Your combi boiler can heat the radiators perfectly and still leave you with no hot water. That’s because the two jobs run through different parts of the same unit. Central heating runs as its own sealed circuit. Hot water, meanwhile, is produced separately, on demand, the instant a tap opens. When the part responsible for hot water develops a fault, heating carries on unaffected. That’s true whether the culprit is the diverter valve, the plate heat exchanger, or a sensor. The taps, meanwhile, run cold.
This combination — hot radiators, cold taps — is one of the most common combi boiler faults in Southampton homes. It’s rarely as serious as it feels standing in a lukewarm shower. Most causes are water-side. That means a plumber can diagnose them, and often fix them, without any gas work at all.
It’s also a genuinely different pattern to a system or regular boiler. Those store hot water in a separate cylinder and can develop faults on either side. A combi has no cylinder. Instead, hot water is made instantly by diverting the boiler’s output through a plate heat exchanger the moment a tap opens. That’s a completely separate process to the central heating circuit. It’s exactly why one can fail while the other keeps working normally.
Why does this happen on a combi boiler?
Diverter valve stuck in heating mode. The diverter valve redirects the boiler’s output to the hot water circuit the moment a tap opens. If it’s stuck, worn, or jammed — often from limescale or a failed actuator — the boiler keeps heating radiators and never switches over. Taps run cold or lukewarm, however long you leave them running.
Blocked or scaled plate heat exchanger. Hot water passes through a compact plate heat exchanger with narrow internal waterways. In hard water areas, limescale gradually builds up inside these channels. This restricts flow until the boiler can no longer keep up with the tap. Heating is unaffected, though, since it doesn’t rely on this component.
Faulty thermistor. A thermistor measures water temperature and tells the boiler how hard to fire. If it fails or reads incorrectly, the boiler can misjudge the water as already hot. It may back off, or shut the hot water function down as a precaution. The heating circuit’s own sensors, meanwhile, carry on working normally.
PCB fault. The printed circuit board controls which functions the boiler runs and when. A fault affecting the hot water side of its logic — without touching heating — can selectively knock out hot water. The radiators carry on as normal. This is one to leave to a qualified engineer rather than guess at.
Low incoming water pressure or flow rate. A combi boiler needs a minimum flow of water before it will fire up to heat water. If mains pressure has dropped, or something else in the house is drawing water at the same time, the boiler may never sense enough flow. It won’t switch into hot water mode.
Airlock. Trapped air in the pipework feeding the hot water side can stop water flowing smoothly. The boiler then can’t detect demand and fire correctly. The sealed central heating circuit, by contrast, is far less prone to airlocks once properly filled. It keeps running fine.
Plumber or Gas Safe engineer?
What a plumber can check. On a combi, water-side covers the diverter valve, the plate heat exchanger, pressure, flow rate and airlocks. There’s no separate cylinder to consider. Even so, the same fault-finding approach applies. We check these first, since they’re the most common cause and don’t need Gas Safe work.
What needs a Gas Safe engineer. Anything involving the ignition system, burner, or a genuinely gas-related PCB fault is only ever legally worked on by a Gas Safe registered engineer. We diagnose the fault fast, checking water-side components before assuming the worst. We hand off to a Gas Safe engineer for anything that needs one.
Quick checks first
- Check the pressure gauge. Most combi boilers should read between 1 and 1.5 bar when cold. Well below that, low pressure could be stopping hot water from firing properly.
- Open the tap fully and give it a moment. A tap that’s only partly open, or that’s just been turned on, may not draw enough flow for the boiler to register demand.
- Look for a fault code. Most combi boilers show a code or symbol on the front panel when something’s wrong. It’s worth noting down, as it usually tells an engineer or plumber exactly where to start.
- Check the condensate pipe in cold weather. This narrow plastic pipe carries condensation away from the boiler and can freeze solid in sub-zero weather. That’s a common reason a combi cuts out completely over winter.
Frequently asked questions
Can a combi boiler heat the house but not produce hot water? Yes. A combi boiler runs two separate jobs from the same unit. It heats water for the radiators, and heats water on demand for the taps via a plate heat exchanger and diverter valve. One of these can fail while the other keeps working normally.
Is no hot water from a combi boiler always a gas problem? No. Most cases are water-side faults — a stuck diverter valve, a scaled heat exchanger, low pressure or an airlock. A plumber can diagnose and often fix these without any gas work. It’s only when the fault is genuinely gas-side, such as an ignition or PCB fault, that a Gas Safe registered engineer is legally required.
What should I check before booking a repair? Check the boiler’s pressure gauge — it should read 1 to 1.5 bar when cold. Make sure the tap is fully open and given a moment to run. Look for a fault code on the display. In cold weather, check the condensate pipe hasn’t frozen. These take a couple of minutes and often point straight at the cause.
Is your combi boiler heating the house but leaving you with cold taps? Our boiler breakdown page covers how we diagnose and fix these faults across Southampton. Get in touch through our contact form and we’ll get back to you as quickly as we can. See our pricing page for how we work out costs before anything’s booked in. If you have a system or regular boiler rather than a combi, see our wider guide to boilers with no hot water for the causes specific to those setups. We cover this fault across Winchester combi boilers, Romsey heating repairs and Fareham plumbing emergencies. If a burst pipe caused your sudden leak instead, our first 10 minutes guide covers what to do. Our home insurance claim guide covers what happens next.