Radiator pinhole leak
Check the steps below first — if you're not confident, get it fixed safely today.
Post a job — we'll find you an engineer →⚠️ Do not use leak sealer products in your heating system — they can block the boiler heat exchanger and cause a far more expensive repair. A leaking radiator must be replaced, not patched.
A pinhole leak weeping through your radiator panel is never good news — it means the steel inside has corroded beyond repair, and there's no quick fix. This typically happens in older heating systems where sludge (iron oxide) has built up internally, eating away at the metal from the inside out. It's particularly common in homes that haven't had a system flush or inhibitor treatment in years, or where the boiler hasn't been serviced regularly. The good news is that replacement radiators are affordable and the job is straightforward for a qualified plumber. This guide walks you through isolating the problem safely, protecting your floors from staining, and what to expect when sourcing and fitting a replacement. If you're unsure about closing valves or need help identifying the radiator size, it's worth calling a professional rather than risk water damage to your home.
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Most likely cause & what to check
A pinhole leak in the body of a radiator is caused by internal corrosion from sludge (iron oxide) — it is a permanent failure of the steel panel.
Isolate the radiator by closing both valves — TRV fully to 0, and count and note the number of turns to close the lockshield valve.
Dry the surrounding area with towels. Radiator water is dark brown-black sludge and will permanently stain carpets and flooring — put down towels and a dust sheet immediately.
Call a plumber to replace the radiator. Bring the valve measurements (centre distance between valves, and whether 15mm or 22mm tails) to help source the correct size replacement. Standard single-panel radiators cost £30–£120; labour is £80–£150/hour.
When the new radiator is fitted, consider adding central heating inhibitor to the system (Fernox F1 or Sentinel X100) to prevent future corrosion — cost £10–£20.
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Frequently asked questions
Can I use leak sealant or a radiator repair kit to patch a pinhole leak?
Absolutely not — leak sealers and radiator additives will block your boiler's heat exchanger and can cost £500+ to repair. A corroded radiator has failed structurally and must be replaced; patching is not an option.
Why is my heating system sludgy and brown inside?
Sludge forms from oxidised steel particles circulating in old or untreated heating systems. Without a corrosion inhibitor or regular flushing, iron oxide accumulates and attacks radiator panels from within, eventually causing pinholes and leaks.
How much does a new radiator cost and how long does replacement take?
A standard single-panel radiator costs between £30 and £120, with labour at £80–£150 per hour depending on your location and radiator size. Most straightforward swaps take 2–3 hours including isolation, draining, removal, and fitting the new unit.
Will adding inhibitor stop future leaks in my other radiators?
Inhibitor (such as Fernox F1 or Sentinel X100, costing £10–£20) prevents new corrosion but cannot reverse damage already done to existing radiators. It's a worthwhile investment to protect remaining radiators and your boiler once the leaking unit is replaced.
What happens if I don't replace the radiator and just accept the leak?
The leak will worsen, water damage to floors and walls will spread, your heating efficiency will drop, and sludge will circulate further through the system damaging the boiler and other radiators. Early replacement is far cheaper than addressing secondary damage.
Do I need to drain the whole heating system to replace one radiator?
No — a qualified plumber will isolate just that radiator using the TRV and lockshield valves, then drain only that radiator into a bucket or drain pan. The rest of your system stays pressurised and warm.