One radiator cold?
Check this before you bleed it
Where the cold part sits tells you what is actually wrong, and in two cases out of three bleeding it will not help.
The first cold week of the year is when the phone starts, and a good proportion of those calls start the same way, which is that one radiator is cold and somebody has already bled it, and it has not helped.
Bleeding is the reflex, and it is a fair one, because it is the one bit of heating maintenance most people have been shown. The problem is that air is only one of three things that make a radiator go cold, and the other two do not respond to a bleed key at all, and there is a way to tell them apart before you touch anything that takes about ten seconds a radiator.
The top-or-bottom check
Put your hand flat on the radiator, once near the top and once near the bottom, and notice which part is cold, because where the cold sits tells you what is going on inside, and that is all there is to the check.
warm at the bottom, cold at the top
That is air, and bleeding it is the right answer.
Air rises, so it collects at the highest point in the radiator and pushes the hot water down. The bottom stays warm because water is still circulating through it. This is the case people picture when they think of a cold radiator, and it is the only one of the three where a bleed key solves the problem.
hot at the top, cold at the bottom
That is sludge, and there is no air in it to let out.
Iron oxide and debris are heavier than water, so they settle in the bottom of the radiator and build up into a sediment layer that water cannot get through. The top stays hot because water is still flowing across it. Bleeding a radiator like this achieves nothing, because there is no air in it to let out, and if you keep opening the vent you are only letting pressure out of a system that needed it.
A radiator like this wants the system cleaned, and if one radiator has settled out like that it is usually worth looking at the rest of them, because they have all been running on the same water.
cold all the way across
That is usually the valve.
If the radiator is cold top and bottom while the others are hot, water is not reaching it at all, and that normally points at one of the two valves. A thermostatic valve pin can stick, particularly on a radiator that has been shut off all summer, and a lockshield can be closed or the system can be badly balanced so that the radiator furthest from the boiler never gets its share.
Sometimes it is neither valve. Where one radiator, or a few of them, do not heat up at all and the valves are open, there may be a blockage in the radiator or in the pipework feeding it, and that is not something a bleed key or a valve adjustment will shift. That one wants looking at properly.
If it is air, here is how to bleed it properly
Four steps, and the first two are the ones people skip.
Turn the heating off.
With the pump running you are trying to vent a system that is actively pushing water around, and you will get water before you have got the air out.
Let it cool.
It does not need to be stone cold, just cool enough that you are not about to catch hot water on your hand.
Start with a quarter turn.
Put the key on the vent at the top of the radiator and turn it about a quarter of a turn anticlockwise. You should hear air. Sometimes it takes a bit more than a quarter turn before the air starts to come, so if nothing happens open it a little further, but keep it to small turns so that you can close it quickly when you need to.
Close it as soon as water comes.
The moment the hiss turns to water the air is out, so close it there.
Then check the pressure
This is the step that gets missed, and it is the reason some people bleed a radiator and end up with no heating at all.
A sealed system runs under pressure, and every time you bleed a radiator you let some of that pressure out. Do a few radiators in one go and you can drop the system below the point where it will run. The system will not work again until you top it back up.
So after bleeding, go back to the boiler and look at the pressure gauge. People often say around one bar. We prefer to keep it at roughly 1.5, and anywhere between 1.5 and 2.0 is fine as well.
When checking the boiler pressure be sure to check it when the boiler is not working and has been stable for a few minutes (no heating or hot water) as the pump affects the reading of the pressure.
A system losing a bit of pressure once or twice a year is normal and not worth worrying about. If it is losing pressure regularly then something is letting water out somewhere, and that is a different conversation.
One to keep an eye on
If you find yourself bleeding the same radiator every few weeks, the air itself is only the symptom, because it is getting in from somewhere and something is letting it in. A system that is properly sealed should not be making air on an ongoing basis, so a repeat bleed is something to get investigated before it becomes a routine you settle into.
"I had three engineers out to look at one of my radiators that wasn't fully functioning and none of them were able to fix it. Advice from replace the radiator to take the flooring up which would have created unnecessary work and cost and I wasn't a fan off! Ravi was able to diagnose the issue without all the rubbish that the others engineers had advised. I now have a fully functioning radiator keeping the hallway warm."
"The power flush of my heating system resulted in improved radiator output almost immediately, and the leaky pipe was repaired quickly and efficiently with no collateral damage to the surrounding area. They also cleaned up upon finishing, which was an added bonus."
The short version
Feel the radiator top and bottom before you pick up the key. Cold at the top is air and a bleed will fix it, cold at the bottom is sludge and a bleed will not, and cold all over is usually a valve or sometimes a blockage, and a bleed will not touch either of those.
Every system is different, and if something does not look like any of the three, or the pressure will not hold, it is worth speaking to somebody qualified before working through it by trial and error.
Quick answers
Why is my radiator cold at the top but warm at the bottom?
Why is my radiator cold at the bottom but hot at the top?
Why is my radiator cold all over when the others are hot?
What pressure should my boiler be at?
Do I need to turn the heating off before bleeding a radiator?
Is it normal to bleed radiators every few weeks?
Does bleeding a radiator lower my boiler pressure?
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