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Thread: closed loop hot water

  1. #1
    DIY Junior Member
    Join Date
    Nov 2009

    Default closed loop hot water

    I have a oil fired boiler that I get domestic hot water and a hot water heat closed loop. To work on changing my pressure regulator,(a Watts 12-15 psi with a quick fill lever.) and my circulating pump; can I turn off my supply to pressure regulator and drain the closed loop while still allowing domestic hot water to operate. I do not know what the inside of the boiler is like; can the loop be drained of water, and the boiler be firing, with no damage to loop? Thanks.

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    New England


    Probably not. If it is setup with a primary/secondary loop, maybe. The boiler needs to be under pressure and the water lines full when it is firing or you'll get flash steam, and probably break something. If the primary loop can be isolated, then you could work on the secondary loop, but I'm not sure this is often setup to allow this.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    Network Engineer rmelo99's Avatar
    Join Date
    Jul 2006


    I would say no but it depends on where your fill valve is piped into your system.

    Usually they are near the expansion tank directly into the hydronic loop. I have a weird setup where I have a seperate tapping on my boiler where the auto-feed is plumbed seperate from the supply/return pipes.

    In theory if I had a shutoff valve before and after it I would be able to do what you are asking about.

    Not sure why your replacing yours but if you have one that is broken I've seen certain models that have union type fittings that allow you to replace the body/mechanism w/o any soldering.

    Anytime I work on my boiler I flip the little red switch to cut power. I don't want to risk it firing in a non-normal condition.


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