Boiler switched on, working, but no hot water from the taps? That’s rarely a fault with the core burner. Instead, the fault almost always sits in a component that controls or delivers hot water. It’s a genuinely common fault, especially on combi boilers. In most cases, it doesn’t mean the boiler needs replacing.
Below are the seven most common reasons this happens. We’ll cover what a plumber can check and fix on the spot. We’ll also cover when a Gas Safe registered engineer needs to take over instead.
Why is my boiler on but there’s no hot water?
Roughly in order of how often each one turns out to be the cause:
Diverter valve stuck. On a combi boiler, the diverter valve switches water between your heating circuit and hot water circuit. When it wears out or sticks — usually in the heating position — the radiators carry on working. The taps, though, run cold. That’s because the valve never redirects flow to heat the water. As a result, this is one of the most common faults behind “heating works, hot water doesn’t.”
Airlock in the system. Trapped air in the pipework can stop water circulating properly. It affects the heat exchanger or the hot water circuit. It often shows up after recent work on the system, such as bleeding a radiator. Hot water becomes patchy or disappears altogether, although the boiler fires normally.
Broken thermostat or PCB. The boiler’s electronic control board (PCB) and its thermostat govern when the burner fires. They also govern how hot the water gets. If either has failed, the boiler can look like it’s running fine, with the display lit and the pump on. Yet it may never properly call for the burner to heat the water.
Low water pressure. Most boilers need system pressure of around 1 to 1.5 bar to fire properly. That’s true for both heating and hot water. If pressure has dropped below that, plenty of boilers will lock out. As a result, some simply refuse to fire until it’s topped back up.
Scale build-up on the heat exchanger. In hard water areas, limescale gradually coats the plate heat exchanger. This is the part that transfers heat into your hot water. As it builds up, flow through the exchanger narrows. Hot water runs lukewarm at best. The boiler often starts making a rumbling or “kettling” noise as it works harder to compensate.
Frozen condensate pipe (seasonal). Condensing boilers get rid of waste water through a plastic condensate pipe, usually routed outside. In freezing weather, water sitting in that pipe can freeze and block it. That trips the boiler into a fault lockout, cutting off heating and hot water together, not just hot water on its own. It’s one of the most common causes of a winter boiler breakdown across the UK. It usually just needs thawing out rather than a repair.
The pilot light or ignition has failed. Older boilers with a standing pilot light can fail to light the main burner. So can the electronic ignition system fitted to newer ones. This happens although the boiler is powered up and the fan or pump has kicked in. From the outside the boiler looks “on.” It just isn’t actually heating anything.
Plumber or Gas Safe engineer: who deals with what
Most “boiler’s on but no hot water” faults are water-side problems, not gas faults. That’s good news. It means a plumber can usually diagnose and fix them without any gas work at all.
What a plumber checks and fixes: diverter valves, system pressure, pumps, hot water cylinders, immersion heaters, airlocks, and the condensate pipe. We check these first, since together they cover most “no hot water” calls. None of it requires Gas Safe registration.
What needs a Gas Safe registered engineer: anything on the gas side — the pilot light, ignition system, gas valve, burner, or flue. That’s a legal requirement, not just good practice. Only a Gas Safe registered engineer may legally work on these parts. If our diagnosis points that way, we’ll say so plainly. We won’t attempt a fix we’re not qualified, or legally permitted, to carry out.
Quick checks before you get in touch
A couple of minutes on the basics can save a wasted visit. At the least, it tells us what we’re dealing with before we arrive.
- Check the pressure gauge. Most combi and system boilers should read between 1 and 1.5 bar when the heating’s cold. Well below that, and low pressure is very likely your problem.
- Check the thermostat and hot water programmer. Confirm the hot water setting is actually switched on and the programmed times cover when you need it. It’s an easy setting to knock by accident.
- Check the fuse box and any RCD. A tripped fuse or a switched-off spur will stop a perfectly healthy boiler from firing at all. It’s the simplest thing to rule out first.
- Check the condensate pipe in cold weather. If temperatures have been below freezing, look for ice in the white plastic pipe running from the boiler to an outside drain. A frozen condensate pipe is one of the most common causes of a winter lockout.
Frequently asked questions
Can I fix a boiler with no hot water myself? Some checks above are safe to do yourself — the pressure gauge, the thermostat and programmer settings, and the fuse box. A stuck diverter valve, an airlock, or a control board fault usually means opening up the system. That’s best left to a plumber, so nothing gets damaged or left unsafe.
Is a boiler with no hot water a gas emergency? Not on its own. A boiler that’s switched on but not heating water is almost always a water-side fault, not a gas leak. That said, if you ever smell gas, hear unusual noises from the flue, or notice a yellow flame instead of blue, stop using the boiler immediately. Contact the National Gas Emergency Service straight away.
Why does my combi boiler heat the radiators but not the hot water? This is the classic sign of a stuck diverter valve. The valve switches the boiler’s output between the heating circuit and the hot water circuit. When it jams on the heating side, the radiators keep working while the taps run cold. It’s one of the most common combi boiler faults. It’s usually a straightforward repair too.
Been through the checks above and still without hot water? Get in touch through our contact page. We’ll help you get to the bottom of it. For more on how we handle boiler breakdowns generally, see our boiler breakdown page. If you have a combi boiler specifically, our combi boiler guide covers the diverter valve fault in more detail. That’s the single most common cause. We cover this across Totton call-outs, Winchester boiler repairs and Romsey emergency plumbing. If a burst pipe caused your sudden leak instead, see our first 10 minutes guide for the immediate steps to take.