Radiator cold on one side
Check the steps below first — if you're not confident, get it fixed safely today.
Post a job — we'll find you an engineer →A radiator that's warm on one side and cold on the other is one of the most common heating complaints we see in older homes and systems that haven't been maintained properly. This pattern typically indicates that water isn't flowing freely across the entire radiator body, which means you're not getting the heat output you're paying for. The culprits are usually straightforward: an airlock, a lockshield valve that's been closed too far during system balancing, or a buildup of sludge and magnetite inside the radiator restricting the flow. The good news is that many of these problems are fixable with basic tools and a bit of patience, though persistent issues across multiple radiators suggest your whole system needs professional attention from a Gas Safe registered engineer.
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Most likely cause & what to check
If the radiator is warm on the side where the flow pipe enters and cold on the other side, the most likely causes are partial sludge blockage or a flow restriction.
First, try bleeding the radiator — sometimes a partial airlock can cause this pattern.
Check the lockshield valve at the cold end — it may have been closed too far during balancing. Remove the plastic cap and open the lockshield valve a few more turns using an adjustable spanner.
If bleeding and adjusting the lockshield valve does not resolve it, there is likely sludge inside the radiator body blocking flow. You can try isolating and removing the radiator to flush it out with a hosepipe, or consider a power flush (£300–£600 for a whole system).
If multiple radiators show this problem, the whole system may be sludged and a full power flush by a heating engineer is recommended.
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Frequently asked questions
Why is my radiator cold on one side but hot on the other?
This happens when water can't flow freely across the radiator, usually due to a partial blockage from sludge, an airlock, or a lockshield valve that's been adjusted too tightly. The warm side is where water enters, and the cold side shows where flow is restricted. It's a sign your heating system needs some TLC, but it's easily diagnosed and often straightforward to fix.
Can I fix a one-sided cold radiator myself?
Yes, you can try three things as a first step: bleed the radiator to remove any trapped air, check and gently open the lockshield valve at the cold end, and ensure the thermostatic valve (TRV) isn't stuck or set too low. If none of these work, the radiator likely needs flushing or your system needs a power flush — that's when a professional engineer becomes necessary.
What's the difference between bleeding a radiator and flushing it?
Bleeding removes trapped air from a radiator using the bleed screw at the top; it takes minutes and costs nothing. Flushing involves isolating the radiator and running water through it to clear sludge and debris — it's more involved and may require removing the radiator or calling an engineer for a full system power flush.
Is a power flush worth the cost?
If multiple radiators show the same cold-on-one-side pattern, yes — it suggests your whole system is sludged and a power flush (typically £300–£600) will restore efficiency and performance across all radiators. For a single problem radiator, try the simpler fixes first; power flushing makes most sense as a preventative measure in systems over 10 years old with poor maintenance history.
How do I know if my lockshield valve is closed too far?
The lockshield valve is the one without a numbered dial, usually hidden under a plastic cap at the opposite end from the TRV. If it's only cracked open a turn or two, water flow is severely restricted and that radiator won't heat properly — try opening it another 2–3 turns using a spanner and see if the cold side warms up.
Will sludge come back after I flush a single radiator?
It may, because the sludge is circulating through your entire system — flushing one radiator is a temporary fix. If sludge is a recurring issue, a full system power flush and magnetic filter installation is the long-term solution that stops it happening again.