Quick Answer:
If your radiators are warm but your taps run cold, the boiler is still firing, so the fault sits in the part of the system that makes hot water. Start with the free checks below (programmer, temperature setting, pressure). If they all look fine, the usual suspects depend on your boiler type:
- Combi boiler: a stuck diverter valve, a scaled-up plate heat exchanger, or a faulty flow sensor
- System or regular boiler with a hot water cylinder: a faulty cylinder thermostat, a stuck motorised valve, or a failed immersion heater
- Only one tap affected: usually an airlock, a blocked aerator or a worn mixer cartridge, not the boiler
Some of these are quick checks you can do yourself. Anything inside the boiler needs a Gas Safe registered engineer.
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First, Which Type of Boiler Do You Have?
The cause of your problem depends almost entirely on how your home makes hot water, so it’s worth a minute to work out which setup you have.
- Combi boiler: no hot water tank and no cold water tank in the loft. The boiler heats water directly from the mains whenever you open a tap. This is the most common setup in smaller homes and flats.
- System boiler: a hot water cylinder (usually in an airing cupboard) but no cold water tank in the loft.
- Regular (heat-only) boiler: a hot water cylinder plus a cold water tank in the loft. Common in older homes.
Not sure? Look for a large insulated cylinder in an airing cupboard. If there isn’t one, you almost certainly have a combi. The model name on the front of the boiler will also confirm it.
Five Checks to Try Before You Call Anyone
These take a few minutes and rule out the simplest causes, which are surprisingly common.
- Check the programmer or timer. Make sure hot water is switched on and set to “on” or “constant”, not “off” or heating-only. Power cuts and clock changes often reset these.
- Check the boiler’s hot water temperature setting. On many combis there’s a separate dial or menu for hot water temperature. If it’s been turned right down, the water will stay cold or lukewarm.
- Check the cylinder thermostat (if you have a cylinder). It’s usually a dial on the cylinder itself. A sensible setting is around 60°C. If it’s set very low, turn it up and give the water time to heat.
- Look at the boiler display for a fault code, and check the pressure gauge. Pressure is normally between 1 and 1.5 bar when the system is cold. If the pressure is low or a fault code is showing, see our guide to a boiler losing pressure.
- Run a hot tap fully for a minute or two. On a combi, listen for the boiler firing up. If it doesn’t fire, or you hear it start and stop, note exactly what happens. It will help the engineer diagnose the fault faster.
If none of these fixes it, read on. The pattern of your symptoms points to the likely cause.
Why Your Heating Can Work When Your Hot Water Doesn’t
A boiler does two jobs: heating your radiators and heating your water. The two share the same burner, but they use different valves, sensors, pipework and controls. That’s why one can fail while the other carries on.
It’s also a useful clue. If the radiators heat up normally, the burner, pump and gas supply are almost certainly working, so a boiler that isn’t heating your water is probably down to a fault on the hot water side. Telling an engineer exactly what works and what doesn’t can save diagnosis time on the call-out. Regular boiler servicing also helps catch worn valves and sensors before they fail.
Combi Boiler: Common Causes
Diverter valve fault
A combi uses a diverter valve to send the boiler’s heat either to your radiators or to your hot water. If the valve sticks or fails in the heating position, the boiler never switches over to hot water.
- Radiators heat as normal but taps stay cold or lukewarm
- You may notice radiators warming up when you open a hot tap
- Needs an engineer: it’s an internal component
Scaled or blocked plate heat exchanger
On a combi, the plate heat exchanger transfers heat from the boiler to your tap water. Limescale can clog its narrow passages, especially in hard water areas, which includes much of Hertfordshire. The heating circuit is separate, so radiators can carry on working while hot water output drops.
- Hot water that runs lukewarm, or starts hot and fades
- Reduced flow from hot taps compared with cold
- Often fixed by a descale or replacement of the part, which needs an engineer
Faulty flow sensor or temperature sensor
The boiler uses sensors to detect when a hot tap is opened and to control the water temperature. If one fails, the burner may not fire for hot water at all.
- Boiler doesn’t fire when you open a hot tap, though it fires for heating
- Sometimes accompanied by a fault code
Low mains water flow
A combi needs a minimum flow of water through it before it will fire. A partly closed isolation valve, a blocked filter or low mains pressure can stop it firing for hot water while heating still works.
- Check that the boiler’s water isolation valve is fully open
- If cold taps around the house also run weakly, the issue may be your mains supply
System or Regular Boiler With a Cylinder: Common Causes
Stuck or failed motorised (zone) valve
Most cylinder systems use a motorised valve to open the hot water circuit when the cylinder calls for heat. If it sticks closed, the boiler can fire for heating but never send heat to the cylinder.
- Radiators heat normally, but the cylinder stays cold
- The pipe leading to the cylinder stays cool while the boiler is running
- Needs an engineer to replace the valve or its motor
Faulty cylinder thermostat
The cylinder thermostat tells the system when the water has reached temperature. If it fails, it may never call for heat, or may switch off too early.
- Water cold or only lukewarm, even after hours of running
- Check the dial setting first (around 60°C is typical)
- A genuinely faulty thermostat needs replacing by an engineer
Immersion heater trouble
Many cylinders also have an electric immersion heater as a backup. If your hot water relies on it, whether by choice or because the boiler side has failed, a tripped switch, blown fuse or burnt-out element will leave you with cold water.
- Check that the immersion switch is on and the fuse hasn’t tripped
- Replacing the element is a job for a qualified electrician or plumber
Programmer or timer set incorrectly
On these systems, hot water and heating are controlled separately. It’s very common for hot water to be switched off on the programmer while heating stays on. Re-check this before assuming a fault.
No Hot Water From Just One Tap?
If hot water is fine everywhere except one tap, the boiler is very unlikely to be the problem, because a boiler or cylinder fault would normally affect every hot tap in the house. Likelier causes:
- An airlock in that section of pipework, especially after plumbing work or draining down. Running the hot tap at full flow for a few minutes sometimes clears it.
- A blocked aerator or limescale in the tap spout. Unscrew it, soak it in a descaler and refit it.
- A worn mixer cartridge or thermostatic valve in a mixer tap or shower. These often fail in one position and need replacing.
- A faulty non-return or isolation valve on that supply.
If it affects the shower only, the shower’s own thermostatic valve or cartridge is the first thing to suspect. A plumber can usually replace these quickly.
Quick Guide: Symptom to Likely Cause
| What you notice | Boiler type | Likely cause | DIY check? |
| Radiators hot, taps cold, radiators warm when a tap opens | Combi | Diverter valve | No, engineer needed |
| Hot water lukewarm or fades, weak flow from hot taps | Combi | Scaled plate heat exchanger | No, engineer needed |
| Boiler doesn’t fire when a hot tap opens | Combi | Flow sensor, low mains flow or isolation valve | Check isolation valve is open |
| Cylinder stays cold, pipe to cylinder is cool | System or regular | Motorised valve | No, engineer needed |
| Cylinder cold or lukewarm after hours | System or regular | Cylinder thermostat or programmer | Check dial and programmer |
| Cylinder cold even though the boiler is working | Any with immersion | Immersion heater or fuse | Check switch and fuse |
| One tap or shower cold, others fine | Any | Airlock, aerator or cartridge | Run tap, clean aerator |
When to Call a Gas Safe Engineer
The checks above are safe for most households, but if you still have no hot water and your heating works, it’s time to book professional boiler repairing. Anything involving the inside of the boiler, gas, or sealed system pipework must be done by a Gas Safe registered engineer. Call one if:
- You’ve run through the checks and there’s still no hot water
- You suspect a diverter valve, heat exchanger, motorised valve or sensor fault
- The boiler makes unusual noises, such as banging, whistling or repeated clicking
- A fault code is showing that you can’t clear by checking pressure
- The pressure keeps dropping, which points to a leak
- The problem started suddenly with no obvious cause
- You aren’t comfortable making checks around the boiler or cylinder
If you smell gas, treat it as an emergency. Turn off the gas at the meter if you can do so safely, open windows, avoid switches and naked flames, and call the National Gas Emergency Service on 0800 111 999.
LRT Heating is Gas Safe registered and covers boiler repairs across Hertfordshire, including Hertford, Cheshunt and Broxbourne, where we’re based. Tell us what your boiler does and doesn’t do, and we can often narrow down the fault before we arrive. You can read more about our boiler repair service, or see our winter boiler checklist to help avoid breakdowns when the cold arrives.
Get a Free Quote · Call 01438 601616
FAQs
1. Why is my heating working but not my hot water?
The hot water side of a boiler uses different parts from the heating side. On a combi, the usual culprits are the diverter valve, plate heat exchanger or a sensor. On a system or regular boiler, check the programmer, cylinder thermostat and motorised valve. An engineer can confirm which it is.
2. Can I fix a diverter valve fault myself?
No. The diverter valve is an internal boiler component, and work on it should be done by a Gas Safe registered engineer.
3. Why does only one tap have no hot water?
A boiler or cylinder fault would normally affect every hot tap. If only one is cold, look at that tap or its pipework: an airlock, a blocked aerator or a worn mixer cartridge.
4. Why is my hot water lukewarm instead of cold?
Lukewarm water often points to a scaled heat exchanger on a combi, or a cylinder thermostat set too low or failing on a system boiler. High demand can also overwhelm a combi if several taps run at once.
5. How much does it cost to fix?
It depends on the part and your boiler, so we’d rather quote after finding the fault than give a figure that doesn’t fit your system. Get a free quote and we’ll tell you the price before any work starts.
6. Can regular servicing prevent this?
It can help. An annual service lets an engineer spot a worn diverter valve, sluggish motorised valve or early scale build-up before it fails completely. See our boiler servicing page, or our care plans, which include an annual service.