Read Time: 7 minutes
It’s a cold morning, the radiators are warm, and the shower runs ice cold. When your heating works but there’s no hot water, your boiler hasn’t died. Usually one specific part or setting has stopped doing its job.
Some causes take two minutes to check yourself. Others, like a stuck diverter valve, need a Gas Safe engineer. This guide covers the likely causes, the safe checks worth trying first, and when to book a boiler repair service in London.
Quick answer: On a combi boiler, this is most often a stuck diverter valve, a scaled plate heat exchanger or a faulty sensor. On a system or regular boiler with a hot water cylinder, check the programmer, the cylinder thermostat and the motorised valve. Settings, pressure and cold water supply are safe to check yourself. Anything inside the boiler casing is a job for a Gas Safe engineer.
First, Which Type of Boiler Do You Have?
The cause depends on how your home makes hot water.
- Combi boiler: heats water on demand when you open a tap, with no tank. This is very common in London flats and smaller homes.
- System or regular boiler: heats a stored hot water cylinder, usually in an airing cupboard.
Find a cylinder and you have the second type. If there’s no cylinder, you have a combi.
Five Safe Checks Before You Call Anyone
- Check the hot water setting. Many boilers have a separate hot water temperature control or mode. Make sure it hasn’t been turned down or switched off, especially if the controls were changed over summer.
- Check the programmer. If you have a cylinder, confirm the hot water channel is on and set to an “on” period.
- Check the pressure. With the system cold, the gauge normally sits between 1 and 1.5 bar. If it keeps dropping, read our guide on boiler pressure that won’t stay up.
- Check your cold water. Run a cold tap elsewhere in the house. A combi needs decent mains flow before it fires for hot water, and a weak flow or a partly closed stop tap can stop it.
- Read the display. A fault code narrows the problem down fast. Our guide on what your boiler’s error code is really telling you will help.
If everything looks fine, reset the boiler once, following the manufacturer’s instructions. If nothing changes, stop there. Repeated resets won’t fix a failed part and can hide the real fault.
Why a Combi Boiler Gives Heating but No Hot Water
1. A Stuck Diverter Valve (The Most Common Cause)
Think of the diverter valve as a traffic controller. It sends the boiler’s heat either to your radiators or to your taps. If it sticks in the heating position, the radiators warm up while the taps stay cold.
A telltale sign is hot water that only appears when the central heating is on. The valve sits inside the sealed boiler, so this is a repair for a Gas Safe engineer.
2. A Scaled or Blocked Plate Heat Exchanger
The plate heat exchanger passes heat from the boiler’s water to the fresh water at your taps. London is a hard water area, so limescale can build up inside it over time. Sludge from older radiator systems can clog it too.
When it clogs, you may get lukewarm water, or water that runs hot and then turns cold after a minute or two. Banging or kettling alongside it often points to scale or sludge. Our guide to a boiler making noise before winter explains which sounds matter.
3. A Faulty Flow or Temperature Sensor
Sensors tell the boiler when a hot tap is open and how hot the water is. If one fails, the boiler never gets the signal to fire for hot water, even though the heating runs normally.
4. A Blocked Cold-Water Inlet
Many combis have a small filter on the cold water inlet. If debris blocks it, flow drops and the boiler may not switch to hot water mode. Cleaning it means isolating the water and opening the boiler, so leave it to an engineer.
5. A Control Board Fault
A circuit board problem is less common. It usually shows up as a fault code or erratic behaviour rather than a clean “heating yes, hot water no” pattern.
If You Have a Hot Water Cylinder
Combi parts like the diverter valve don’t apply here. Instead, look at:
- The motorised (zone) valve: it controls whether hot water from the boiler reaches the cylinder. If it sticks or fails, the heating may work while the cylinder stays cold.
- The cylinder thermostat: if it’s faulty or set too low, the boiler may never be told to heat the tank.
- The programmer: a wrong setting is easy to miss, so double-check it.
- The pump, air or sludge: poor circulation can stop heat reaching the cylinder.
If your cylinder has an immersion heater, it can be a short-term stopgap. Ask your engineer first, because running it costs more than heating through the boiler.
What Not to Do
- Don’t remove the boiler casing. Work on gas appliances must be done by a Gas Safe registered engineer.
- Don’t keep resetting the boiler. It won’t fix a failed part.
- Don’t ignore safety warnings. If you smell gas, turn off the supply, open windows and call the National Gas Emergency Service on 0800 111 999. If a carbon monoxide alarm sounds, leave the property and get help immediately.
What Will It Cost to Fix?
Prices vary by make, model and parts availability, so treat this as a guide rather than a quote. UK cost guides for 2026 put a fitted diverter valve at roughly £170 to £350, depending on the boiler and how much needs stripping out. Heat exchanger work generally costs more, and London labour rates can push the total higher.
Whoever you choose, ask for a written quote that names the part and separates labour from parts. That makes it easy to compare options.
Repair or Replace?
A single failed part on a boiler in decent condition is usually worth repairing. Replacement starts to make sense when:
- the boiler is old and parts are hard to find
- it has had several repairs in a short time
- the fault involves a major component such as the heat exchanger
If replacement is on the table, read our guide on whether your existing radiators and pipework will work with a new boiler before you get quotes.
How to Stop This Happening Again
- Book an annual service. An engineer can spot early signs of scale, sludge or a sticking valve. Most manufacturer guarantees, including Worcester Bosch’s, expect regular servicing, so check your paperwork. Our Worcester Boiler Service London team can help.
- Fit a system filter if your heating system doesn’t have one, to catch debris before it reaches key parts.
- Act on early warnings like lukewarm water, odd noises or a falling pressure gauge.
Need Help? Call Boiler Services London
If your heating works but there’s still no hot water, don’t wait for it to get worse. Our Gas Safe registered engineers have served London homeowners for over 8 years, covering boiler repair, servicing and installation, including Worcester Boiler Repair London.
Call 07877767776 or visit boilerserviceslondon.co.uk to book.
Frequently Asked Questions
On a combi boiler, the usual culprits are a stuck diverter valve, a scaled heat exchanger or a faulty sensor. With a cylinder, check the programmer, cylinder thermostat and motorised valve first.
No. It’s inside the sealed boiler, and gas appliance work must be done by a Gas Safe engineer.
If there’s no gas smell, no carbon monoxide alarm and no leak, the heating is generally fine to use while you wait. Still, get the fault looked at before it gets worse.
In winter, or with children, elderly people or vulnerable residents at home, it can be. A same-day emergency boiler repair in London is worth asking about.