Proper bleeding method - hot water cast iron radiators

Old 11-10-11, 06:54 PM
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Proper bleeding method - hot water cast iron radiators

hi folks. I am reading conflicting information on the web about how to bleed radiators and searched these forums pretty thoroughly to no avail, so I though I would ask in this forum.

I have pressurized oil boiler with cast iron radiators, 3 stories, sitting at about 15 psi right now.

I see bleed em hot, bleed em cold, bleed em on lower floor first, bleed em on upper floor first, bleed em when calling for heat, bleed em powered down. So I thought I would ask some experts.

Thanks in advance for your advice!
Old 11-10-11, 07:15 PM
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It can be hard to bleed them hot only, as air doesn't flow water well.
So, rule of thumb has always been bleed the from the lower point up, with the circ off.
If the circ is on, and you drop pressure you may (and likely will) pull air in and that doesn't help bleeding rads now does it.

15 PSI may not be enough pressure for 3 floors.
Old 11-10-11, 09:34 PM
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Quite simply, there is NO "standard method" of bleeding the air from a hot water heating system, you have to try all to be certain of removing all the air. I have seen systems that worked best when bleeding cold, when bleeding hot, when the circulator was running and when the circulator was stopped.

My suggestion would be to try running the circulator for a short while without firing the boiler and then stop the pump and bleed. It won't make any difference whether you start at the top or the bottom. Then run both the circulator and the burner to heat up the water and try bleeding again, this time with the pump running.

You need a minimum of 4 psi at the top-most part of the system so make a rough measurement in vertical feet from the bottom of the boiler to the top-most part and divide by 2 then add 4. The result is a good average number for the cold pressure of the system. This method, quick and dirty, gives a slightly higher pressure than actually needed but as long as the hot pressure is no higher than about 27 psi you are fine.
Old 11-11-11, 03:08 PM
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I feel that bleeding a system with cast iron radiators is simpler than a system with fin-tube baseboard. After your pump has run a couple minutes, most of the air gets trapped in the top of your radiators. Go around let the air out each one. If you have a lot of air in the radiators from refilling the system, you should bring it up to 25 psi. This will help push the air out of the radiators more quickly. The pressure will decrease as the air comes out. If your pressure is still a higher than desired when done bleeding, let some water out through the boiler drain. With a 3rd floor, you are probably looking to have around 18 psi when cold.

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