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Boiler cold water return - thermal shock

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by diyer, Feb 16, 2013.

  1. diyer

    diyer Member

    Joined:
    Mar 29, 2008
    Messages:
    13
    Loc:
    NH
    I am looking to install a Buderus G115WS oil boiler to replace my old Tarm. I am looking for a method to avoid the thermal shocks when the circulator starts up.

    The boiler is 100 ft outside my house in NH. There are times when everything will be cold (maybe below 32F) when the circulator in the house turns on. The boiler is always hot (160F+) and the circulators will pass 50 gallons of cold water into the 12 gallon boiler.

    The usual system bypass loop only slows down the flow into the boiler, but the inlet water will still be icy.

    Has anyone worked out a solution for something like this?

    I was thinking that I could run a 28VAC line out to the boiler to start a pump on the boiler short circuiting the outlet to the inlet mixing with the cold water when the inside circulators start.


    Thanks
    Carl


    coldshock.jpg

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  2. PassionForFire&Water

    PassionForFire&Water Minister of Fire

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    Jan 14, 2011
    Messages:
    582
    Loc:
    Billerica, MA
  3. ewdudley

    ewdudley Minister of Fire

    Joined:
    Nov 17, 2009
    Messages:
    1,697
    Loc:
    Cayuga County NY
    Our house is a converted gravity system with big pipes and cast iron rads that are over-sized now that the place has been tightened up. So when zones do activate they've normally been idle for a while and there's a big slug of cold water. With the boiler aquastat in the top of the oil boiler water jacket, by the time the boiler aquastat could see the cold water the boiler was shocked and chilled.

    So we added a boiler loop with injection, just as you've drawn, and as Marc has identified as a possibility. I added an aquastat on the boiler return to disable the load circulator whenever boiler return is below 160 degF (or 150 or 140 if you prefer). With a 1.25 inch boiler loop a typical 75 watt circulator will flow well over 15 gpm, which tempers the cold return water nicely.

    But when I did mine I couldn't find a thermostatic mixing valve for less than $250, and I had a salvage pump and aquastat laying around. For $125 the mixing valve Marc identified looks like a nice way to go.
  4. diyer

    diyer Member

    Joined:
    Mar 29, 2008
    Messages:
    13
    Loc:
    NH
    Marc and EWDudley,

    Thanks for the suggestions. It looks like I will need to add a pump near the boiler. Currently the circulation pump is in the house, 100 ft away. The $125 LK820 mixing valve instructions looks like you need the pump on the boiler side, otherwise you would just pump cold water against the cold inlet side. I can't see it working without the pump on the boiler side.

    If I need to add the pump, I'll feed the 28VAC from the house pump switch through an aquastat to a 28V solenoid to turn on the injection pump. This will only run the injection pump when the house is pumping, sort of a slave pump. This will be low cost, simple, and use standard pumps/aquastats/relays that I use elsewhere.

    The mixing pumps/electronic controls are nice, but if they fail, they can be costly and unavailable in an emergency.

    Carl

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