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Mixing valve do-over

lmnoq Member Posts: 3
I had some thermostatic mixing valves on my system I tried replacing last year and failed miserably. It's time for take-two and I want to make sure I don't blow it this time :)

The set-up I have is a main closed-loop exchanged to the open loop to my boiler. The installer went pump-crazy so I have a pump going off for each loop. I swapped my thermostatics out with Taco iseries only to realize that the water bypassed my floor loops completely because they don't have check valves like the thermostatics did. I put a swing check in but apparently, they don't work if you mount them upside down... Is there a better way to do this with a pump-heavy system? I was thinking of maybe moving the mixing valve before the pump and "pulling" through it instead of trying to force through and then pull return water. The whole thing doesn't seem to be the best design since wouldn't you need some sort of back-pressure on the return to even get cool water through the valve? I've got two floor loops - i haven't even touched the second one yet... Attaching pics of the whole setup

I pretty much just replaced the mixing valve and then added the swing check - the whole original design of this seems backwards to me. What prompted me to even mess with it in the first place was super-hot water was getting to my floors...

Once I figure out how to fix this mess i'd like to get rid of that other thermostatic as well..



  • Gordan
    Gordan Member Posts: 891
    That's not how you pipe a mixing valve

    You always pump from the mixed outlet of the valve toward the zone, and let the mixing valve decide what proportion of the flow is pulled from the system supply, and what portion from the zone return.
  • kcopp
    kcopp Member Posts: 4,092
    you can put....

    the largest pump on there and you still can't jamm BTU's into a mix valve. Pull out from the mixed side of the valve......as Gordon said.
  • lmnoq
    lmnoq Member Posts: 3

    That's exactly what I was looking for - I just wanted to be certain before I ripped everything apart and did it over. One last thing - when moving that valve pre-pump should I throw a spring check on the return line to make sure it doesn't pull from the main line? I'm torn on this - won't the pump be putting enough flow through to ensure that won't happen?
  • Gordan
    Gordan Member Posts: 891
    edited September 2012
    How you pipe the zone to the system is the same

    with or without the mixing valve. If you would normally use a flow check on the zone supply and return in order to prevent ghost flow when the zone circ is off, then you should do it here as well. Flow checks are only necessary between the heat source (system loop here) and the emitters. You won't need one on the bypass line that goes from the zone return to the "cool" port of the mixing valve, but the flow checks should either both be between the mixer/bypass and the system loop, or between the mixer/bypass and the zone supply/return. (Let's say that you did it wrong - using an integral check valve on the circ on the supply side, and a flow check between the bypass and the system loop on the return side; then ghost flow could occur from system supply, from mixer hot port to mixer cold port, through the bypass, and up the return line to the zone emitters.)

    Anyway, here's an application guide with many nice diagrams about how to properly pipe mixed circuits. Please take time to study it; never mind that it doesn't exactly match your system, the relevant details are the same.
  • Gordan
    Gordan Member Posts: 891
    The simple reason

    is that there's no way to make water flow from the zone return into the cold port of the mixer - the pressure differential goes the other way. So the mixer doesn't mix.
  • lmnoq
    lmnoq Member Posts: 3

    Thanks for the guide - I'll dig into it a bit (the link didn't work but i found it through google) I've been looking for exactly that but my searches have been yielding mostly sales-type stuff. I guess my concern on this is making sure I don't end up pulling hot from the system loop through the return line and back through the mixing valve instead of cool return water. (i know i'm not doing a great job of explaining this :) ) The installer did not put any flow checks in this system when it was built aside from the ones built into the pumps. I would assume that changes once I tee off and put the mixing valve in place though.
  • clammy
    clammy Member Posts: 2,829
    just wodering

    Whats that zone valve doing on the inlet side of your pump on the first photo and are you using it's end switch to make that circ .Not to bust your you know what but just wonder if you are a HO taking this project on if so contact a pro to help you out before your end up re piping it multi times and wasting money  just a suggestion because from the looks of it some one was lacking the know how and experence to make a fairly simple piping set up happen properly which only makes me futher think what else did he think he was doing correct .No insulate meant to you or your mech just a suggestion .I recently looked at a job that has been re piped 3 times by the same installer and it is still wrong and the sysytem still does not work properly and thats about 50 grand later when you don't have a clue after the first few attempts and still do not have a clue then it is time to get some one who has more then a clue and pay the piper or plumber .Peace and good luck clammy
    R.A. Calmbacher L.L.C. HVAC
    NJ Master HVAC Lic.
    Mahwah, NJ
    Specializing in steam and hydronic heating
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