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bleeding the radiators in an old house

My son just moved into an old house in Pittsburgh and was told that he needs to bleed the radiators.
Since he grew up in Georgia, we never had boilers so the questions are:
1. Does he bleed each individual radiator "after" starting the boiler up for the first time or "before"?
2. Or I guess the correct question is, what is the step by step procedure and how often does he do this? (He says he has a special key that he was told is to be to bleed the radiators.
Thanks for the help.

Re: bleeding the radiators in an old house

The radiators should be bled until a little water comes out of each radiator's bleed valve so that any air is completely evacuated--this means trying to bleed them both before & after the boiler is turned on.

First, make sure the FRESH WATER SUPPLY VALVE near the boiler is turned ON (this supplies enough water pressure to the piping system so that air can be forced out & there is only water left inside the pipes/radiators---the altitude/pressure gauge on the boiler should read 12 psi or more to indicate that enough water pressure is reaching all the radiators.

Take a small cup to catch any water that comes out (or scrunch a wad of paper toweling & place it under the bleed valve to prevent wetting the floor).

Start at the HIGHEST RADIATOR in the system---this would be on the 3rd or 2nd floor if multi-family, or highest point if a one-story split-level.

Quite often, there are pockets of air somewhere in the piping, so the procedure should be repeated after the heat has been turned on for 10 minutes or so---most if not all of the air will gravitate toward the HIGHEST RADIATOR, so always bleed the high rad several times until only water comes out.

You should hear air hissing when you turn the bleed valve key counter-clockwise---if one or more of the rads don't hiss or produce any water, check the PRESSURE/ALTITUDE GAUGE at the boiler---it should read 12 psi (this is approx. 1 BAR (.827) on european-style gauges) or more---if not, additional water can be added via the WATER INLET VALVE discussed previously to get up to 12 psi.

Symptoms of air inside the radiators/piping is a "sloshing" or "running water" sound inside the pipes.


Re: bleeding the radiators in an old house

Thanks for the great info.

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