All radiators cold except one or two
Check the steps below first — if you're not confident, get it fixed safely today.
Post a job — we'll find you an engineer →There's nothing more frustrating than paying to heat your home only to find most radiators stone cold whilst one or two are burning hot enough to blister your hand. This common problem strikes many UK households, particularly older systems or those that haven't had regular maintenance. The good news is that it's usually fixable without calling an engineer — and this guide walks through the diagnostic steps that resolve it in most cases. The issue stems from your heating system taking shortcuts: instead of distributing hot water evenly around the property, it's funnelling everything through the path of least resistance. This happens because of closed valves, trapped air, or internal sludge that's clogged the pipework. We'll cover the checks you can do yourself first, and when it's time to ring a qualified heating engineer.
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Most likely cause & what to check
When most radiators are cold but one or two are scorching hot, the system is not circulating correctly. The boiler is likely firing and the pump is running, but water is taking the path of least resistance through only a couple of radiators.
Check all radiator valves are open — both the TRV (set to 3 or 4) and the lockshield valve. A common cause is several lockshield valves having been fully closed at some point.
Bleed all radiators, starting with those furthest from the boiler — trapped air can cause severe flow restriction.
Check the pump speed setting at the boiler — on some systems the pump speed can be adjusted. A higher speed setting will push water through a sludged system more effectively as a temporary measure.
If all valves are open and the radiators are bled, the system likely has significant sludge blockage. A heating engineer can carry out a power flush (£300–£600) to restore circulation.
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Frequently asked questions
Why are some radiators boiling hot and others completely cold?
Your heating pump is working, but water is following the easiest route through just one or two radiators rather than circulating evenly around the whole system. This happens when valves are closed, air is trapped, or sludge has built up inside the pipes and blocked most of the circuit. The hot radiators are the ones on the 'easy' route, so they're getting all the heat.
Is it dangerous to have radiators that won't heat?
No, it's not dangerous — but it's inefficient and costly. Your boiler is working harder than it needs to, burning more fuel to heat water that's mostly bypassing your rooms. If you've got unheated areas, you're also at higher risk of condensation and damp in those spaces during winter.
How do I know if my lockshield valve is closed?
The lockshield valve is the one with a plastic cap on the opposite end of the radiator from the TRV (the one with numbers 1–5 on it). Gently turn the valve head anticlockwise; if it won't turn at all, it's fully closed. You should be able to feel it open with just a quarter to half turn — don't force it.
Can bleeding radiators fix a system that's completely blocked with sludge?
Bleeding helps with trapped air, but it won't clear sludge blockages — that requires a power flush from a qualified engineer. However, it's always worth bleeding first, as air restriction is a much simpler fix and costs nothing. If bleeding doesn't help and all valves are open, sludge is the likely culprit.
How often should I bleed my radiators?
Most systems benefit from bleeding once a year, ideally in autumn before the heating season starts. If you're regularly finding air in your radiators, it suggests a leak somewhere in the system that needs investigating by a plumber.
What does a power flush actually do?
A power flush uses high-velocity water and specialist chemicals to dislodge decades of rust, sludge, and debris from inside your pipework and radiators. Once flushed, an inhibitor is added to the system to prevent new sludge forming. It's expensive but can transform a sluggish old system and typically adds years to its life.