A pinhole leak at the top of a radiator. Repair or replace?

[saveasteading]

Current house, not the project: We saw a paint blister on the back of a rad, just over the top edge. Of course we popped it, and some mucky water sprayed the wall.

How much water? negligible, 1/20 of a teaspoonful.

There is perhaps 50mm x 30mm of loose paint which has rubbed off easily.

Strangely there does not appear to be any leak other than that initial spray. But there is probably air at the top....in which case where does the water spray come from?

 

I'm guessing rust caused in the air gap at the top. I don't think we've bled it for a year.

And that if I bled the rad, a pinhole would become apparent and leak.

 

Patch or new rad?

 

 

[JohnMo]

Bin fit a new one

[MPx]

Yes agree with JohnMo - bin it and fit a new one.  It wont have just rusted in the one place - could be like rice paper over a pin cushion.  And while you're at it inject some Fernox or your choice of inhibitor into the system to delay the rust process in the rest of the kit.  I'm guessing its not had a dose for some considerable time if rusting through....

[saveasteading]

It should be dosed. The whole circuit was looked at not so long ago. 

Is this a common thing? Maybe the whole house is a concern.

[JpManning]

It's fairly common, and it usually tells you more about the system than about that one radiator. Pressed steel rads nearly always fail from the inside, and the top is the classic spot because that's where air or gas collects. Steel sitting in that pocket with a film of moisture on it corrodes much faster than steel below the waterline, and inhibitor does very little up there.

So the real question is why there was a pocket at the top. A few things worth checking before you worry about the whole house:

How often do the other rads need bleeding, and is it a long hiss or barely anything? Rads that need bleeding every few weeks point to air getting in, or gas being produced by corrosion.

When you drain the old one, look at what comes out. Black sludge is magnetite, which is normal wear and manageable with a magnetic filter and a clean. Orange or brown water suggests oxygen is getting in somewhere, and that's what eats rads.

If it's open vented, have a look at the feed and expansion tank in the loft. A ball valve that's constantly topping up, or the pump pushing water over the vent pipe, keeps pulling fresh oxygenated water into the system. If it's sealed, think about how often you've had to repressurise it. Regular top ups mean a slow leak somewhere and fresh water going in every time.

Get a sample tested for inhibitor level rather than assuming it's fine. Most inhibitor manufacturers offer a testing service, and a cheap dip test kit will give you a rough idea in the meantime.

If the others rarely need bleeding, the water is black rather than orange and the inhibitor reads OK, I'd treat it as one tired rad (how old is it?) and just replace that one. If several bleed often and the water is rusty, it's worth tracking down the air ingress first, otherwise the new rad will go the same way.

[saveasteading]

2 hours ago, JpManning said:

how old is it?

probably 15 years. I'll give them all an airing, but generally it isn't a problem.

This radiator did leak from a valve for a while so may also have been letting in air there or elsewhere.

[JpManning]

17 hours ago, saveasteading said:

probably 15 years. I'll give them all an airing, but generally it isn't a problem.

This radiator did leak from a valve for a while so may also have been letting in air there or elsewhere.

Thanks, that helps. An earlier leak at a valve would explain it. While it was weeping, the system was losing water and taking fresh water in, and air can get drawn in at a joint like that too, so the top of that rad would have been sitting in an air pocket for a while.

At 15 years old I'd just replace it rather than try a repair. When you fit the new one, use new valves or at least repack and reseal the old ones, so you don't end up with the same problem. Once it's back on, bleed it again after a day or two of running, and keep an eye on the pressure or the loft tank for the next few weeks. If the others bleed quickly with only a short hiss, the rest of the system is probably fine, but a fresh dose of inhibitor and a magnetic filter if you don't already have one are worth doing.

[Nickfromwales]

On 06/10/2026 at 19:26, saveasteading said:

Current house, not the project: We saw a paint blister on the back of a rad, just over the top edge. Of course we popped it, and some mucky water sprayed the wall.

How much water? negligible, 1/20 of a teaspoonful.

There is perhaps 50mm x 30mm of loose paint which has rubbed off easily.

Strangely there does not appear to be any leak other than that initial spray. But there is probably air at the top....in which case where does the water spray come from?

 

I'm guessing rust caused in the air gap at the top. I don't think we've bled it for a year.

And that if I bled the rad, a pinhole would become apparent and leak.

 

Patch or new rad?

 

 

Are there 2 f&e tanks in the attic, or just one? 
 

If it’s very old it may have a primatic HW cylinder and you cannot use inhibitor if so. 

[saveasteading]

4 minutes ago, Nickfromwales said:

just one? 
 

It has been there 30 years. Just a plastic tank with expansion and overheating release pipe, and a ball valve.

 

The hot tank is newer, because the old one failed for some forgotten reason. It is in the airing cupboard next to the boiler. So it's easy to look at if info would be of use.

[Nickfromwales]

5 minutes ago, saveasteading said:

It has been there 30 years. Just a plastic tank with expansion and overheating release pipe, and a ball valve.

 

The hot tank is newer, because the old one failed for some forgotten reason. It is in the airing cupboard next to the boiler. So it's easy to look at if info would be of use.

So 100% defo one tank for central heating AND HW? Newer (old) systems would have a fish tank sized f&e for heating, and a bigger 50/75 gallon tank for HW. 
 

If you have one tank for both then do NOT put inhibitor in or there’s a chance you’ll be bathing and washing in inhibited water. 
 

Primatic cylinders use a one-way bellows to feed the f&e into the heating and they fail and can let the two bodies of water mix.

[saveasteading]

I like advice to not do anything. Thanks.

 

Before tackling the rad, is there any check I could do other than probing for weaknesses?

Maybe it is the tiniest pinhole..... there was such a tiny amount of water in the paint blister.

Radiators are designed and manufactured crured to hold water after all.

Release any air and see if it leaks and how much?

Then patch with Snibbo. 

 

Or is that just delaying a bigger issue?

[-rick-]

Rather than spend time on current radiator. Just go buy a new one. Cheap (unless unusual type), easily available from places like Screwfix. Time spent fiddling now is just a waste. Fitting a new like for like rad shouldn't take particularly long.