Bleeding Your Radiators Before Winter

Confirm it's air, not something else

Bleeding only fixes air, and air sits at the top. A radiator that is cold at the top and hot at the bottom needs bleeding. If it's cold at the bottom and warm at the top, that's sludge, and bleeding won't fix it. If it's cold all over, check that both valves are open first. If it has a thermostatic valve (TRV) with a numbered head, turn it to max. Make sure it's actually a hot-water system: many older US apartments have steam radiators, with one pipe and a small chrome vent that hisses. Those are not bled with a key. Spend your time on this check. It decides whether the rest of the job will help at all.

Why heat on, then heat off

Run the heating first so you can feel where the cold spots are. Then switch it off and let the radiators cool for 30–60 minutes before opening anything. Hot water can scald. A running pump can also pull air into the system instead of pushing it out, or spray water across the room. Use the waiting time to find your bleed key, gather the rags, and read the boiler pressure gauge.

Order and the knack of the valve

• Work on the lowest floor first, starting with the radiator farthest from the boiler, then move upstairs. Air travels upward, so this order stops you re-bleeding.
• Turn the key counter-clockwise only a quarter to half turn. You'll hear a hiss, usually for 10–30 seconds. Keep a rag and cup under the valve.
• Close the valve as soon as you see a steady stream of water with no sputtering. Snug it closed, but don't force it. Overtightening strips the small brass pin.
• If a valve won't budge, don't grab pliers. Add a drop of penetrating oil, wait 10 minutes and try again. If it's still stuck, leave it for a heating tech.

Shortcut: if you don't have a key, many modern valves take a flat-head screwdriver. Hardware stores sell universal keys for a few dollars, and it's worth buying two so one stays with the boiler.

Pressure is the step people skip

Every radiator you bleed lets out a little water, so sealed systems lose pressure. Check the boiler gauge when cold. Most US residential boilers sit around 12–15 psi (0.8–1 bar). Most UK and Australian combi boilers sit around 1–1.5 bar (15–22 psi). Go by the green zone or the label on your boiler. What you do next depends on your system:

• US boilers with an automatic fill valve: these usually refill on their own within a few minutes. Only open the manual fast-fill lever if the gauge stays low.
• UK and Australian sealed systems: top up with the filling loop.
• UK open-vented systems: if the boiler has no gauge and there's a small tank in the loft, it refills itself and there's nothing to top up.

Renters should report low pressure to the landlord instead. You'll know the job worked when every bled radiator is evenly hot from top to bottom within about 20 minutes of restarting, the gurgling has stopped, and the pressure holds steady the next day. If the pressure keeps dropping over several days, there's a leak, and that needs a heating tech.