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Community and Q&A

DHW Thermasiphon vs active

z6YwoAQwLx | Posted in Mechanicals on

I recently installed a water heating element in my englander 3500 wood burner. Because the 80 gal storage tank and the wood burner are both in the basement and about 12 ft apart I opted to go with an active circulation system. Nothing fancy just a Grundfos pump with an aquastat. The w shaped Heating element is mounted horizontally in the firebox and the 1/2 copper plumbing runs across the ceiling between the tank and heater. To my surprise this system thermalsiphons pretty effectively. My question is how will this effect the performance. the output pipe seems to be hotter with the start stop action of the active system before it inevitably starts siphoning. If I override the aquastat to cycle the pump it temporarily goes back to cycling. I’ve just put this system into service this week but so far it has supplied enough hot water for our family of five. I am also looking to put a solar collector on the roof this spring to supply hot water during the months we do not run the wood burner. I am still getting accustomed to this system so any insight from experienced users would be appreciated.


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  1. GBA Editor
    Martin Holladay | | #1

    1. Don't waste electricity running a pump.

    2. Be sure that you have at least two P/T relief valves, because these systems can explode.

    3. Built a sturdy wooden stand to raise your storage tank close to the ceiling.

    4. Slope your pipes between the wood stove and the storage tank to encourage thermosyphoning.

    5. If your storage tank has an internal heat-exchange coil, you can run your solar collector fluid through the heat exchanger.

  2. z6YwoAQwLx | | #2

    Thanks Martin,
    I do have a P/T valve on each "end" of the loop, as well as a watts automatic air bleeder on each leg of the upper transfer pipes. This morning the loop was back to cycling. I think what is happening is that the system thermosiphons until it builds enough hot water to lower the stratification level in the tank slowing or stopping the thermosiphon until the aquastat hits 150 and the cycling resumes. I'm new to this but that is what makes sense to me based on what I'm seeing. My tank is a 80 gal. Marathon electric unit and I'm pretty sure it doesn't have any kind of auxiliary coil. I like the idea of raising the tank and not running the pump at all. Maybe I should run it as is an get a little more familiar with it and then rework several things at once like raising the tank, plumbing in the solar collector that I still have to build, and possibly replacing the tank with a coil equipped unit. I'm also kind of regretting installing the heating element horizontally, I just didn't think I would get a good thermalsiphon.

  3. kevin_in_denver | | #3

    Thermosiphons work best with 1" or larger pipe. And avoid thermal "traps"

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