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Recirc line with mixing valve

fricard
fricard Member Posts: 18
This is a drawing of how I would pipe a recirc line with a mixing valve. I’ve done it this way for 20 years and never had a complaint. Today I was doing some research and it seems I’ve been doing it wrong. What I don’t understand is why it’s wrong. If I’m using an aquastat and its set lower than the mixing valve setting it should use the cooler water in the recirc line and once the aquastat reaches temperature the pump will shut off before the temperature has a chance to overshoot. The mixing valve has built in checks so once the pump shuts off there shouldn’t be anymore circulation. 
Everything I’m reading says the temperature will “creep” doing it this way. 

Comments

  • Larry Weingarten
    Larry Weingarten Member Posts: 2,284
    Hi, Does the mixing valve have a check in the cold supply? If not, what about putting a check in the cold line, a few feet downstream of where it ties into the mixing valve? That would insure that cold into the mixer stays cold and maintains a temperature differential, helping to avoid "creep". With a little luck, smarter folks will answer you :p

    Yours, Larry
  • hot_rod
    hot_rod Member Posts: 15,037
    it should not creep as long as you are turning the recirc off on temperature.
    It could creep if the pump was running 24/7, what happens after a period of time is you have the mixed return water temperature at the cold port of mixing valve, so warm on one side, hot on the other,the valve doesn’t see any cold until a faucet is opened, to be able to blend back to the desired temperature, that is how they creep. Mix valves need about a 25 degree difference between the hot supplied to H port and the mixed outlet. To work properly and regulate as designed.
    Temperature in a loop can also droop if no hot is being added to the loop. A balance valve allows you to add just enough flow to the loop to overcome pipe temperature drop, the rest should go back to the tank.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • fricard
    fricard Member Posts: 18
    so this will work?
  • hot_rod
    hot_rod Member Posts: 15,037
    I would add the check tailpieces, both hot and cold. They prevent crossing over in the valve if ever a pressure imbalance. 1017 valves suggest checks, 1070 POU valves must have them included to meet listing

    A check on the cold would prevent the recirc from pushing into the cold supply, that depends on the pump size a bit. Also a thermal expansion tank if you check the cold supply or have a backflow device on the main supply.

    If you have a long loop of uninsulated copper for example, the far end will still drop below what the aquastat will be seeing at the tank. It really depends on how the loop was piped, and what you or the customer expects. With today’s tiny ECM recirc pumps,running 24/7 should not cost a lot. The cost is more an issue if none of the piping is insulated. Then you basically have a Hydronic heating loop😁

    Energy codes like you to shut down the recirc, anti legionella concerns would suggest 24/7 operation, soooo
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • fricard
    fricard Member Posts: 18
    Thank you. In this case there is a check valve on the main. I will install an expansion tank. I just didn’t draw it. I always use a mixing valve with built in checks also.