BoilerBuddy · symptom guide
Radiators not heating — the five things that cause it.
One radiator cold at the bottom or all radiators cold while the boiler fires is the system telling you the heating side is not moving heat round. On a combi the five reasons, ranked most-likely-first, are a thermostatic radiator valve (TRV) stuck or slipped, an air-lock in the system, magnetite sludge in the pipes, a diverter valve stuck on the hot-water side, or a two-port / three-port zoning valve that's failed. The cards below walk through each, what it looks like, and the line for when to stop and call a Gas Safe engineer.
Could this be carbon monoxide?
If anyone feels unwell, or you notice signs of CO, stop troubleshooting the boiler and leave the property.
Stop troubleshooting now if you smell gas, anyone has carbon-monoxide symptoms, or the boiler looks unsafe.
Do not reset, inspect, diagnose or keep troubleshooting the boiler. Treat the risk as real until the emergency services or a Gas Safe registered engineer takes over.
Smell of gas
Leave immediately. Do not switch electrical items on or off, use flames, investigate the boiler, or try to find the source.
Carbon-monoxide symptoms
Headache, dizziness, nausea, weakness, confusion, breathlessness, chest pain, or more than one person feeling unwell can indicate carbon monoxide.
Unsafe boiler conditions
Treat smoke, scorching, visible damage, overheating, leaks, repeated lockouts, or a boiler that will not stay lit as unsafe. Do not reset or open it.
Leave now — get everyone outside and seek emergency help.
From outside the property, call the National Gas Emergency Service on 0800 111 999. Call 999 if anyone is seriously unwell, has severe symptoms, or needs fire and rescue.
Why this happens
Five causes, ranked by how often we see them on a UK system.
If the boiler fires and the pump runs, the burner and the PCB are alive. The fault is on the central-heating side, and almost always lands on one of the five below — TRV, air, sludge, diverter, or zoning valve. Each has a different physical sign and a different fix; the cards describe each.
TRV stuck or slipped on the body
The thermostatic radiator valve (TRV) is the small valve head on one end of a radiator that controls a single room. If the head has been knocked or the pin has slipped inside the valve body, that radiator stays cold even when the others in the house heat up fine. Lift the cover and confirm the pin is still seating inside the valve body, not floating free; if it has slipped, a new TRV head is a five-minute, safe-at-home fix.
Safe at home · replace the TRV head
Run BoilerBuddy check →Air in the system
Air trapped at the top of a radiator or inside the pipework stops the hot water circulating through that part. The classic signs are a radiator that is cold at the top and warm at the bottom, gurgling or tapping noises when the heating fires, or a slow climb on the pressure gauge after a recent top-up. A single bleed usually clears it; if it keeps coming back over weeks, treat that as a sign the system has lost pressure, not a steady-state air-lock.
Safe at home · bleed (one-off)
Run BoilerBuddy check →Magnetite sludge in the pipes
Older steel-pipe systems build up black magnetite sludge inside the radiators and pipes, especially in the lower sections of the system where flow is slowest. The classic sign is a radiator cold at the bottom and warm at the top — the opposite of an air-lock — and the cold patch returns within days of a chemical clean. Sludge is not a safe-at-home fix: the cure is a power-flush by an engineer, sometimes a replacement radiator if the channels have already blocked.
Engineer territory · power-flush or replace
Run BoilerBuddy check →Diverter stuck on the hot-water side
On a combi, the diverter valve decides whether the burner feeds the central heating or the hot tap circuit. If it sticks on the hot-water side, the boiler fires and the pump runs but no (or only lukewarm) water makes it to the radiators while the taps keep running hot. The same symptom means a separate two-port or three-port valve head has failed on a system or regular boiler. Engineer territory; the fix is a single part swap (diverter, valve head, or actuator).
Engineer territory · part replacement
Run BoilerBuddy check →Two-port / three-port zoning valve failed
On a system or regular boiler with separate upstairs and downstairs zones, a motorised valve decides which circuit the pump sends hot water through. A failed valve head usually shows up as one zone cold while the other heats fine, the same way as a TRV but at the manifold. Engineer territory; the fix is a new valve head or actuator, often picked up and fitted in the same visit as a related repair.
Engineer territory · valve head / actuator
Run BoilerBuddy check →Safe at home
Four checks to do now, before you call anyone.
These are exactly the checks we would walk you through before booking an engineer. None of them requires opening the boiler or touching the gas side. Start with the TRV — it is the single most common reason a single radiator is cold — and work through the list.
Lift the TRV cover and check the pin
On the cold radiator, lift the cover on the thermostatic radiator valve (TRV). The pin inside should sit flush with the valve body. If the pin has slipped or is missing, the TRV is the cause and a new TRV head is a five-minute, safe-at-home fix.
If the radiator is still cold after these four, stop here and book an engineer.
Bleed the cold radiator once
If the radiator is cold at the top and warm at the bottom, it is air-locked. Hold a cloth under the bleed screw, open the bleed a quarter-turn with a radiator key, and close it the moment water appears. The radiator should fill within a minute; if the cold patch returns within days, treat that as a sign, not a fix.
If the radiator is still cold after these four, stop here and book an engineer.
Read the pressure gauge and the cold-water loft tank
On a sealed system the pressure gauge should sit between 1.0 and 1.5 bar when cold. Under 1.0 bar and the boiler may lock out on a heating demand too — top up via the filling loop. On a system boiler, lift the loft hatch and confirm the cold-water tank is full and the float valve is not stuck open.
If the radiator is still cold after these four, stop here and book an engineer.
Walk the bleed vs sludge call
Cold at the top, warm at the bottom = air, treated with a single bleed. Cold at the bottom, warm at the top = magnetite sludge, not a bleed. Our bleed vs sludge walkthrough at /guides/cold-radiator-bottom walks you through the call, with a clear "stop and call an engineer" line at every step.
If the radiator is still cold after these four, stop here and book an engineer.
Know when to stop
Stop if you notice
- A smell of gas, or a yellow, orange or sooty flame.
- A carbon monoxide alarm is sounding, or someone has possible CO symptoms.
- Smoke, scorching, visible damage, overheating or a leak around the boiler.
- The boiler keeps locking out or will not stay lit.
Try this first
Before anything else, rule out the TRV and the air-lock.
Two of the five causes above are easy to clear from the front of the radiator without opening the boiler. Check the TRV pin is still seated, and bleed the cold radiator once if it is cold at the top. If both look right, the cause is on the system side and the bleed-vs-sludge walkthrough is the natural next step.
8 steps. Yes / no at each. Stops the moment a step says it should.
Where the safe-at-home line stops
If any of these apply, stop and call a Gas Safe engineer.
These are the lines that mean the fault is past the safe-at-home boundary. Any of them — leave the boiler off, leave the heating off, and book a registered engineer today, not next week:
- The cold patches return within days of a chemical clean or a power-flush, or the cold patch sits at the bottom of the radiator (the opposite of an air-lock) — that is magnetite sludge, sealed-component territory, and clearing it needs a registered engineer.
- The boiler keeps locking out, you can smell gas, your CO alarm is sounding, or the boiler is leaking near the diverter, the pump, or the heat exchanger — stop, leave the property, and read the emergency safety guide before doing anything else.
Next step
Run the full triage in about a minute.
Tell BoilerBuddy what your boiler is doing. We will return a ranked list of likely causes, the safe checks to do first, and the exact line that means "call a Gas Safe registered engineer".
FAQ
The five questions this symptom raises.
Need a qualified engineer?
BoilerBuddy can pass what you've worked out to a local Gas Safe engineer — symptoms, checks, and the bits that point to the likely cause.
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Engineer handover
Send this sheet with you to the visit.
Print or email it to your engineer so they arrive already knowing what your boiler is doing and what you've already tried.