Radiators warming up when you run a hot tap almost always comes down to one of two faults, and which one applies depends on your boiler type. On a combi boiler, hot water and heating share one internal diverter valve. When sludge, black magnetite from years of internal corrosion, jams that valve part-way, heat bleeds into your radiators even though you only asked for a shower. If you have a separate hot water cylinder, the boiler usually isn’t the problem at all. The fault sits outside it, in a motorised zone valve in the airing cupboard, most often a Honeywell or Drayton unit on an S-Plan or Y-Plan system. You can check the zone valve yourself in under a minute. A stuck-open diverter valve inside a combi casing needs a Gas Safe engineer. We see both faults regularly in Victorian conversions across West and North West London, where old airing cupboards and heating systems left standing all summer make valve failures common.
It’s a confusing fault to live with. You turn the tap on for a two-minute wash. By the time you’re done, the hallway radiator is too hot to touch. The cause isn’t the same for every boiler. The first job is working out which system you’ve got.
Key takeaways
- On a combi, sludge jamming the internal diverter valve part-way is the usual cause, not a faulty thermostat or programmer.
- On a system boiler with a separate cylinder, the fault sits outside the boiler, in a motorised zone valve in the airing cupboard.
- The tell competitors skip. Test hot water on its own first. If it’s fine, the boiler works, and the fault sits in the controls, not the appliance.
- The airing cupboard lever test tells you the answer in seconds. Zero resistance means the return spring has failed and the valve is stuck open.
- Testing the lever is safe to do yourself. Replacing either valve is Gas Safe registered work.
How a combi boiler bleeds heat into your radiators
A combi boiler heats water for both your radiators and your taps. It can only do one job at a time. A motorised diverter valve inside the casing decides which. Ask for hot water and the valve should swing fully across, sending the primary water through the plate heat exchanger and away from the heating circuit.
The trouble starts when the valve doesn’t swing all the way. Central heating systems are sealed. They never get fresh water. Over years, the steel in your radiators corrodes internally and produces magnetite, a black sludge that circulates with the water. That sludge is gritty. It settles inside the diverter valve body and stops the spindle moving cleanly from one port to the other.
When the valve jams part-way, it never fully closes off the heating circuit. Ask for hot water and some of that heat still bleeds through to the radiators. You get a warm towel rail or a hot hallway radiator at the exact moment you only wanted a shower. Left alone, it gets worse. The valve wears further, hot water can run lukewarm, and the boiler works harder heating two circuits when it should only be heating one. A boiler repair visit at this stage usually means clearing or replacing the valve cartridge, not the whole boiler.
If you have a hot water cylinder, look outside the boiler
Not every home has a combi. If you’ve got a separate hot water cylinder tucked away in an airing cupboard, you’re on a system boiler, or a heat-only boiler paired with a cylinder, usually wired as an S-Plan or Y-Plan. On this setup, the boiler just makes hot water. It doesn’t decide where that water goes.
That job belongs to a motorised zone valve, usually a Honeywell or Drayton unit fitted on the pipework near the cylinder. An S-Plan uses two separate valves, one for heating and one for hot water. A Y-Plan uses a single mid-position valve that splits flow between the two. Either way, the valve has a small electric motor and a return spring that should snap it shut the moment demand stops.
When that spring fails, or the valve’s motor seizes, the valve can stick part-open toward heating. Ask for hot water and the boiler fires as normal. But heat keeps flowing to the radiators too, because the valve never fully closed that path. Nine times out of ten on a system boiler, this is a zone valve fault, not a boiler fault. It’s worth ruling the boiler out early so you’re not paying for the wrong repair. Valve replacement and any related central heating work is a controls job, separate from the boiler itself.
The airing cupboard test you can do yourself
You don’t need to be Gas Safe registered to check a zone valve from the outside. You’re not touching gas. You’re not opening anything sealed. Here’s what to do.
Wait until the heating and hot water are both switched off, so nothing is calling for either. Open the airing cupboard and find the zone valve. It’s a small grey or white box, roughly the size of a fist, fitted directly onto a pipe near the cylinder. On the side, there’s a metal lever with a manual override switch.
Gently push the lever. On a healthy valve, you’ll feel resistance straight away. That’s the return spring doing its job, holding the valve shut until the system calls for it. If the lever moves freely with no resistance at all, the return spring has failed. The valve is stuck open. That’s very likely your bleed-over fault.
This test tells you which fault you’re dealing with before an engineer arrives. It saves time on the visit and means you’re not guessing over the phone.
Symptom triage table
Use this to work out what’s happening before you call anyone.
Combi boiler: radiators warm up when a tap runs
Diverter valve jammed part-way by sludge
No. Inside the sealed casing, Gas Safe engineer only.
System boiler: radiators heat up while the cylinder is heating
Motorised zone valve stuck open
Partly. You can test the lever yourself; replacing the valve is an engineer job.
Zone valve lever has zero resistance when pushed
Broken return spring inside the valve head
Yes to test. No to repair; the valve head needs replacing.
Hot water runs lukewarm while radiators run hot
Heat bleeding into the wrong circuit instead of the plate heat exchanger
No. Needs a full diagnostic, not a DIY fix.
Boiler locks out on an overheat fault
Valve jammed fully, water can’t circulate
No. Turn the system off and call a Gas Safe engineer.
What’s safe to check yourself, and what needs a Gas Safe engineer
A few checks are worth doing before you book anyone. Look at the pressure gauge. It should sit between 1.0 and 1.5 bar cold. Check your room thermostat or programmer hasn’t been left on constant heat. Test the zone valve lever, as above, if you have a cylinder.
What you can’t safely do is take the boiler casing off. Inside it there are gas connections, combustion seals and electrical components. Those must only be handled by someone Gas Safe registered. That covers replacing a diverter valve cartridge, working on the actuator motor, or anything involving the boiler’s internal water circuits. It’s the law, not just good practice.
Why this fault turns up so often in West London properties
We see this constantly in Victorian conversions. Airing cupboards in the flat conversions around Queen’s Park often still have the original S-Plan zone valves fitted when the property was split into flats decades ago, and those valves seize up after standing still all summer.
In Kilburn, where we’re based, mansion block flats and terrace conversions bring their own mix of old pipework and cramped cupboards. A zone valve fault can sit undiagnosed for months because nobody’s looked at it. Kensal Rise’s terraced conversions off Chamberlayne Road show the same pattern, a cupboard boiler with pipework routed around the conversion rather than planned for it.
Notting Hill’s stucco-fronted houses, most long since divided into flats, tell much the same story. A valve that hasn’t moved since spring seizes up. Nobody notices until the heating comes on with the shower.
Frequently asked questions
Why do my radiators get hot when I turn on the tap?
On a combi boiler, it’s almost always a diverter valve jammed part-way by sludge inside the sealed heating circuit. The valve should close off the radiator circuit when you want hot water. When sludge stops it moving fully, heat bleeds into your radiators instead of staying at the tap.
My boiler isn’t a combi. I have a hot water cylinder, so what’s causing it?
The fault usually isn’t inside the boiler at all. On S-Plan or Y-Plan systems, a motorised zone valve near the cylinder controls where heat goes. When its return spring fails, the valve sticks open and lets heat reach the radiators when only hot water was called for.
How do I test my zone valve myself?
With the heating and hot water switched off, find the valve near your hot water cylinder and gently push the manual lever on the side. A healthy valve resists straight away. If the lever moves freely with no resistance, the return spring has failed and the valve is stuck open.
Can I fix a stuck diverter valve or zone valve myself?
No. Testing a zone valve lever from outside is safe, but replacing either component means working on sealed hydraulics, and on a combi it means opening the boiler casing. That’s Gas Safe registered work only, by law, not a DIY repair.
Will this fault damage my boiler if I leave it?
It can. A jammed diverter valve makes the boiler heat two circuits instead of one. That wastes gas and strains the valve further until hot water starts running unreliably. Some systems eventually lock out on an overheat fault. It’s worth fixing before it gets there.
Does hard water cause this fault?
No. This is a central heating fault caused by sludge, magnetite from internal corrosion in the sealed heating circuit. Hard water and limescale affect the hot water side instead, things like tap flow and the plate heat exchanger, not the diverter or zone valve.
Get your diverter or zone valve diagnosed
If your radiators heat up when you run a tap in Queen’s Park, Kilburn, Kensal Rise, Notting Hill or anywhere across West and North West London, our Gas Safe registered engineers (657972) can tell you which valve is at fault and put it right.
- WhatsApp us. Book in 60 seconds
- Call 020 8051 2124. our line is answered 24/7 and we’ll get your details to an engineer
- Book online: boiler repairs
We cover West and North West London. Heating that just works, no drama.
About the author Adam Said is a Gas Safe registered domestic gas engineer (657972) at The Happy Heating Co, diagnosing boiler hydraulic faults, replacing motorised valves and repairing heating systems across West and North West London.
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