This article contains affiliate links. We may earn a commission if you buy through our links, at no extra cost to you. Last reviewed: June 2026.
If your radiators are warming up but your taps and shower run cold, the most common cause in a combi boiler is a faulty diverter valve, the component that decides whether heated water goes to your radiators or to your hot taps. The reassuring news is that hot water failing while heating still works almost never means a dead boiler. It points to a specific fault in the hot water side of the system, and several of the possible causes are simple checks you can safely make yourself before calling anyone out.
This guide runs through the quick checks to try first (settings, pressure, power and, in winter, a frozen condensate pipe), then explains the diverter valve in plain terms, including a safe five-minute test to confirm it, and is honest about which repairs you can do yourself and which are strictly a Gas Safe registered engineer’s job. In a combi boiler, heating and hot water run on separate circuits sharing one boiler, which is exactly why one can fail while the other carries on working.
★★★★★
4.8 from 14,000+ reviews
Boiler not heating water: the short answer
Start with the simple checks: confirm the hot water setting or timer has not been switched off, check the pressure gauge reads 1.0 to 1.5 bar, make sure the boiler has power, and in cold weather rule out a frozen condensate pipe. If the heating works fine but your taps stay cold after all that, the likely cause is a stuck diverter valve, which is an internal part that only a Gas Safe registered engineer should repair. Replacing one typically costs around £200 to £350.
Quick checks to try first
Before assuming the worst, work through these safe checks in order. Any one of them could restore your hot water in minutes with no engineer needed.
- Check the hot water setting and timer. It sounds obvious, but the hot water can be switched off or scheduled off without you realising, particularly after the clocks change, a power cut, or a failing back-up battery resets the timer. Confirm the hot water is set to on and the schedule is correct. Combi boilers have separate controls for heating and hot water, so one can be off while the other is on.
- Check the pressure gauge. If the pressure has dropped below 1 bar, the boiler may stop heating water. Top it back up to 1.0 to 1.5 bar following our guide on how to repressurise a boiler. If the pressure keeps dropping, see why your boiler is losing pressure.
- Check the boiler has power. A blank display can simply mean there is no power to the boiler. Check the switch and the fuse or RCD on your consumer unit, and rule out a power cut.
- Rule out a frozen condensate pipe. In cold weather, a frozen external condensate pipe makes the boiler lock out, cutting both heating and hot water, usually with a fault code. This is a common winter problem with a simple home fix, covered in our guide on how to thaw a frozen condensate pipe.
- Reset the boiler. After ruling out the above, a reset can clear a one-off glitch. Follow the reset procedure in your boiler manual, then monitor it for 10 to 15 minutes to see whether normal operation returns.
Heating works but no hot water? It is usually the diverter valve
If the heating is working perfectly but your hot taps stay cold, and you have ruled out the quick checks above, the most likely cause is a faulty diverter valve. The diverter valve is a mechanical part inside a combi boiler that acts like a traffic director, sending heated water either to your radiators or to your taps and shower, depending on what is being asked for. It is unique to combi boilers, because they heat water on demand rather than storing it in a cylinder.
Over years of use, central heating sludge (black iron oxide) builds up in the system and can cause the valve to seize, often leaving it stuck in heating mode. When that happens, the boiler keeps heating your radiators but can no longer divert hot water to your taps. The classic symptoms of a failing diverter valve are taps that stay cold while the heating works, water that comes through only lukewarm, or hot water that appears only when the central heating is switched on.
One thing sometimes worth trying as a temporary measure is turning your room thermostat right down to zero before opening the hot tap, which removes the demand for heating and can occasionally nudge the valve back across. If it works, treat it as a sign the part is on its way out rather than a fix. A diverter valve must be cleaned, freed or replaced by a Gas Safe registered engineer, as it sits behind the boiler casing and involves the boiler’s internal workings. Replacement typically costs around £200 to £350 including labour, though this varies by boiler and region.
It is worth knowing that a faulty diverter valve often only reveals itself in the warmer months, when the heating is off and you suddenly notice the hot water is not working properly. That is simply because the fault was masked while the heating was running.
Other internal causes (engineer needed)
If the diverter valve checks out, a few other internal faults can stop a combi boiler heating water, all of which need a Gas Safe engineer to diagnose and repair:
- A faulty thermostat or temperature sensor can stop the boiler detecting demand for hot water, so it never fires for the taps.
- A faulty printed circuit board (PCB), the boiler’s electronic brain, can prevent it switching into hot water mode at all.
- An airlock or blockage in the hot water side can restrict flow so the boiler does not register that a tap has been opened.
Quick diagnosis table
Match what you are seeing to the most likely cause and whether it is a safe home fix or an engineer’s job.
| What you are seeing | Likely cause | DIY or engineer? |
|---|---|---|
| Heating works, hot taps cold | Stuck diverter valve | Engineer |
| Hot water only when heating is on | Diverter valve sticking | Engineer |
| Water comes through only lukewarm | Diverter valve or sensor | Engineer |
| No heating and no hot water, low pressure | Low system pressure | DIY (top up) |
| No heating and no hot water, blank display | No power to the boiler | DIY (check fuse) |
| No heating and no hot water in cold weather, fault code | Frozen condensate pipe | DIY (thaw pipe) |
| Hot water schedule or setting off | Settings or timer | DIY (adjust) |
What you can safely do, and what you cannot
To keep it simple, here is the dividing line. You can safely check and adjust your hot water settings and timer, check and top up the pressure, check the boiler’s power supply, thaw a frozen condensate pipe, and reset the boiler. You should never remove the boiler casing or attempt to clean, free or replace the diverter valve, sensors or circuit board. These are internal gas appliance repairs, and under UK regulations they must be carried out by a Gas Safe registered engineer. Attempting them yourself is unsafe, can cause further damage, and may invalidate your boiler warranty and home insurance.
Diverter fault on an aging boiler? Get a fixed-price replacement quote
A failed diverter valve, or repeated hot water faults, on a boiler that is already 10 or more years old is a classic point to weigh repair against replacement. Once you are spending £200 to £350 on a diverter on top of an aging boiler, a new one may be the better value. Before committing to either, it is worth knowing the real cost of a replacement. Both Heatable and BOXT show real installed prices for your home in under 2 minutes, with no obligation and no home survey before quoting. Our boiler lifespan guide covers the repair-versus-replace decision in detail.
- Online quote in ~90 seconds
- Broader credit acceptance via panel lenders
- Save My Quote for later comparison
- 4.8 stars from 14,000+ Trustpilot reviews
- Online quote in ~60 seconds
- Longest 0% finance term in segment
- Worcester Bosch shareholder backing
- 5 stars from 59,000+ Trustpilot reviews
Common questions about no hot water
Why is my heating working but I have no hot water?
In a combi boiler this almost always points to a fault on the hot water side rather than a dead boiler, most commonly a stuck diverter valve that is no longer directing heated water to your taps. Before assuming that, check the hot water setting and timer, the pressure, and the boiler’s power, as any of these can cause the same symptom and are safe to check yourself.
What is a diverter valve?
A diverter valve is a mechanical part inside a combi boiler that directs heated water either to your radiators or to your hot taps and shower, depending on demand. It is only found in combi boilers, because they heat water on demand rather than storing it in a cylinder. Over time, sludge can cause it to stick, usually in heating mode, which leaves you with working radiators but cold taps.
Can I fix a diverter valve myself?
No. The diverter valve sits behind the boiler casing and is part of the sealed gas appliance, so under UK regulations only a Gas Safe registered engineer may clean, free or replace it. You can safely run the five-minute pipe-feel test to confirm the fault, but the repair itself must be left to a professional. Attempting it yourself is unsafe and may invalidate your warranty and insurance.
Why does my hot water only work when the heating is on?
This is a classic sign of a failing diverter valve. The valve is stuck in the position for heating and cannot move on its own to satisfy hot water demand, so you only get hot water when the whole system is already running and the heat exchanger is hot. It needs a Gas Safe engineer to free or replace the valve.
Why is my hot water only lukewarm?
Lukewarm water from a combi boiler often indicates a partially stuck diverter valve letting some heat leak to the radiators, or a faulty temperature sensor. Both are internal faults that a Gas Safe engineer should diagnose. Before calling out, it is worth confirming the hot water temperature setting on the boiler has not simply been turned down.
How much does it cost to replace a diverter valve?
A diverter valve replacement in the UK typically costs around £200 to £350 including labour, though it varies by boiler model and region. On a boiler that is already 10 or more years old, it is worth weighing that repair cost against replacing the boiler, especially if other faults are appearing.
Is no hot water an emergency?
No hot water on its own is an inconvenience rather than a safety emergency, and the boiler shutting down the hot water side is usually it protecting itself. The exception is any situation involving a smell of gas, which should be treated as an emergency by leaving the property and calling the National Gas Emergency Service on 0800 111 999.
This guide was last updated in June 2026. It is general information drawn from manufacturer and trade guidance for UK combi boilers and does not replace your boiler manual or a professional inspection. Repair cost figures are indicative only and vary by boiler, region and engineer. Internal boiler repairs must be carried out by a Gas Safe registered engineer. If you smell gas, call the National Gas Emergency Service on 0800 111 999. We are not a credit broker, lender or installer. Heatable and BOXT handle quoting, finance and installation directly.