Bleed Radiators With the Heating On or Off?

Why the heating should be off when you bleed

Bleeding a hot, running system is the most common mistake behind a radiator that is still cold the next day. The reasons are simple.

• Scalding: water in a working system commonly runs at 50 to 80°C, and some boilers allow up to about 85°C. A bleed valve spits it out under pressure, straight at your hand
• Air stays put: with the pump running, air is being swept round the circuit. Once the pump stops, it floats to the highest point of each radiator, which is where the bleed valve is
• You can pull air in: on some systems, open vented ones especially, the pump lowers the pressure at certain radiators below the air outside. Open a bleed valve there with the pump running and air is sucked in rather than let out

The right way to do it, step by step

You need a radiator key (brass, square socket, sold at any hardware shop), or a flat screwdriver for the slotted valves on many newer radiators, and an old towel.

• Put the heating on for 20 to 30 minutes so every radiator warms up. Note which ones are cold at the top: those have air in them
• Switch the heating off at the programmer or thermostat, and turn the boiler to standby if you can. Wait until the radiators are cool enough to touch comfortably, usually around an hour
• Start downstairs, at the radiator furthest from the boiler, and work towards the boiler, then do the upstairs ones the same way
• Hold the towel under the bleed valve at the top corner. Turn the key anticlockwise a quarter to half a turn. You will hear a hiss
• Close the valve as soon as a steady dribble of water replaces the hiss. Do not unscrew it further; the pin can come out
• When all are done, check the boiler pressure gauge and top up if needed, then turn the heating back on

If the water that comes out is black or dark grey, that is magnetite sludge. Bleeding will not fix it, and it is a sign the system water needs treating.

Check the boiler pressure afterwards

This is the step most people skip, and it is the one that causes the call the next morning. On a sealed system, the kind with a pressure gauge on or under the boiler, every bit of air and water you let out drops the pressure. Most combi and system boilers want about 1 to 1.5 bar when cold. Below roughly 0.5 bar, many will lock out and show a low-pressure fault.

Top up through the filling loop, usually a braided hose or a pair of small taps under the boiler. Open slowly, watch the gauge rise to about 1.2 bar, and close. Worcester Bosch, Vaillant, Baxi and Ideal all place the filling loop slightly differently, and some Worcester models use an integral key. If the loop will not fill, our guide to a boiler filling loop not working helps.

An older open vented system with a small tank in the loft refills itself, so there is no gauge to check. Just make sure the loft tank has water in it.

Towel rails, upstairs radiators and the boiler vent

Heated towel rails are often the highest point on a circuit, so they collect air first. Their bleed valve is usually a small screw at the top of one leg or in the top cap. Bleed them last, and expect a little more air than from a normal radiator.

Radiators on the top floor of a tall London terrace or a maisonette above a flat collect most of the air in the building. If only those are cold at the top, bleeding them is normally all that is needed. If the ground floor ones are also affected, the pressure is probably low or the pump is struggling, and bleeding alone will not solve it.

Many system and combi boilers also have an automatic air vent on top of the pump inside the casing. You do not need to touch it and should not open the boiler to find it. If it is working, small pockets of air clear on their own after a few hours of running, which is why a mild gurgle in the radiators after bleeding often fades by the next day.

What if the radiator will not bleed?

Sometimes you open the valve and nothing comes out, not even air. On a sealed system that usually means the pressure is at or near zero, so there is nothing pushing out. Top up the pressure first, then try again. On an open vented system it can mean the loft tank is empty because its ball valve has stuck.

If the valve itself will not turn, do not force it with pliers. Old bleed screws seize, and a snapped one leaves an open hole in a pressurised radiator. A drop of penetrating oil and a few hours usually frees it; if not, it needs a replacement bleed valve, which is a quick job for an engineer.

If the radiator bleeds but stays cold at the bottom, the cause is not air. Radiator not heating up goes through sludge, stuck valves and balancing in order.

Bleeding every week is a warning sign

A radiator that needs bleeding once or twice a winter is normal. One that fills with air again within days is telling you something is wrong. The two usual causes are a small leak that lets water out and fresh, oxygen-rich water in when you top up, and corrosion inside steel radiators that produces hydrogen gas. Never hold a flame near a bleed valve to find out; an engineer can test a water sample for corrosion and inhibitor level instead.

Both problems get worse if left. Corrosion makes sludge, sludge blocks radiators and the boiler heat exchanger, and a slow leak keeps drawing more oxygen in. If the boiler pressure keeps dropping between top-ups, read boiler losing pressure every day, and if you would rather have it found and fixed, book central heating repair. The £60 call-out covers the visit and the repair price is agreed with you on site before any work starts.