Radiators go cold in four or five different ways and they aren't the same fault. Where the cold sits tells you most of it before anyone touches anything.
Cold at the top, cold at the bottom, cold all over, one radiator or the whole house. Each one points somewhere different, and two of them you can sort yourself for nothing.
This page covers which is which, what to try, and what it should cost if it does need someone.
Before anything else, put the heating on, leave it twenty minutes, then go round and feel every radiator top and bottom. It takes five minutes and it decides everything that follows.
One radiator cold and the rest fine. The problem is at that radiator. Air, sludge, or its valve.
All of them cold or lukewarm. Now it's upstream of the radiators, so you're looking at the boiler, the pump, the controls, or a system full of muck.
Upstairs fine and downstairs cold, or the other way round. That's zoning, so it's valves and wiring rather than radiators at all.
That walk round saves more time than anything else on this page.
That's air, nearly every time.
Air collects at the highest point, and inside a radiator that's the top. So the bottom is hot, the top is cold, and the line between them is roughly level across the panel.
Bleed it. Key into the square valve at the top corner, quarter turn, cloth underneath, and close it the moment water comes out instead of air.
Two things worth knowing. You'll lose a bit of pressure doing it, so check the gauge after and top up if it's dropped. And if the same radiator wants bleeding every few weeks, air is getting into the system from somewhere, which is a different problem and worth looking at properly.
Hot at the top and cold along the bottom is usually sludge.
Magnetite is what you get when steel radiators and water live together for years. It's black, it's heavy, and it settles in the bottom of the radiator where the water moves slowest. The hot water runs over the top of it and the bottom never warms.
Here's how to tell it apart from air. Run your hand down the panel and find the line where it goes cold. Air gives you a level line straight across. Sludge doesn't, it follows the shape of what's settled in there, and it's usually a few inches up from the bottom rather than halfway down.
I wouldn't call it certain though. I've felt a radiator cold along the bottom, changed a sticking valve at the other end, and had it come up fine. Not enough flow through a radiator looks a lot like sludge, because there isn't enough hot water getting through to reach the bottom of it. And honestly, restoring the flow may only be hiding it, because the sludge is still sat in there either way.
The pipes tell you which. Feel both pipes where they go into the radiator. If both are hot and the panel is still cold at the bottom, water is going in and coming out, so the blockage is inside the radiator. If one of them is cold, water isn't getting through properly and it's the valve you want to look at first.
Either way, check the valves before anyone starts talking about flushing. It's free, and it's the quicker answer if it's the right one.
The rest of the house is fine and this one's stone cold all over. That means it isn't getting any water at all, so it's one of its two valves.
The TRV pin, and this one costs nothing. The thermostatic head sits on a small spring-loaded pin. Over the summer with the heating off, that pin seizes down. Come October the head turns, the numbers move, the stat is working perfectly, and the pin never lifts.
Pull the head off, there's a knurled ring or a nut holding it. Underneath you'll see the pin. Press it with your thumb and it should spring back. If it's solid, free it off with a bit of gentle persuasion and a wiggle until it moves freely, then put the head back on.
The lockshield at the other end. Plastic cap, needs a key or grips. If someone's closed it during decorating and forgotten, that radiator gets nothing and nobody can work out why.
Now it isn't the radiators at all, so work outwards from the boiler.
If it's firing, the pump's running, and they're still only lukewarm, you're into circulation. Either the system is choked, or the flow temperature is set too low.
That last one is worth checking before you spend anything. A lot of boilers now leave the factory with the flow temperature turned down, which is better for efficiency but means the radiators never get properly hot. That's a setting rather than a fault, and it catches a lot of people who've just had a new boiler fitted.
That isn't the radiators at all.
Most houses with two zones have a motorised valve for each, usually in the airing cupboard or near the cylinder, and each zone has its own thermostat. If a floor's dead, either that valve isn't opening or nothing is telling it to.
Worth knowing how to think about it. The valve, the wiring centre, the programmer and the stats are all one organism. People go straight to the valve because it's the thing you can see and it's the thing that gets replaced. What you want to do instead is follow the call for heat through the zone until you find where it stops.
And there'll usually be no fault code for any of this. The boiler is working perfectly. It just hasn't been asked.
This one isn't a fault at all.
Water takes the easiest route. So the radiators nearest the boiler get the flow and the ones furthest away get whatever's left over. Balancing is deliberately closing down the easy ones a little, at the lockshield, so the far ones get their share.
A system that's never been balanced will heat the hall and the kitchen beautifully and leave the back bedroom lukewarm forever. Nothing is broken. It's just never been set up.
Here's the test. Shut the TRVs on everything else and see whether the problem radiator comes up. If it does, it's balancing. If it still won't, something is restricting that branch and that's a different job.
Past that point it's testing rather than looking. What matters then is the order things get checked in, because the same cold radiator comes from four or five different places and people start by replacing the cheapest one. Getting that order wrong is how somebody ends up paying for a power flush when a lockshield had been shut since they decorated.
Most of what's on this page costs nothing. Bleeding a radiator, freeing a pin, opening a lockshield. If somebody's quoting you for those, something has gone wrong with the conversation.
Freeing a stuck TRV pin. Nothing, you can do it yourself. If someone comes out for it, you're paying a callout and a few minutes.
A new TRV fitted. Around £70 to £140 per radiator, part and labour.
Balancing the system. Around £100 to £200. It's an hour or two of somebody going round patiently with a thermometer, and it's worth it if the far rooms have never been right.
A magnetic filter fitted. Around £100 to £200. If you're having a flush anyway, have the filter done at the same time. Flushing a system and not fitting a filter is cleaning the carpet and leaving the dog outside in the mud.
A power flush. An independent will usually be £400 to £700 depending on the size of the system, with about £550 the middle of it for a three or four bed. It's a day's work.
British Gas sit a long way above that. Their own website starts at £755 for a three bed, and I've seen quotes north of £1,500 for an eight radiator house. That isn't necessarily a rip-off, you're paying for the name and the guarantee behind it, but it is two or three times what a good independent charges for the same day's work. If you're holding a number like that, get one other quote before you agree to anything.
One more thing to watch for. A power flush is sometimes the right answer and sometimes it's the easy thing to sell. If one radiator is cold and the other nine are fine, that isn't a system problem. Ask what they found that made them recommend it, and ask about that one radiator first.
I'm Carlo Cennerazzo, twenty-three years in the trade. Go round and feel them like I've described, then point your phone at the ones that are wrong and at the boiler, and I'll tell you which of these it is and what it should cost to put right.
£20, no callout charge, same day when there's a slot free. A written report follows afterwards, so you've got it in black and white rather than trying to remember.
And if you've already had someone out and you're holding a quote you're not sure about, that's worth a call too. I've no interest in who does the work, so I'll tell you straight whether the price is fair and whether it's even the right job.
Smell gas, or hear your CO alarm?
Don't book a call. Leave the property, then ring the National Gas Emergency Service on 0800 111 999. In Northern Ireland it's 0800 002 001.
Burning smell, sparking, or scorch marks indoors?
Turn the circuit off at the consumer unit if it's safe to do so, and get an electrician out. If there's fire or anyone's in danger, ring 999.
Power cut, or a damaged cable or substation outside?
Ring 105. It's free, 24 hours, and puts you straight through to your local network operator.