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thermostatic mix valves and check valve requirements

I'm going to use a Am series thermo mixing valve 3 way connected on a secondary loop to provide the reduced temp water for my radiant circuits. circuit is the standard  - tees off the primary, mixing valve, pump off "mix" flow to manifold and zone valves with return to TMV cold and return tee.  I see may references to using a check valve on the "cold" TMV input and some references to using one on both the hot and cold inputs.

What kind of flow is a check valve trying to control here?



  • Brad White
    Brad White Member Posts: 2,393
    edited November 2010
    Gravity Flow

    Using dual flow checks is to prevent gravity flow, but frankly, given small tubes and usually low elevation differences, I tend to find that over-kill. I dropped thermal U-trap of at least a foot should break that habit.

    I think the concern is the precision that radiant floors need to work well; a degree is a lot of heat given the overall scale of temperatures involved, compared to conventional heating systems with higher temperatures.

    Given that many radiant floors use constant or nearly constant circulation during the heating season (circulator running off of ODT with slowly varied water temperatures), I find the return-side flow check to be redundant.

    I am open to hearing what others have to say, always.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    Thermal trap...

    My house has radiant heat in the downstairs zone, and baseboard in the upstairs zone. The near-boiler piping is primary-secondary for the heating, but the indirect hot water heater is just tied across the supply and return at the boiler. The piping is mainly just above the boiler that is in the garage. That is where the closely spaced Ts, the air separator, the tie-in to the expansion tank are. Just below that pipe is a Taco 007-IFC going down to the floor level to enter the slab. The drop is about 5 feet. Now the IFC gizmo is a spring check valve, but I understand it is a pretty weak spring. On the return is no check valve at all. It seems to me that 5 feet drop and the weak IFC spring check prevents circulation when that circulator is not running. Similarly, the 5 foot rise on the return is enough there. It seems that for me, hot water does not like to go down, and cold water does not wish to rise. For my radiant, the water is not constant flow in this warm weather (over 41F outside at the moment), but when it gets really cold it can run 18 hours per day (it does have outdoor reset).

    Upstairs could be a problem, but it is not. Another Taco 007-IFC pushes the hot water up, probably about two feet. Max temperature of 135F. 3/4" tubing and 28 feet of 3/4" Slant/Fin. But about 75 feet of 1/2" horizontal tubing.

    I did notice in John Siegenthaler's book that he showed a Flow Check valve in both the supply and in the return of systems, and I asked him why two flow checks. He said (I hope I am getting this right) that with large diameter pipes you could get circulation in just the one pipe with the heat going up the center and the cold coming down the edge. He said the smaller the pipe, the less likely that was to happen.
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