Nothing worse than a cold radiator when you're paying for the heating. Here are the five most common reasons and what to do about each one.
1. Trapped Air (Cold at the Top)
The symptom: Radiator hot at the bottom, cold at the top.
Why it happens: Air rises and gets trapped at the top of the radiator. The air pocket stops hot water filling the whole radiator.
The fix: Bleed the radiator.
- Turn heating off and wait 10 minutes
- Use a radiator key on the bleed valve (top corner)
- Turn anti-clockwise until air hisses out
- Close when water starts dribbling
- Check boiler pressure and top up if needed
Takes: 5 minutes
This is by far the most common radiator problem. Do all your radiators at the start of the heating season as routine maintenance.
2. Sludge Buildup (Cold at the Bottom)
The symptom: Radiator hot at the top, cold at the bottom (or in the middle).
Why it happens: Over time, rust and debris (sludge) accumulates in the bottom of radiators. It stops water circulating.
The fix: Depends on severity.
Mild cases:
- Add a radiator sludge remover to the system
- Run heating for a few days
- Drain and refill system
Moderate cases:
- Remove and flush the individual radiator
- Reconnect with fresh water
Severe cases:
- Professional powerflush (whole system)
- Or replace badly affected radiators
Prevention: Adding inhibitor to your system prevents sludge. Most systems should be treated.
3. Stuck Thermostatic Valve (Radiator Completely Cold)
The symptom: One radiator completely cold while others work fine.
Why it happens: The pin inside the thermostatic valve (TRV) gets stuck in the closed position, usually from not being used over summer.
The fix: Free the stuck pin.
- Remove the TRV head (usually unscrews or pulls off)
- Find the pin underneath - it should move freely up and down
- If stuck, gently work it loose with pliers
- Spray with WD-40 and work up and down
- Replace TRV head
- Turn valve to max and test
Pro tip: At the end of each heating season, turn all TRVs to max. This keeps the pins moving and prevents sticking.
4. System Imbalance (Some Radiators Hotter Than Others)
The symptom: Radiators nearest the boiler are roasting, radiators furthest away are lukewarm.
Why it happens: Water takes the path of least resistance. Without balancing, it flows through nearby radiators and skips distant ones.
The fix: Balance your radiators.
This involves adjusting the lockshield valve (the one without numbers, usually at the bottom opposite the TRV) on each radiator to control flow.
Basic method:
- Turn off heating, let radiators cool
- Open all lockshield valves fully (anti-clockwise)
- Turn heating on
- Starting from the radiator nearest the boiler, partially close the lockshield valve
- Radiators further from the boiler should have lockshield more open
Proper method: Use thermometers on the pipes and balance so there's a 12°C difference between flow and return pipes at each radiator.
This is fiddly work - call a heating engineer if you're not confident.
5. Low Boiler Pressure
The symptom: All radiators lukewarm or heating not working at all.
Why it happens: Pressure has dropped below working level. Usually from small leaks or recent bleeding.
The fix: Repressurise the system.
- Find the pressure gauge on your boiler (should read 1-1.5 bar)
- Locate the filling loop (braided hose with valves, often underneath)
- Open valves slowly to let water in
- Watch the gauge rise to 1.2-1.5 bar
- Close valves
Warning: If pressure keeps dropping, you have a leak somewhere. Check radiator valves, joints, and connections. Persistent pressure loss needs investigation.
Less Common Causes
Faulty pump: If no radiators heat up, the circulation pump might have failed. Listen for it running - should hear a gentle hum.
Diverter valve stuck (combi boilers): Hot water works but heating doesn't? The diverter valve might be stuck in hot water position.
Blocked pipes: Rare but happens. Usually affects radiators on one section of pipework.
Air lock in system: Air trapped in pipes, not just radiators. May need draining and refilling the whole system.
DIY vs Call a Professional
Do it yourself:
- Bleeding radiators
- Freeing stuck TRV pins
- Topping up pressure
Consider professional help:
- System balancing (if you want it done properly)
- Powerflushing
- Pump or valve replacement
- Persistent pressure loss
Maintenance Tips
Keep your radiators working well:
- Bleed annually before winter
- Run heating in summer occasionally (once a month for 15 minutes)
- Keep TRVs turned up when not in use
- Add inhibitor when system is drained
- Get your boiler serviced annually
Radiator problems beyond DIY? We diagnose and fix heating issues across Exeter. Call 01392 964094 or get a quote.
Sam Hembury
Sam is the founder of Hembury Contracting, providing professional handyman services across Exeter and Devon. With years of experience in property maintenance, he shares practical tips to help homeowners tackle common tasks.
