Quick answer
To bleed a radiator, turn the heating off and let the system cool, then open the bleed valve at the top corner a quarter turn with a radiator key. Air hisses out; close the valve when water runs in a steady stream. Start on the lowest floor and work up, then check the boiler gauge and top up to about 12 to 15 psi cold. Hot-water systems only, never steam.
You bleed a radiator by letting trapped air out through the small valve at its top corner until water follows. It takes a radiator key, a rag and about two minutes per radiator, and it is one of the few jobs on a boiler system a homeowner can safely do. Two conditions apply: the system must be hot water, not steam, and the boiler pressure has to be checked afterwards.
Hot-water radiators only: steam is never bled, and baseboard is purged
Bleeding is for hot-water systems, where the radiators are full of water under 12 to 15 psi. A steam radiator is full of air when cold and steam when hot, and it clears its own air through an automatic vent on the side. Opening anything on a steam radiator while the boiler is firing releases steam at over 100 °C. If each radiator has two pipes and a small square-headed or slotted valve near the top, the system is hot water and you can carry on.
Hydronic baseboard heaters are a continuous loop of finned copper pipe, and many have no bleed valve at all. Air in a baseboard loop is pushed out at the boiler by closing a valve and forcing water through one zone at a time into a hose, which is called purging. It involves the fill valve, the zone valves and the boiler drain, and it is a technician's job unless you have been shown how on your own system.
The steps, in order, for a hot-water radiator
- Turn the heating off at the thermostat and wait until the radiators are cool. With the circulator stopped, air settles at the top of each radiator, and the water that follows it will not scald.
- Read the boiler pressure gauge and note the number. If it is already under 12 psi, bleeding will lower it further, so plan to top up.
- Start with the lowest floor and work upward, finishing with the highest radiator in the house. Air migrates up, and doing the top floor first means doing it twice.
- Hold a rag or cup under the bleed valve and turn the key counter-clockwise a quarter to half a turn. Do not unscrew the pin all the way; it is small, and the water behind it is under pressure.
- Listen for the hiss. When the air stops and water comes in a steady stream with no spitting, close the valve snugly by hand.
- Move on to the next radiator, then turn the heat back on and feel across the top of each one after half an hour.
Old cast-iron radiators and newer panel radiators can take different key sizes, and some bleed valves have a slot for a flat screwdriver instead.
After bleeding: top the boiler back up to 12 to 15 psi cold
Every bit of air you let out lowers the system pressure, so the gauge check is part of the job. Most boilers in Ontario homes have an automatic fill valve, a bell-shaped brass valve on the cold-water line into the boiler, which should restore about 12 psi by itself provided the shut-off feeding it is open. If the gauge stays low, lifting the fast-fill lever on that valve for a few seconds brings the pressure up; let go the moment the needle reaches 12 to 15 psi.
Combi and wall-hung boilers may have a manual filling loop or a fill button instead, and the owner's manual shows which. Do not go past 15 psi cold: pressure climbs as the water heats, and the relief valve opens at 30 psi. A loop filled with glycol antifreeze, common where pipes run through a garage or out to a snow-melt slab, should not be topped up with plain water again and again, because each top-up thins the freeze protection.
Air that comes back every few weeks is a service call, not more bleeding
A sealed heating system should need bleeding once after it has been opened for work, and rarely after that. If the same radiators fill with air again within weeks, something is feeding air or fresh water in.
- A leak: every litre that drips out is replaced through the fill valve by fresh water, and fresh water carries dissolved air that comes out as soon as it is heated. Look for green or white crust on valves and fittings, and see why a boiler keeps losing pressure.
- A failed automatic air vent: the small brass can near the boiler or on the air separator is meant to release air by itself. When its float sticks or its cap is screwed down tight, the air travels on to the radiators.
- A waterlogged expansion tank: with no air cushion, pressure swings hard on every cycle, the relief valve spits water, and the make-up water brings new air with it.
Repeated bleeding hides these faults and keeps adding oxygen to the system, which rusts cast iron and steel from the inside. Book the visit instead.
Stop and call if any of these happen
- Nothing comes out of an open bleed valve, air or water: the pressure is too low to reach that radiator, or the valve is blocked with paint or scale.
- The bleed screw will not close and keeps dribbling.
- The water that comes out is black and gritty, a sign the system needs a flush more than a bleed.
- The boiler gauge will not come back up to 12 psi, or it climbs past 25 psi once the heat is running.
- A radiator stays cold after bleeding; one cold radiator has other causes.
If you would rather not do it yourself, radiator bleeding and balancing covers every radiator in the house plus the pressure check. A visit to find out why air keeps returning starts with the $90–$150 diagnostic, credited to the repair, and any part is priced in writing first. ZK Mechanical handles that work and wider boiler repair from its Burlington shop; book a time that suits you.