How to Bleed a Radiator (And Why Yours Is Cold at the Top)
It's the first properly cold week of the year, the heating's been on an hour, and one radiator is doing that infuriating thing where the bottom is roasting and the top is stone cold. The good news is that a radiator cold at the top is the easiest fix in home heating, two minutes with a ninety-cent key.
The part most guides skip is that where the cold spot sits tells you what's wrong, and bleeding only fixes one of the three patterns. So before the how-to, a quick diagnosis.
Why is my radiator cold at the top, bottom or all over?
• Cold at the top, hot at the bottom: trapped air. Air rises to the top of the radiator and blocks hot water from filling it. Bleeding fixes this, carry on reading our guide to bleeding your radiator below.
• Cold at the bottom or in the middle, hot at the top: sludge. Rust and debris settle at the bottom where the key can't reach them, and no amount of bleeding will shift it. That's a flushing job.
• Cold all over while others work: usually a valve problem, often a thermostatic valve with a stuck pin, or the system needs balancing. Bleeding won't hurt, but it's rarely the cure.
If it's either of these two scenarios, you will require a plumber to diagnose and fix the issue.
How to bleed a radiator in six steps
You'll need: a radiator key (any hardware shop, under a couple of euro), a cloth, and a small bowl or jug.
Turn the heating off and let the radiators cool. Bleeding a hot system sprays scalding water, so give it a good half hour.
Find the bleed valve: a small square pin inside a round nut at one top corner of the radiator.
Hold the cloth and bowl under the valve, insert the key, and turn anti-clockwise a quarter to half a turn. Never open it fully, because getting it closed again against escaping water is a fight you don't want.
Listen for the hiss. That's the trapped air leaving, and it usually takes well under a minute.
The moment water dribbles out steadily instead of spitting, close the valve snugly. Hand-tight plus a nudge, not swung on, or you'll damage the seal.
Doing the whole house? Start with the radiator furthest from the boiler and work towards it, downstairs before upstairs, which moves the air out of the system in the right order.
Do I need to repressurise the boiler after bleeding a radiator?
Here's where Irish houses differ from the UK-centric guides. If you have a sealed system with a pressure gauge on the boiler, which includes every combi, bleeding releases pressure and you'll likely need to top it back up to around 1 to 1.5 bar using the filling loop, as the gauge drops below 1 the radiators upstairs stop heating at all.
If you have an older open-vented system fed by a tank in the attic, which plenty of Dublin homes still run, there's no gauge and nothing to repressurise, because the tank tops the system up by itself. If you're not sure which you have, a pressure gauge on the boiler is the giveaway.
Why is my radiator cold at the bottom?
A radiator that's cool across the bottom while the top cooks is carrying a layer of magnetite sludge, the black iron oxide that builds up as a heating system corrodes from the inside. It sinks, it insulates the bottom of the radiator from the hot flow, and it steadily clogs valves and wears the pump.
The fix is cleaning the system, either a chemical cleanse or a full power flush depending on how far gone it is, and it pays for itself in rooms that actually heat and a boiler that isn't fighting porridge.
Why does my radiator keep needing bleeding?
A healthy sealed system shouldn't need regular bleeding, so air that keeps coming back is getting in or being made somewhere: a weeping joint drawing air in, a pump or vent pulling air into an open-vented system, or active corrosion producing gas inside the radiators.
Bleeding treats the symptom while the cause quietly gets worse, which is the point to have a plumber look at the system rather than buy a better radiator key. The same goes if bleeding releases water but no air and the radiator stays cold, or if every radiator upstairs is cold at once, which usually points at pressure or the pump rather than air.
Need Help with Cold Radiators in Dublin? Get In Touch
Read the cold spot, bleed the airlocked ones in the right order, check the pressure if your system has a gauge, and treat repeat airlocks or cold bottoms as the system telling you something.
If the radiators still won't behave, or you'd rather someone else caught the scalding water, our plumbers handle radiator repairs, valve replacements and system flushes across Dublin all winter.
Radiators cold where they shouldn't be? Call Abbington on 087 465 8338 and we'll have the heat back where it belongs.
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