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Gas Furnace or Boiler


1. Should I let my forced hot water heating system cool down before trying to purge the trapped air from the drainage spigots?

2. How do I manually manipulate the Watts self-feeding (auto-makeup) water valve to control pressure so it doesn't go above 25 PSI during the actual air purging process?

3. To purge air from a forced hot water system, you advised another homeowner to "close all of the shut-off valves" as one of the steps to follow just before opening the drain spigot to drain water from each heating loop. My heater has three shutoff valves: 1. the valve to shut all water off to the house, at the pump location 2. the shut-off valve just before the pressure regulating valve that is part of each heating loop 3. a shut off valve that supplies fresh water directly into the side of the boiler, labeled "cold water inlet". Do I shut all these shut-off valves off before I drain the water from each loop?


Dear Roy:

Regarding your first question, I would suggest letting your forced hot water heating system cool down before purging the air out of the system. There are two reasons for this: 1) as the water cools down dissolved air will be released from the water, which will then allow you to purge this air out as well; and 2) less risk of being scaled by hot water. However, if it is not practical for you to let your system cool completely down, then you can still purge the system, but just be careful of the hot water.

Regarding your second question, does your Watts auto-fill valve look like the one pictured here with the release lever on top:

If not, here is the contact information for Watts Valves: who can tell you specifically for your particular model of valve.

And regarding your third question, you mentioned that you are looking to drain your system, but do you mean actually draining the system, or just purging the air?


Hi again, and thanks for the advice thus far. In answer to your first question, my Watts fill valve looks exactly like the one pictured, with the release lever on top, at:

I just don't know how to operate the valve to regulate the water
pressure when purging the air from each forced hot water loop.

To respond to your second question, I am interested in purging the air from the system through the drain spigots. I am trying to accomplish this by flushing all of the water plus air out of the spigot for each heating loop, then filling each loop with fresh, but not too cold water, that hopefully has no new air in it. As noted in my question 3, I appear to have three shutoff valves to consider at the boiler.

After the spigot draining and new water fill-up is done, I plan to bleed air from the air vent in the system to get out any additional air that may be released.

Your thoughts in reply to my last two questions would be most
appreciated. Any source of very detailed steps as to how this draining and refilling procedure can most safely and effectively be accomplished would be most appreciated.

Thank you


Dear Roy:

Regarding your first question, yes, it sounded like you had something like a Watts Fill Valve and Backflow preventer valve, and from what you've said, it turned out that the link I sent you was exactly the one you had. Great!

Then if you look at your valve, the small lever on the top of the valve operates the bypass for the pressure reducer for the valve. So as you lift up on the lever, it opens the valve to street pressure from your water supply. So as you start to purge the water from one of your heating system loops, you lift the lever to increase the pressure to get stronger flow as you are purging that line. But let the lever down if the pressure in your system starts to go too high (however, if you isolate you boiler properly, as described below, then you won't have to worry about over-pressuring your system).

Now regarding your second question, the exact procedure for specifically which valves to open and close as you are purging your baseboard heating system of air will depend on how your specific system has been piped. But in general, to be safe, you will want to:

1) turn off your boiler and heating system.
2) make a note of which valves are open, and which are closed.
3) close the valves that allow you to isolate your boiler from the rest of the system (so that you don't get a pressure spike that causes your boiler's pressure relief valve to lift, as this weakens it).
4) connect a drain hose to safely drain hot water from the system.
5) follow your pipes around starting with the Watts fill valve, and open only those valves necessary to allow the fresh water that is coming into the system from the Watts valve to flow through the particular loop that you are trying to purge (that is, one loop at a time), and then out the drain hose that you have connected.
6) close all of the valves, and then repeat opening the valves needed to purge each loop.
7) disconnect the drain hose and close this valve.
8) return all of your valves to their original position (especially the valves that you used to isolate your boiler).

If it helps you to think of it this way, if you have turned off your boiler and heating system, and then properly closed the valves to isolate your boiler, and properly placed your drain hose to a drain or outdoors, then you can fairly safely purge your system without hurting your boiler or scalding yourself.

And when you go to start you system back up, you of course just need to be sure to put the valves back to their original positions.

Also, here are some additional suggestions that you might find helpful:

First, if you haven't had your annual inspection done yet on your boiler, then when the service technician comes out to do your inspection and service, you can ask them to show you exactly how to purge your specific system, when they can be physically there to point out which of your valves does exactly what.

The other thought is that you can call the toll-free phone number for the company that manufactures your Watts valve. I've talked to them before, and they are very helpful regarding how to purge, backflush and drain baseboard heating systems where their valves are installed. And here is their toll-free phone number in Vashon, Washington: 866-361-4782.

Hope this is helpful.


Thank you so much for your very helpful and thorough advice!

I now feel comfortable that I will be able to safely purge the air from the forced hot water heating loops. Thank you also for the advice to get on-site advice during the annual inspection of the boiler. The contact to the Watts manufacturer is also a great lead.

You have been so much help to me! Your site is clearly the best, and I have been searching a long time through the Internet, and thus have a strong basis for a comparison!

Take care

Gas Furnace or Boiler