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Heating · RadiatorsRadiator Bleeding & Balancing
Radiator bleeding and balancing is two jobs on one visit: bleeding lets trapped air out so each radiator heats to the top, and balancing throttles the radiators nearest the boiler so the far ones get their share of hot water. Bleed with the pump off, lowest floor first, then bring the system back to about 12–15 psi cold.
A radiator that is hot along the bottom and cold across the top has air in it, and one that gurgles when the heat comes on has air passing through. Bleeding is done with the boiler off and the circulator stopped, so the air has settled at the high points. The order is lowest floor first, working outward from the boiler, top floor last. How to bleed a radiator walks through a single radiator if you want to try one yourself.
Pressure is the step homeowners skip. Every bit of air released lowers the pressure, and a system left under about 12 psi cold may not lift water to a third-floor radiator at all. After bleeding, the system is topped up through the fill valve to roughly 12–15 psi cold for a two-storey house, a little more for three storeys, and the top floor is bled once more. A gauge that will not hold points to a leak or an expansion-tank fault, explained in why a boiler loses pressure.
Balancing fixes a different complaint: every radiator is full of water, yet the ones near the boiler are scorching while the back bedroom is lukewarm. Water takes the easiest path, so the near radiators take most of the flow. Each radiator has a second valve on its return side, the lockshield, which is set with a wrench instead of a handle. We close the near ones down in steps and measure the temperature drop across every radiator until the drops match.
Air that comes back within weeks is not a bleeding problem. It points to a failed automatic air vent, an air separator that no longer separates, a waterlogged expansion tank, or a slow leak drawing fresh, air-laden water in through the fill valve. We see it most in the pre-war brick houses of Westdale, Dundas and Old Oakville, where systems converted from gravity circulation hold a great deal of water in oversized pipes. Those parts are boiler repair work, and you can book the visit here.
Bleed, balance or repair: what each symptom means
- Cold across the top, hot at the bottom: air in that radiator, so bleed it
- Gurgling or rushing water when the heat starts: air moving through the piping, so bleed the whole system in order
- Near radiators very hot, far rooms lukewarm, all of them full of water: the system needs balancing
- Cold along the bottom, warm at the top: sludge, not air, and that radiator needs flushing
- Air back within weeks of bleeding: a failed auto-vent, air separator or expansion tank, or a leak pulling in fresh water
How it works
Cold pressure, the expansion tank's air charge, the fill valve, the air separator and the automatic vent. Bleeding radiators on a system with a waterlogged tank only has to be done again.
Circulator off, lowest floor to highest, with the pressure topped up between floors so the upper radiators can fill. Seized or painted-over bleed screws are freed or replaced as we go.
Lockshields are set with the system running, the temperature drop is measured across each radiator, and the final valve positions are written down so a later repair does not lose them.
Common questions
How often do radiators need bleeding?
A healthy closed system needs it rarely: a check at the start of the heating season, and often nothing comes out but water. Needing to bleed the same radiators every few weeks is a symptom, not maintenance. Fresh water is getting in or the air-removal parts at the boiler have failed, and the system will corrode faster until that is fixed.
Can I bleed the radiators myself?
Yes, on a hot-water system. Turn the boiler off, let it cool, and use a radiator key with a cloth under the bleed screw, starting on the lowest floor. Watch the boiler gauge afterwards. Call us if the pressure will not come back up, if a bleed screw is seized, or if black water or no water at all comes out.
How do I know if my radiators are steam or hot water?
A hot-water radiator has two pipe connections and a small bleed screw at a top corner, and the boiler has a circulator pump and reads 12–15 psi. A steam radiator often has one pipe and a bullet-shaped vent halfway up the side, and the boiler has a glass water-level gauge. Steam radiators are never bled; a cold one needs its vent or its pitch looked at.
Why is it always the top-floor radiator that goes cold?
Air rises, and the top of the system is where pressure is lowest. If the cold fill pressure is set for a two-storey house and the radiator is in a finished attic, water barely reaches it and any air in the system collects there. Raising the fill pressure a little and checking the expansion tank usually cures it. Radiators cold upstairs has more.
Will balancing lower my heating bill?
Somewhat, and mostly indirectly. A balanced system lets you set the thermostat for the room you are in instead of overheating the front of the house to make the back bedroom bearable. The main gain is comfort: every room reaches temperature at about the same time. For per-room control after balancing, look at thermostatic radiator valves.