If your radiators are warm but the taps run cold, the boiler itself is usually working, so the fault is somewhere in the part of the system that sends heat to your hot water. On a combi that’s often the diverter valve or a flow sensor. On a system with a cylinder, it’s more likely the timer, a cylinder thermostat or a motorised valve. A few settings are worth checking first. After that, it’s a job for our heating and hot water repairs team in Sidcup, Bexley and across South London.
First, work out which system you have
The cause depends on how your hot water is made.
Combi boiler. No hot water tank. The boiler heats water instantly when you open a tap. If you’re not sure, our explainer on what a combi boiler is will help.
System or regular boiler with a cylinder. Hot water is stored in a tank, often in an airing cupboard. It might be a vented cylinder fed from a cold tank in the loft, or an unvented cylinder running at mains pressure.
Safe checks on a combi boiler
- Open the hot tap fully. Some combis need a minimum flow rate before they fire for hot water. A tap only slightly open may not trigger it.
- Check the hot water setting. Make sure hot water hasn’t been switched off or turned right down on the boiler’s controls.
- Check the pressure gauge. Low pressure can stop a boiler working properly. Your manual shows the right range.
- Try another tap. If one tap is cold and the rest are fine, the problem may be the tap or a mixing valve, not the boiler.
If the boiler fires for heating but does nothing when you open a hot tap, the diverter valve may be stuck or the flow sensor may have failed. Both are internal parts and need a Gas Safe registered engineer.
Safe checks on a cylinder system
- Check the programmer. Make sure the hot water channel is on and the clock is right. Power cuts and clock changes catch people out.
- Check the cylinder thermostat. It’s a dial strapped to the side of the tank. If it’s been knocked down, the water won’t get hot enough.
- Check the immersion heater. If you have one as a backup, switching it on can confirm the cylinder itself is fine while you wait for a repair.
- Feel the pipes. If the pipe into the cylinder coil stays cold while the heating runs, the motorised valve may not be opening.
With an unvented cylinder, don’t touch the safety valves, the expansion vessel or the tundish. If water is dripping from the tundish or discharge pipe, call an engineer. Our article on unvented cylinder servicing explains why these parts matter.
When it needs an engineer
Book a repair if:
- The checks above haven’t helped
- The boiler shows a fault code when you run a hot tap
- Hot water comes and goes, or turns cold after a few seconds
- There’s water around the cylinder or the boiler
- You can hear the motorised valve buzzing or clicking but nothing changes
Common fixes include replacing a diverter valve, flow sensor, motorised valve head or cylinder thermostat. These are routine repairs for a qualified engineer. Where we supply genuine branded parts, they come with a 12-month guarantee.
Hot water problems in Sidcup and Lewisham homes
Many older houses around Sidcup and Bexley still have a cylinder in the airing cupboard and a cold water tank in the loft, with controls that have been added to over the years. That’s where we often find a valve or wiring fault after decades of service. Flats and newer homes in Lewisham are more likely to have combis, where the diverter valve is the usual suspect. The hard water across much of South London and Kent can also scale up combi heat exchangers over time.
Frequently asked questions
Why does my combi give heating but not hot water?
The most common cause is a faulty diverter valve, which switches the boiler’s heat between radiators and taps. A failed flow sensor is another. Both are inside the boiler and need a Gas Safe registered engineer. Before calling, check the hot water setting and try opening a hot tap fully.
Can I use my immersion heater instead?
If you have a cylinder with an immersion heater, yes, as a temporary measure. It heats the water electrically, so it costs more to run than gas, but it keeps hot water going until the repair. Combis don’t have an immersion heater, so this option only applies to cylinder systems.
Is no hot water an emergency?
Usually not, unless there’s a leak or a vulnerable person at home who needs it. Most hot water faults can be booked as a standard repair. If water is pouring from the cylinder or boiler, turn off the water and call straight away, as that needs urgent attention.
Taps still running cold?
If you’ve tried the simple checks and there’s still no hot water, give us a call on 020 3722 5095 or drop us a message and we’ll find the fault and get your hot water back.