The radiator that was cold at the top
Cold at the top, hot at the bottom. Anna had put a jumper on and mentioned it twice and I had said I’d look at it, twice, over about five weeks. It took four minutes and a fifty-pence key.
Anna mentioned it in the middle of October. Then again about a fortnight later, wearing a jumper, in a way that was still generous.
I said I’d look at it both times. I looked at it on the 28th of November.
Four minutes. A key that costs about fifty pence, and it lives in the box with the other three things.
Cold at the top means air
This is worth knowing because it’s almost always the same thing and it’s the easiest fault in a heating system.
Hot water goes in and circulates. Any air in the system rises, because air does, and it collects at the highest point of each radiator and sits there taking up room. So the bottom of the radiator, where the water is, is hot, and the top third, where the air is, is cold.
That’s it. That’s the whole diagnosis, and you can do it with a hand.
Doing it
Heating off, and left to cool. Not warm. Off for an hour.
Two reasons. If the pump’s running you’re pushing water round while you open a hole in it, which just moves the air somewhere else and you’ll be doing the same radiator again next week. And the water in there is hot enough to hurt you.
Cup under the valve — the little square one at the top corner. Cloth on the floor under the cup.
Key on, quarter turn, no more. You’ll get a hiss. Let it hiss. When the hiss turns to a dribble of water, that’s the air out and the water arriving, and you close it. Not tight, just closed.
The cup
I did the first one without a cup, holding a bit of kitchen roll, on the basis that it would be a few drops.
Radiator water is not clean. It is black-brown and it smells of pennies, and it went down about eighteen inches of wall and onto a carpet Anna had had cleaned in the September.
Use a cup.
The order
Downstairs first, then up. Furthest from the boiler first on each floor.
Air wants to go up. If you work bottom to top you’re pushing it the way it already wants to go and each one you do helps the next. Do the top floor first and you can end up bleeding the same downstairs radiator twice, which I have done, in a previous house, in a bad mood.
Afterwards, check the pressure
Bleeding lets pressure out of a sealed system. Do six radiators and you can drop it enough that the boiler sulks.
There’s a gauge on the front of most combi boilers and it wants to sit somewhere around one to one and a half bar when it’s cold. If it’s dropped below, there’s a filling loop — usually a braided silver hose underneath — and you open it slowly until it comes back up and then close it properly. If you don’t know which one it is, that’s a fine thing to not do yourself.
When it isn’t air
If it’s cold at the bottom and hot at the top, that’s the other way round and it isn’t air. That’s sludge — corrosion debris settled in the bottom, blocking the flow — and no amount of bleeding will touch it. That’s taking the radiator off and flushing it outside, or getting somebody in with a power flush.
One radiator like that, you can live with it. Several, and the system’s telling you something and it’s worth paying somebody.
Five weeks
The actual job was four minutes.
I’ve written it down mostly so that next October, when somebody mentions that a room is cold, I remember what the job costs before I say I’ll look at it. Same as the washing machine, which I also left for five weeks, and which also took ten minutes.
Things people email to ask
- Why is my radiator cold at the top but hot at the bottom?
- Trapped air. Air rises to the top of the radiator and sits there, holding the hot water out of the upper third, which is why the pattern is so consistent. It is the easiest heating fault there is to fix — a bleed key, a quarter turn, and about four minutes — and it costs nothing.
- How do you bleed a radiator properly?
- Turn the heating off and let everything cool. Put a cup under the valve at the top corner and a cloth on the floor. Open the valve a quarter turn with the key until air hisses out, wait until water comes instead of air, and close it. Then do the next one, and check the boiler pressure when the whole house is done.
- What order should you bleed radiators in?
- Downstairs before upstairs, and within a floor, furthest from the boiler first. Air collects at the highest points in the system, so working from the bottom up pushes it in the direction it already wants to go. Doing the top floor first can leave you bleeding the same downstairs radiator twice.
- What if the radiator is cold at the bottom instead?
- That is usually sludge rather than air, and bleeding will do nothing for it. Corrosion debris settles in the bottom of the radiator and blocks the flow, and the fix is either taking the radiator off and flushing it outside or having the system power-flushed. If several radiators are cold at the bottom, it is a system problem and worth getting somebody in.
Tom
Carpenter. Writes when something in the house breaks and he has an opinion about why. Types with two fingers, which is why there are fewer of his.
If this was any use
The chair I fixed four times
Three of those fixes were wrong and one of them was wrong in a way that took a year to show up. What I should have done on the first evening, written down so I stop doing it.
Three tools fix nine things out of ten
People ask me what to buy and expect a list of twenty. It’s three, plus a torch, and the money is better spent on one decent version of each than on a set of forty in a plastic case.
The washing machine wasn’t broken
It wouldn’t drain, it was eleven years old, and we’d already looked at new ones. Then I took the filter out. There was a 2p, a hair clip and something I have chosen not to identify.
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