank near the water meter). If it's empty or very low, your cylinder won't have water to heat in the first place. Look for a stuck ball valve inside the tank, which prevents it from refilling—gently lift the arm to see if water flows in.
Can I fix a stuck motorised valve myself?
A stuck motorised valve occasionally frees itself if you gently tap the body with a soft mallet whilst the boiler is calling for hot water, but this is rarely permanent. If tapping doesn't work within a couple of attempts, the valve needs professional replacement—it's not a DIY repair and requires Gas Safe certification.
How long should I leave the immersion heater on for a full cylinder?
A standard immersion heater should bring a typical cylinder from cold to usable temperature in 45–60 minutes, though larger cylinders may take longer. Check the water temperature after an hour by turning on a hot tap—if it's still not warm, the immersion element itself may have failed and needs replacing.