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Thread: Use a Thermostatic Mixing Valve in a Solar Hot Water heat dissipator

  1. #1
    DIY Junior Member Herman Hartzler's Avatar
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    Jun 2010

    Default Use a Thermostatic Mixing Valve in a Solar Hot Water heat dissipator

    This question only makes sense in the context of solar hot water heating. But it is also a plumbing question.

    One solar manufacturer sells a heat dissipator unit whose purpose is in the summer to get rid of excess heat produced by an evacuated tube solar hot water panel used with closed loop glycol. This heat dissipator can be plumbed as a side track of sorts. You can use a 3-way solenoid operated valve to divert the water to the dissipator. The controller can take care of that. That's probably the standard way.

    But now we come to my question. The literature on this dissipator says, "Alternatively a thermostatic mixing valve/tempering valve set to a suitable temperature can be used, providing mechanic, automatic regulation of flow through the heat dissipator." So how would that be plumbed? When the TMV is used in a typical DHW application, you would have hot (H) in and cold (C) in and warm (W) out. If I see it right, for my solar dissipator, I would need to plumb the TMV in reverse... so that the main incoming line would connect to W, the normal outgoing line would connect to H and the diversion line going to the dissipator would connect to C. So the water would be flowing in the opposite direction through the valve from the usual. Is that correct? If not what is correct? And why?

  2. #2
    Forum Admin, Expert Plumber Terry's Avatar
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    Aug 2004
    Bothell, Washington
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    Last edited by Terry; 06-18-2010 at 02:11 PM.

  3. #3
    Moderator & Master Plumber hj's Avatar
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    Aug 2004
    Cave Creek, Arizona


    Unless there is actual flow through the valve and not just circulation in a closed loop, a thermostatic valve is not only useless, but can be damaged. Many thermostatic valves have integral check valves to PREVENT what you are trying to do. If the "mixed" water was too hot, it would flow into the "cold" port, and if too cool through the hot one, but there would be NO modualation of the temperature by mixing "hot" and "cold" flows UNLESS water was being "dumped" somewhere so "new" water could flow into and through the valve.

  4. #4
    Aspiring Old Fart, EE, computer & networking geek Mikey's Avatar
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    Nov 2005
    Central Florida


    I've got the too-much-heat problem, but was wondering if it'd be possible to build a sunshade/reflector for the collector -- like a big Venitian blind -- that could be adjusted based on the temperature of the water coming in to the collector.

  5. #5
    DIY Senior Member chris8796's Avatar
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    Jan 2005


    I know its late, but I would think the mxing valve would be plumbed before the solar panel. W would connect to the panel. H would be the return from the tank. C would be feed from the line coming off the panel. Tee into the line coming off the panel, one connection goes to tank the other goes to the dissapator and connects to the mixing valve. A mixing valve is a sliding valve with a thermostatic element. When the return line is hot (tank saturated with heat), the sliding valve responds by increasing the portion from C. In this case, pulling hot water through the dissipator on the way to the mixing valve. If the return line below the mixing temp, the valve remains closed and no water runs through the dissipator. It just seems odd, since your C water originates with the hottest water (which cools somewhat going through the dissipator). The trick is the valve doesn't care about the temp of H and C, it justs adjusts the ratio to more C as the mixed water temp increases.

    The pump should be before the T, so both legs have equal pressure.
    Last edited by chris8796; 07-28-2010 at 09:10 AM.

  6. #6
    Extreme DIY Homeowner Scuba_Dave's Avatar
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    Jan 2009
    South of Boston, MA


    I have several solar heat dissipators
    19,000g swimming pool, 450g hot tub, 300g ho tub
    DIY Handyman (not 4 hire)
    I have enough to do to my own house

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