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Question:

How to bleed a boiler? Should the boiler be off or on? How to know when bleeding is completed? Normal how long?

I know there's definitely air in it because I can hear water running inside the pipes and sometime making bubble or gurling sound when the boiler is running. It's a 2 zones. I did try to bleed it until I see clear and hot water come out. But I don't if I did it right. Any help is greatly appreciated.

Answer:

Bleeding your system is dependent how your system was designed and installed. There should be valves to release air at the tops of older, cast iron radiators. Most newer systems have automatic air release valves mounted near the boiler itself. Before bleeding the system, make sure it's properly pressurized. For example, my boiler requires 12 to 14 psi in the system when it's cold. If I lose that pressure I'll hear water running as if there's air in the system due to cavitation, whether there's actually air in the system or not. If your system pressure is correct, you can flush any hot water heating system by opening the water feedline fully (ensuring you bypass any pressure reducing valve), and connecting a hose to a drain bib on the boiler's plumbing. Open the drain bib fully and let the water flow through the system for several minutes. The high velocity of the water should push any air out of the system - even if there are no air bleed valves installed anywhere on the system. Good luck with it!

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