The first chilly evening of autumn arrives, you turn the heating on, and the taps run hot but the radiators stay cold. It’s a common problem when the heating comes back on after summer. Start with the simple things: the thermostat, the programmer and the boiler pressure. If those are fine, the likely cause is a stuck diverter valve on a combi, a motorised valve on a system or regular boiler, or the pump. Those need a boiler repair, and I cover Bognor Regis and nearby.
Simple checks first
- The thermostat. Turn it up a few degrees above room temperature. If it runs on batteries, change them.
- The programmer or timer. Check it’s set to call for heating, not just hot water, and that the clock is right.
- The boiler pressure. Most sealed systems need about 1 to 1.5 bar when cold. Low pressure can stop the heating working properly.
- The radiator valves. Make sure the TRVs aren’t turned right down and haven’t stuck closed over the summer.
- Any fault code on the boiler display. Look it up in your manual.
If one radiator is cold but the rest are fine, it’s more likely to be that radiator than the boiler. My guide to thermostatic radiator valves explains how a stuck TRV pin can cause this.
Combi boilers: the diverter valve
A combi boiler heats both the radiators and your hot water, but not at the same time. A diverter valve inside the boiler switches the hot water between the two.
If the valve sticks in the hot water position, you get hot taps and cold radiators. Diverter valves can stick after a long summer of not switching, or wear out over time. It’s an internal boiler part, so it’s an engineer job.
System and regular boilers: the motorised valve
With a hot water cylinder, a motorised valve (or two) directs hot water to the radiators or the cylinder. If the valve’s motor or actuator fails, or it sticks, the heating side may never open.
You’ll usually find the valve in the airing cupboard near the cylinder. A manual lever on the side may move it temporarily, but that’s not a fix. Replacing the actuator or valve head should be done by an engineer.
The pump
The pump pushes hot water around the radiators. If it has seized after the summer, or is starting to fail, hot water may reach the cylinder or taps but not get round the heating. A pump that’s humming but not moving water, or a boiler that overheats and locks out, can both point to it.
Airlocks and sludge
Air trapped in the system can stop water circulating, especially after work on the pipes. Sludge in older systems can restrict flow enough to leave radiators cold. Bleeding radiators deals with air. Sludge needs the system cleaning.
Heating problems in Bognor Regis homes
Many homes around Bognor Regis, Felpham and Middleton-on-Sea have had combis fitted in place of older systems, and some still have motorised valves and cylinders from the original setup. Whatever you have, a quick test of the heating in early autumn, before you really need it, gives time to sort faults out. Getting the flow temperature right helps too, as my guide on what boiler flow temperature to use explains.
Frequently asked questions
Why does my combi give hot water but no heating?
The most common cause is a stuck or faulty diverter valve inside the boiler. Thermostat settings, programmer settings and low pressure can also be to blame, so check those first. If they’re all fine, the diverter valve or pump will need looking at by a Gas Safe engineer.
Can I fix a diverter valve myself?
No. The diverter valve is inside the boiler casing, and only a Gas Safe registered engineer should work inside the boiler. You can safely check the thermostat, programmer and pressure, and reset the boiler once, but anything beyond that is an engineer job.
Is it worth repairing an old boiler with this fault?
Often, yes. A diverter valve or pump is a common repair. If the boiler is old, has had several faults recently or parts are hard to get, it may be worth weighing up a replacement instead. I’ll give you an honest view either way.
Heating still not coming on?
If you’ve checked the basics and the radiators are still cold, give me a call on 07545 916929 or drop me a message and I’ll come and get your heating working again.