Quick Answer
Your combi boiler is built to send hot water to the taps before the radiators every time. A part called the diverter valve does the switching, so when it sticks or a related part fails, the heating keeps working but the tap water stays cold.
There are four usual culprits: a stuck diverter valve, a sludge-blocked plate heat exchanger, a jammed hot water flow sensor, or a faulty temperature sensor. You can check which one is likely with a simple pipe touch test, described below.
Confirming and fixing the actual part needs a Gas Safe engineer, since it means draining the system and working on electrical components. We run a fixed £120 weekday diagnostic visit across West and North West London, with any repair quoted before we start.
Radiators piping hot, tap running stone cold. It’s a strange fault. It points to one of a handful of known parts inside the boiler.
Key Takeaways
- Four usual causes: a stuck diverter valve, a sludge-blocked plate heat exchanger, a jammed flow sensor, or a faulty temperature sensor.
- The pipe touch test tells you a lot: feel the heating pipe under the boiler while a hot tap runs. If it heats up, the diverter valve is passing.
- This is a combi-only fault: a hot water cylinder has a different zone valve instead, and in our experience that valve sticks more often than a combi diverter does.
- We run a fixed £120 weekday diagnostic across West and North West London, with any repair costed and agreed before work starts.
How a combi boiler decides where the heat goes
A combination boiler only has one burner. It has to heat your radiators and heat water for the taps. Hot water gets absolute priority every time. Open a hot tap, and a flow sensor detects the movement. It tells the boiler’s control board to act.
The board switches a motorised diverter valve. In normal running, that valve sends heated water from the burner around the radiator circuit. When a tap opens, the valve’s spindle moves. It redirects that same heated water through a plate heat exchanger instead, where it warms fresh mains water on its way to the tap. Close the tap and the valve switches back.
Heat in the radiators but nothing at the tap means that switching process has broken down somewhere. Four parts cause almost all of these calls.
The 4 causes of heating that works with no hot water
The motor or the valve’s internal cartridge seizes in the heating position. The burner still fires, but hot water never gets sent to the plate heat exchanger. The heating pipe under the boiler stays hot when a tap runs.
Sludge from the sealed heating circuit clogs the narrow plates on the heating side. Heat can’t transfer into the fresh water fast enough, so the tap runs lukewarm or the boiler cuts out and relights repeatedly.
A small paddle wheel in the cold inlet pipe should spin and tell the board a tap is open. If grit or scale jams it, the board never gets the signal and the burner doesn’t fire for hot water at all.
The sensor on the hot water outlet reports a false high reading. The board thinks the water is already hot enough and shuts the burner down before it reaches the tap.
Stuck or worn diverter valve
This is what we find most often on a “heating fine, no hot water” call. The valve’s motor burns out, or its internal cartridge and O-rings wear until the spindle won’t move fully across. Either way, the result is the same. Heated water keeps circulating through the radiators instead of switching over when a tap opens.
You can’t reset this fault. It needs the valve’s motor or cartridge replacing, which means draining part of the system and working on a 230V component. That’s boiler repair work for a Gas Safe engineer, not a DIY job.
Blocked plate heat exchanger
The plates that transfer heat into your hot water sit on the heating side of a closed loop. That loop collects magnetite sludge over the years, not limescale. It’s rarely a hard-water problem. More often it comes down to a cheap installation with no system protector (inhibitor) ever fitted, letting the heating circuit corrode unchecked from day one. Enough sludge on those plates and the boiler can’t move heat into the fresh water quickly enough. You’ll usually notice this one as fluctuating, lukewarm water rather than a total absence of it, not a dead-cold tap. We’ve written a full breakdown of that specific pattern in why radiators get hot when the tap runs, which covers the related fault where the two circuits cross the other way. A working inhibitor, checked at every annual service, is what keeps this from happening in the first place.
Jammed flow sensor
Suspect the flow sensor before the diverter valve if the burner doesn’t fire at all when you open a hot tap. A jammed paddle wheel means the board has no idea water is moving. Nothing happens at all. This is a common finding on older systems in flats around Kilburn and Maida Vale, where mains pressure and debris in the pipework vary more than in a newer build.
Faulty temperature sensor
Rarer, but it happens. A sensor reports the wrong temperature to the control board. The board thinks it’s an overheat and cuts the burner. The water at the tap is still cold. Diagnosing this one needs the boiler’s fault history read off the control board, since the symptom looks similar to a diverter or flow sensor fault from the outside.
The safe pipe touch test you can do yourself
Turn off the heating and let the system cool
Switch off the central heating (leave hot water on) and wait until the radiators are cold to the touch. This gives you a clean baseline.
Open a hot tap and let it run
Choose a tap close to the boiler and let it run for a minute or two.
Feel the 22mm heating pipe under the boiler
Don’t touch anything inside the casing. Just feel the metal heating flow pipe running out of the underside of the boiler, from the outside.
Pipe heats up while the tap runs
The diverter valve is passing heat into the heating circuit instead of the tap. That points to a stuck valve.
Pipe stays cold and the burner never seems to fire
That points more toward the flow sensor or the burner not being told a tap is open.
This test tells you which direction to point an engineer in. It doesn’t tell you exactly which part has failed. It isn’t a fix. Don’t be tempted to open the casing to look further. Once you’re inside the boiler you’re working around a live gas appliance and a pressurised system. That’s where the job stops being a homeowner check.
Stop and call a Gas Safe engineer if…
- The pipe touch test confirms the diverter valve is passing. Or you’re not confident doing the test at all.
- You can smell gas at any point. Call the National Gas Emergency Service on 0800 111 999 first, and leave the property if you’re told to.
- The boiler is cycling on and off repeatedly when you run a hot tap, since that can mean it’s overheating internally.
- You’ve reset the boiler more than once and the fault comes straight back.
None of these are things to open the casing for yourself. Diverter valves, plate heat exchangers and their sensors sit behind gas and electrical safety interlocks. Only a Gas Safe registered engineer is legally permitted to work on them.
No Hot Water from Your Combi Boiler in West London?
Our Gas Safe registered engineers (657972) run a fixed £120 weekday diagnostic visit across West and North West London. Tell us what the pipe touch test showed and we’ll agree a fixed price for the repair before any work starts.
What our engineer does on the visit
- Checks the fault history on the control board. Most combi boilers log recent lockouts, which narrows down whether the fault sits with the diverter valve, the flow sensor or the temperature sensor before the casing even comes off.
- Checks whether you have a mixer shower, since that changes how some of these faults present at the outlet.
- Connects a temperature probe to the boiler’s hot water outlet and watches the actual flow temperature and the boiler’s operation together, rather than judging by hand-feel at the tap.
- Runs the diverter valve through its motion manually, where the boiler design allows it, to confirm whether the spindle is moving freely or seized.
- Isolates and tests the suspect part with a multimeter, confirming the motor, sensor or cartridge at fault rather than guessing from symptoms alone.
- Agrees a fixed price for the repair before touching anything further, whether that’s a valve head, a sensor, or a heat exchanger.
- Tests the fix under load, running a hot tap for several minutes to confirm the diverter switches properly and stays hot at the tap.
If your boiler is an older model and this is one of several major repairs it’s needed in the last couple of years, ask about the running cost of keeping it. Don’t just fix the fault in front of you. Weigh up the bigger picture. Our heating upgrades team can talk you through whether a repair or a newer, more reliable boiler makes more sense for your home.
Problems that get mistaken for this one
A hot water cylinder system, not a combi. If your home has a separate hot water cylinder, there’s no diverter valve to blame. The equivalent part is a motorised zone valve. It’s a different component with its own failure pattern, covered in our guide to radiators getting hot when the tap runs. In our experience, a stuck valve causing this exact mismatch turns up more often on S-Plan and Y-Plan cylinder systems than on a combi’s internal diverter valve, so don’t assume combi is the more likely fault if your home has a cylinder.
Low or falling system pressure. A boiler with pressure below about 1 bar can lock out for hot water specifically before it locks out for heating, since some models prioritise protecting the heat exchanger. Check the pressure gauge first. It only takes a glance. Our guide on why your boiler is losing pressure covers the safe checks for that separately.
A programmer or timer set wrongly. Rare, but worth a glance. Check the programmer before assuming a part has broken.
We see all four of these on jobs across Kilburn and Fulham, and the pipe touch test above is the fastest way to tell which one you’re dealing with before anyone comes out.
Frequently Asked Questions
Why does my heating work fine but I have no hot water at all?
Almost always a part inside the combi boiler that’s meant to redirect heat to your taps when you turn one on. The diverter valve is the most common culprit. Others include a blocked plate heat exchanger, a jammed flow sensor or a faulty temperature sensor. Radiators keep working. They’re on a separate part of the same circuit.
How can I check which part has failed without calling anyone out?
Turn the heating off, let it cool, then run a hot tap. Feel the heating pipe underneath the boiler from the outside. If that pipe heats up while the tap runs, the diverter valve is passing heat the wrong way. If the pipe stays cold and the burner doesn’t seem to fire, suspect the flow sensor instead.
My shower runs lukewarm and then goes cold. Is that the same fault?
Usually not. A total loss of hot water points to the diverter valve or flow sensor. Fluctuating, lukewarm water that comes and goes is more often a sludge-blocked plate heat exchanger, where the boiler can’t transfer enough heat into the fresh water fast enough and cuts the burner in short bursts.
Can I fix the diverter valve or a sensor myself?
No. Reaching either part means draining part of the sealed system and working around 230V electrical components behind the boiler’s gas-tight casing. That’s Gas Safe scope only. Don’t attempt it yourself, even if you’re confident with tools.
Is it safe to keep using the heating while the hot water is out?
Generally yes, if the heating itself is working normally with no unusual noise, smell or repeated lockouts. The two circuits share a boiler but not the same fault. If you ever smell gas, that’s different. Call the National Gas Emergency Service on 0800 111 999 straight away and don’t wait for an appointment.
How much does it cost to fix no hot water on a combi boiler?
Our standard weekday diagnostic visit is £120 across West and North West London. That covers the full check described above and identifies the faulty part. We’ll agree a fixed price for the repair before any further work begins. No surprises once the part is confirmed.
Why doesn’t the burner even try to fire when I turn on a hot tap?
That points toward the flow sensor rather than the diverter valve. The boiler’s control board relies on that sensor to know a tap has opened at all. If it’s jammed with debris, the board never gets the signal. The burner stays off completely, even though the heating circuit is otherwise fine.
Need a Gas Safe engineer for a combi boiler with no hot water in West London? Tell us what the pipe touch test showed, and we’ll get an engineer to you with a fixed price agreed before any work starts.
- Book online: boiler repairs · Homeowner Plan
- WhatsApp: Message us now
- Call: 020 8051 2124
We’re Gas Safe registered (657972) and cover West and North West London. Heating that just works, no drama.
Related reading:
