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.