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Thread: Newbie Forced-Hot Water Help in Frozen NH

  1. #1

    Exclamation Newbie Forced-Hot Water Help in Frozen NH


    Like 200,000+ of our neighbors we lost power last Thursday. Fearing freezing of the heating system (4-zone, forced-hot water - baseboard radiators, McClain boiler, expansion tank, well-water supply) I opened the spigots on each of the zones and let them drain until the flow stopped. I only gathered about 2 gallons of water (clear - no apparent additives).

    I know nothing about the system other than water in pipes freezes. The interior of the house is about 45 degrees right now and I expect that to remain relatively stable until it starts snowing Tuesday or Wednesday.

    My question is: what will happen when the power comes back on? It appears there is a cold-water feed into the boiler, with a fancy valve (shut-offs on either side). I've read about bleeding the air from the system, but no clue as to where I would do that. I've also read it is difficult to "re-charge" forced-hot water systems - but again, no idea what that means.

    Any assistance would be greatly appreciated.

    Thank you,

    - Casey

  2. #2
    Master Plumber Redwood's Avatar
    Join Date
    Dec 2007


    You need to refill the system and purge the air from it.
    Can you post a picture of the boiler and valves taken from a distance where we can see how they all are plumbed together?

  3. #3


    Thanks for the reply. I don't have a way to take a picture and post at this point as I am currently out of state and the house is empty and still without power.

    The only thing I can tell you about the plumbing is that below each of the 4 spigots is a blade-type shut-off valve.

    Can you tell me what will happen when the boiler fires up if the system hasn't been refilled or the air bled out?

  4. #4
    Network Engineer rmelo99's Avatar
    Join Date
    Jul 2006


    2 gallons doesn't seem like a lot of water to be drained from a boiler's zones.

    I think you to try and get more water out of the pipes and leave the "spigots" open so if you do freeze the expanding water will have somewhere to go rather than burst the pipe. If you do this then you can't run the boiler until you close them and refill the system and purge air.

    If you haven't turned off the main water supply valve then your boiler might have already refilled some of that water b/c most have auto-fills.

    You don't want to run your boiler without water in it. Not sure if you also drained from the spoigot on the boiler near the floor, that will get the water out of the boiler too, not just the zones. Some boilers have low-water cutoffs to prevent them from running w/o water but that isn't standard esp if this is several years old.

    As for refilling, it isn't bad at all. Your biggest problem will be getting all the air out. This just involves some purging, maybe bleeding the radiators and doing it a few times to get the air out.

    This isn't the "best" winterization, but it's better than nothing I guess.

    Just curious, what about the rest of the house plumbing?

  5. #5



    Thanks for the reply.

    RE: Boiler drain
    I haven't touched the boiler drain, so I'm guessing it is still full. Since there's no electricity, the water pump is not running so no water is flowing.

    RE: Refilling
    Won't the pipes refill by themselves since the boiler is connected to the water supply (once the power comes back on)?

    RE: Purging the system of Air
    We have baseboard units (the ones with the fins). Is there a bleeder valve somewhere on the ends?

    RE: Rest of plumbing
    The main house water - sinks, toilets, etc. are inside the house - the lowest temp its been inside has been 39F (when it was 18F outside). My biggest concern is a room over the garage (no bath) but a 15-20" length of baseboard.

  6. #6
    Master Plumber nhmaster's Avatar
    Join Date
    Jul 2008
    S. Maine


    Here's the real bad news. All you did was take the pressure off the system. The piping is all still full of water. With luck it didn't freeze and break, but it got pretty cold up here Friday and Saturday night. You now need to shut the purge valves off and open the fill valve. Fire up the system and see if it circulates. If not, you have either a small amound of air in the system or frozen pipes.


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