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Air gets into a hot water heating system whenever it is filled or topped up, and it collects at the highest point of each radiator. Water cannot reach where the air is, so the top of the radiator stays cold. Bleeding lets the air out. It applies only to hot water systems; a steam radiator has an air vent that does this job by itself.
Check that it is a hot water system
Each radiator or baseboard has two pipes, and at one end, near the top, there is a small valve with a square stem or a slotted screw. The boiler has a pressure gauge. If instead the radiators have a single pipe and a bullet-shaped vent on the side, you have steam, and this guide does not apply.
What you need
- A radiator bleed key, which fits the square stem, or a flat screwdriver for the slotted kind
- A cup or a small bowl, and a cloth
- Ten minutes
Step by step
- Turn the heating off at the thermostat, and give the system twenty or thirty minutes to settle. Bleeding with the pump running draws more air in.
- Start on the lowest floor, at the radiator nearest the boiler, and work through the house, ending upstairs at the radiator furthest away.
- Hold the cup under the valve with the cloth ready. Turn the key counterclockwise about a quarter to half a turn. Do not remove the stem entirely.
- Listen for the hiss. Air comes out first. When water starts to come in a steady stream, close the valve. Do not overtighten.
- Check the boiler pressure when you have finished. Bleeding lets water out of the system, and the pressure will have dropped. Cold, most home systems want about 12 to 15 psi; the gauge often has a marked range. If it is low, the system needs topping up: see below.
- Turn the heating back on and check that every radiator heats through.
Baseboard heaters
Baseboards often have a bleed valve at one end, behind the cover, or a bleeder on the pipe in the basement near where the loop returns to the boiler. Some have none, and the air is removed at an air separator near the boiler. If a whole loop of baseboards stays cold, that is where to look.
Topping up the pressure
Most boilers have an automatic fill valve that keeps the pressure up by itself. If yours has a manual fill, there is a valve on the cold water pipe feeding the boiler; opening it briefly raises the pressure. Watch the gauge and stop at the marked range. If you are unsure which valve it is, ask the plumber to show you at the next service, and do not guess. Overfilling opens the relief valve.
If bleeding does not fix it
- The radiator is cold all over: its valve is closed or stuck, or that loop is airlocked. Check the valve first.
- It needs bleeding every few days: air is being drawn in, or the automatic air vent near the boiler has failed. A plumber can find where.
- Every radiator is cold and the boiler is running: the circulator pump. A professional's job.
- Water keeps coming from the bleed valve after closing: the valve needs replacing, which is a quick job for a plumber with the system drained down to that level.
When to call a plumber
- Pressure that keeps dropping, which means a leak somewhere in the system
- A pump that hums but does not move water
- Radiators that need bleeding again and again
- Any doubt about which valve fills the boiler
Find a licensed plumber near your town.