One radiator not heating: valves, air, or balancing
Quick summary
If all other radiators heat normally, focus on this radiator’s valves, trapped air, and whether it’s being starved of flow.
Safety first
- Radiators can be hot — protect hands and surfaces.
- Don’t force seized valves.
- On sealed systems, bleeding can reduce pressure.
Full checklist: Safety guidance
What to check (in order)
- Confirm other radiators get hot (proves heating works generally).
- Check TRV is open and not stuck; if it has a pin, confirm it moves freely (gentle only).
- Check the lockshield valve is open (note its position before changing anything).
- Bleed briefly if you suspect air (only if safe/you have a key).
- Try a flow test: temporarily turn down a couple of very hot radiators; see if this radiator starts warming.
What the result means
- TRV pin stuck: freeing it can restore flow; recurring sticking suggests replacement.
- Lockshield closed/too restricted: opening may restore flow, but balancing may be needed afterwards.
- Only heats when others turned down: system distribution/balancing issue.
What you can safely do
- Check that the TRV is fully open and the pin is not stuck.
- Confirm the lockshield is not accidentally closed.
- Bleed briefly if you suspect trapped air.
- Turn down a couple of very hot radiators temporarily to see whether flow shifts.
When to call a professional
- You can’t safely isolate/bleed or valves appear damaged/leaking.
- Multiple radiators affected.
- After changes, system becomes noisy/unstable (balancing needed).
Engineer notes
Single-rad diagnosis: verify valve function and pipe temps. If TRV, check head and pin; if still cold, check lockshield seat and consider blockage. If balancing, document lockshield turns and aim for stable ΔT across rads. Watch for microbore constraints and long-run starvation.
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