No Hot Water but Heating Works? Causes and Fixes

No hot water but heating works is one of the most common faults we get called out to. On a combi boiler, the usual culprit is a faulty diverter valve - the part that switches the boiler between heating your radiators and heating your tap water. On a system boiler, suspect the cylinder thermostat or a stuck motorised valve.

This fault pattern tells you a lot. Because the heating side works, you already know the boiler is getting gas, power and water, and that it can fire and burn safely. The problem is almost always in the part of the system that directs heat to your taps rather than the boiler itself. Below we walk through the safe checks you can do yourself in about ten minutes, a symptom table to help you narrow it down, and typical costs for the common repairs. If the boiler is completely dead rather than just failing on hot water, start with our guide to what to do when your boiler is not working instead.

Why a combi boiler loses hot water while the radiators stay warm

A combi boiler heats water on demand. Inside the casing there is a motorised diverter valve that flips between two positions: one sends hot water around your radiators, the other sends it through a plate heat exchanger to warm the mains water heading to your taps. Hot water always takes priority, so every time you open a hot tap the valve should snap across.

Over the years that valve does thousands of cycles, and it is a known wear item. It can stick, the rubber diaphragm can perish, or the little motor that drives it can fail. When it sticks on the heating side you get exactly this fault: radiators piping hot, taps running cold or lukewarm. A partially stuck valve often shows up first as hot water that is warm but never properly hot, or hot water that only appears when the heating is switched off. It is a very common repair on combis over five to ten years old, and it is one of the wear points a regular annual boiler service is designed to catch before it fails completely.

On a system or conventional boiler the plumbing is different - your hot water is stored in a cylinder - so the equivalent suspects are the cylinder thermostat, the motorised zone valve that lets boiler heat into the cylinder coil, or the programmer. We cover those in the checks below.

Safe DIY checks you can do in 10 minutes

Work through these in order. None of them involves opening the boiler casing, and between them they resolve a good share of the no-hot-water calls we attend. If you smell gas at any point, stop and call the National Gas Emergency Service on 0800 111 999 before anything else.

1. Check the boiler pressure

Look at the pressure gauge on the front of the boiler. It should sit between 1 and 1.5 bar when the system is cold. Some boilers keep the heating limping along at low pressure but refuse to do hot water, and many modern models simply lock out with a fault code. If the gauge reads below 1 bar, top it up using the filling loop - the small braided hose or built-in lever under the boiler - until it reaches about 1.2 bar, then reset the boiler. If the pressure keeps dropping over the following days, you have a leak somewhere and it needs looking at.

2. Check the controls, timer and thermostat

It sounds obvious, but check it anyway - we find settings faults more often than you would think, especially after a power cut, a clock change or a visit from the grandchildren. Make sure hot water is actually switched on at the programmer, the schedule covers the time of day you are testing, and any hot water temperature setting on the boiler itself has not been knocked down to minimum. If you have a smart thermostat or app control, check the hot water mode has not been set to off or holiday mode.

3. Check the pilot light or ignition

Older boilers have a permanent pilot light you can see through a small window - if it has gone out, relight it following the instructions on the casing or in the manual, and only ever that way. Modern boilers ignite electronically instead, so listen for the click-click-click of the spark igniter and the whoosh of the burner lighting when you run a hot tap. If the boiler tries to fire for hot water but fails and shows a fault code, note the code down - it will save your engineer time - and try one reset. If it will not hold a flame, stop there.

4. In winter: check for a frozen condensate pipe

Condensing boilers drain slightly acidic water through a plastic pipe, and where that pipe runs outside it can freeze in a cold snap. A frozen condensate pipe usually stops everything, but a partial blockage can cause the boiler to gurgle, fire briefly and cut out - often failing on hot water demand first because that is when it works hardest. If you can see an external white or grey plastic pipe, pour warm (never boiling) water over it, or hold a hot water bottle against it, then reset the boiler.

5. Check for an airlock

If the hot taps splutter, hiss or run at a dribble - especially after plumbing work, a cold water tank refill or the system being drained - you may have an airlock in the hot water pipework rather than a boiler fault at all. Sometimes simply running the affected tap gently for several minutes clears it. If one tap runs hot fine and another does not, the problem is in the pipework to that tap, not the boiler, which is a plumbing job rather than a gas one.

6. System boiler? Check the cylinder thermostat

If your hot water comes from a cylinder, find the thermostat strapped to its side - it should be set to 60 to 65 degrees, which keeps stored water hot enough to control legionella bacteria. If it has been turned down, failed, or worked loose from the cylinder, the boiler thinks the water is already hot and never heats it. Also listen near the motorised valve on the pipework when hot water is calling: a healthy valve gives a soft hum or click. Radiators hot but cylinder stone cold points squarely at the valve or the stat.

Symptom checker: likely cause and who can fix it

When to stop and call a Gas Safe registered engineer

Anything behind the boiler casing is off limits for DIY. Under the Gas Safety (Installation and Use) Regulations 1998 it is illegal for anyone who is not competent and registered to work on a gas appliance, and that includes replacing a diverter valve, because the casing seals the combustion chamber. The Gas Safe Register is the official list of engineers legally allowed to do this work in the UK, and the Health and Safety Executive's domestic gas guidance is blunt about the risks of botched gas work: gas leaks, fires, explosions and carbon monoxide poisoning.

So call an engineer rather than reaching for a screwdriver when:

• ◆The checks above have not restored your hot water - the fault is almost certainly internal, most likely the diverter valve on a combi.
• ◆The boiler shows a fault code that comes back after one reset. Repeated resetting can mask a genuine safety lockout.
• ◆You have to top the pressure up more than once every few months - that is a leak, not a quirk.
• ◆There is any burning smell, sooty staining around the boiler, or a yellow lazy flame instead of a crisp blue one - switch the boiler off and get it checked.
• ◆You smell gas: do not touch the boiler at all. Open windows, avoid switches and naked flames, and call the National Gas Emergency Service on 0800 111 999 first.

When any engineer arrives, ask to see their Gas Safe ID card - every legitimate engineer carries one and will happily show it. All three of us at CAF Heating (Carlton, Simon and Stephen) are Gas Safe registered, and if you are ever unsure you can check any engineer's registration on the Gas Safe Register website before they touch a thing.

How much does it cost to fix no hot water?

It depends on which part has failed. Here is what the common repairs typically look like across the UK. Treat these as typical national ranges rather than quotes - the exact figure depends on your boiler brand, the part price and how accessible the boiler is. We always confirm a fixed price before starting any repair, and you can see our standard prices on our pricing page.

• ◆Repressurising the system, correcting timer settings or thawing a condensate pipe: free if you do it yourself using the steps above.
• ◆Diverter valve repair or replacement on a combi: typically £150 to £350 including parts and labour, depending on the boiler model.
• ◆Motorised zone valve on a system boiler: typically £150 to £300.
• ◆Cylinder thermostat replacement: typically £80 to £150.
• ◆Ignition parts such as electrodes or a gas valve: typically £150 to £350.
• ◆Printed circuit board (PCB) replacement: typically £300 to £500 - on an older boiler this is the point where it is worth weighing repair against replacement.

One honest caveat from thirty years of doing this: if your boiler is 12 to 15 years old and needs a £400 PCB, put that money towards a new boiler instead. Repairs on boilers that age tend to come in clusters, and a modern replacement will run more efficiently from day one. And if your problem is the reverse of this article - hot water fine but radiators cold - the fault list is different, so read our guide to radiators that are cold or not heating.

No hot water in Surrey or South London? We can help

CAF Heating is a family firm - Carlton Fuller and his sons Simon and Stephen, all Gas Safe registered engineers - and we have been fixing boilers across Surrey and South London since 1996. We are based in Horley and cover 19 towns across the area. No hot water with a house full of people counts as urgent in our book, so call us on 07956 876156 and we will talk the problem through on the phone first - sometimes we can get your hot water back with a two-minute conversation and no call-out at all. If it does need a visit, our emergency plumber service covers exactly this kind of fault, and once it is fixed an £85 annual service is the cheapest way to stop it happening again.

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