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Radiators Cold At The Top
The bottom of the radiator is hot and the top half stays cold — air is trapped in the panel.
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What's happening?
Hot water enters a radiator low and circulates up and across the panel. Air is lighter than water, so anything that gets into the system collects at the highest point of each radiator and sits there, keeping the water out. The result is the classic pattern: hot at the bottom, cold across the top, and a radiator that gives out perhaps half the heat it should. Air gets in from repeated refilling, from a leak drawing air in on the suction side of the pump, and from hydrogen produced inside the system as steel radiators corrode. The bleed itself is easy. The question worth answering is why air keeps coming back, because a radiator that needs bleeding twice a year is normal and one that needs it monthly is telling you something.
What usually causes it?
- Air drawn in during a recent refill, drain-down or radiator change
- Hydrogen gas generated by internal corrosion where the system has no inhibitor left
- A leak on the suction side of the pump pulling air in while it runs
- An automatic air vent that has failed open and is admitting air rather than releasing it
- A pump set too fast, drawing air over the top of an open-vented feed and expansion tank
- Pipework routed above the radiator with no vent point, holding an air lock
- A ball valve fault in a loft feed and expansion tank letting the tank run low
Is it dangerous?
Very low. A cold-topped radiator wastes heat and money and nothing more. Take ordinary care when bleeding: the water inside is hot, so use a cloth, and turn the heating off first so the pump is not pushing more air round while you work. Never open a bleed valve fully — a quarter turn is plenty, and a valve wound right out can come free.
Can I fix it myself?
Yes. Turn the heating off and let the radiators cool. Hold a cloth under the bleed valve at the top corner, open it a quarter turn with a bleed key, let the air hiss out, and close it as soon as water appears. Work from the downstairs radiators up. On a sealed system, check the pressure gauge afterwards and top up if bleeding has dropped it below the band. If the same radiator is cold again within weeks, stop bleeding and get the cause looked at instead.
When should you call a heating engineer?
Call a heating engineer if radiators need bleeding repeatedly, if you bleed and get nothing but a trickle of water, or if bleeding does not restore full heat. Repeated air is either corrosion or a leak, and both are worth finding. Also call if the pressure falls each time you bleed and will not hold, or if the water that comes out is black and dirty, which points at sludge as well as air.
What will the heating engineer actually do?
- Bleed and vent the system properly, including high points and towel rails that have no bleed valve
- Test the inhibitor level and top it up or replace it after cleaning
- Check the pump speed setting and the position of the feed and vent on an open-vented system
- Trace and repair air ingress on the pump suction side or at a failed automatic air vent
- Balance the radiators so flow reaches the ones at the end of the circuit
- Advise on a magnetic filter where corrosion is producing gas and sludge
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