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Thermosiphoning in closed loop system

I lost track of this thread. Listen to Hot Rod (closely). Much wisdom there.

The idea of intentional thermosiphoning in a 3/4 inch line to the tune of 2 gpm seems, well, optimistic to me. If I were to make such a system work with a pump, I would get the smallest pump available and install a rat-race bypass around it. The net flow goes to the load, the balance goes back to the pump suction. A little parasitic maybe, but very controllable. Absent this, a pump with a VSD built-in is a good investment. Grundfos has such a line.

For balancing, I would use a Tour and Andersson type metered balancing valve, properly sized. Armstrong and others make these also. Practically linear and with four turns open to closed, you have 1440 degrees of turning range. And as Hot Rod said, a flow meter with that small range is essential.


  • Thermosiphoning not working

    I'm running a wood stove with hot water tubes as a heat source into a 50 gallon water tank (water heater). Its supposed to thermosiphon hot water into the tank. It triggers the T&P even with smaller wood fires, and seems to be very slow to heat the water in the tank. If it is thermosiphoning, its not adequate. I have drained the air from the system, so I dont think that is the problem.

    I'd like to resolve the problem by installing a small circulation pump (A Taco 005) on the cold side of the loop.

    Can I do this? The mfg of the woodstove suggests that I use a 2gpm or less circulator pump. Can I use a gate valve to reduce flow of a slightly larger pump- like a Taco 005, say?

  • Brad White_9
    Brad White_9 Member Posts: 2,440
    Some questions

    What size is the thermosiphon line? If too small you can see that there might not be enough capacity.

    The circulator and flow rates I would take to heart. If too much flow, thinking creosote build-up/condensation.

  • Hi Brad. Thanks for the quick reply!

    The lines are 3/4" copper outside, the stove, and 3/4" stainless tube inside the stove. Its what the manufacturer specs.

    So, can I use a gate valve to slow the flow of an 8gpm pump down to 2gpm? Or should I spend more for a lower capacity pump. (It just bugs my sense of value to pay MORE for a pump that has lower output, but I'll get over it.)

    Thanks again for your help.
  • hr
    hr Member Posts: 6,106
    It's tricky

    to set the flow as it needs to change according to the size of the fire.

    As Brad mentioned too much flow and not enough fire can lead to condensing issues and cresote build up.

    You may need to mess with it a bit to get it right.

    Really without a flowmeter it will be hard knowing your exact flow rate. I would try adjusting while watching the delta t. Temperature in vs temperature out.

    A flowsetter or gate valve would be easiest to adjust and read.

    I'd use a ball valve before a gate valve. Most of the inexpensive circuit setters are ball valves.

    hot rod

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