Bleeding radiators in the right order

When the heating comes on for the first time after summer, the same pattern shows up in many homes: one radiator is cold at the top, or several are, and the pipes gurgle like a kettle being carried upstairs. Air in a radiator is not a fault with the boiler, it is air that has to leave, and it leaves through a small square valve at the top corner. The job takes minutes. Doing it in the right order, with the right expectations, is what this sheet is for.

Cold where? That decides everything

Feel the radiator with the back of your hand, heating on. Cold at the top, warm below is air: the air collects at the highest point and blocks the hot water from convecting round the top. That is what bleeding fixes. Cold at the bottom, warm at the top is usually not air at all, it is sludge, the black iron oxide sediment that can settle at the bottom of older radiators, and no amount of bleeding will move it. That is a flushing question, and this sheet comes back to it at the end. Cold all over while its neighbours warm is a different fault again: a valve that has been closed, a radiator that is not getting its share of the flow, or a part that has stuck, and it is worth getting a heating engineer to look at rather than assuming it is air.

What you need, and the order we do it in

You need a radiator key (some modern valves take a flat screwdriver instead), an old towel, and a jug or bowl. Heating off, bleeding with the pump running pulls fresh air into the system as fast as you expel the old, and hot radiators scald.

A common order: start with the radiator nearest the boiler, on the lowest floor, then work away from it, finishing with the radiator furthest away, usually upstairs at the end of the run. Air rides the water, so pockets gather furthest from the pump last; clearing the run in order means you are not pushing one radiator's air into the next one you open. Some engineers argue for the reverse order, and the honest position is that both work, what matters is that you work in an order, one radiator at a time, rather than opening every valve in the house at once.

At each radiator: key on the valve at the top corner, towel underneath, quarter-turn anti-clockwise. Keep the key on, and when the valve starts to release a dribble of water rather than air, close it. That's the whole job. Water from an older vented system can be dirty, so keep the towel where the water goes, not where you think it will go.

The step everyone forgets: the pressure afterwards

On a sealed combi system, letting air out of the radiators also lets a little pressure out of the circuit. After bleeding a few radiators, check the boiler dial, it can read lower than it did before you started, and that is the boiler's next complaint queued up behind the one you just fixed. For topping back up, follow the specific manufacturer's manual for your boiler: the filling loop procedure and the cold pressure band vary between models, so do not assume one figure for all boilers. Run the heating, and check once more the next morning. Vented systems with a feed-and-expansion tank in the loft are meant to top themselves up, but if that ball valve has stuck, or the tank is empty for another reason, that is one thing to check if the radiators were full of air in the first place.

When bleeding is not the answer

If the same radiator fills with air every month, something is introducing it: a slow leak on the circuit drawing air in, a reaction inside the system eating itself, or the fresh-water ingress of a pressure fault. Radiators cold at the bottom in an older house may be carrying sludge, and the fix there is a proper flush of that radiator or the circuit, which should be assessed by a heating engineer. Neither is a Saturday job with a key, and doing the key job repeatedly while calling it maintenance just delays the real one.

What this sheet does not cover

We have not covered power flushing, stuck valve pins, or system balancing, the fine adjustment of valves that decides which radiator the water visits first. Balancing especially is half craft, and a badly balanced system is why the furthest room stays cold even with every radiator bled. If the house never quite warms evenly, that is the visit to book, with a heating engineer who can assess the system properly.