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Cold Sandwich Eliminator with Circulation Pump- Is this diagram correct?

Installing a mod con combi and want a cold sandwich eliminator, and hot water circulation pump. Problem is, that the electrical panel has no space for a breaker for an electric WH and existing circuits are inadequate. Therefore, plan to use the aquastat on the WH to circulate thru the boiler to maintain the temperature. I think the attached diagram is correct (note that isolation valves are not shown), but want to get some feedback. Also, I want some opinions on the thermostatic mixing valve. It is required or just nice to have? Will it work as shown on the diagram?

Comments

  • Ironman
    Ironman Member Posts: 6,555
    edited June 2018
    I don't see how flow will be induced in the mixed/hot line going to the fixtures unless the recirc pump is running.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • dhurst12
    dhurst12 Member Posts: 6
    Thanks Bob,
    But I'm not sure I understand your concern. The normal flow, if the circulator happened to be off, would follow the dotted lines shown below:



  • dhurst12
    dhurst12 Member Posts: 6
    I think I see a problem with my proposed system. If the aquastat is set at say 130F and the mixing valve is set at say 120F, will the mixing valve totally shut off the hot water flow when the system is trying to recirculate since cold water can't enter the loop.

    Do I need the mixing valve at all?
  • hot_rod
    hot_rod Member Posts: 15,019
    A new ASSE standard in the works that proposes instantaneous WHs can regulate their output as accurately as a thermostatic mix valve and should be able to meet that + or- temperature requirement.

    Also a thermostatic valve needs the hot supply to be about 25° warmer than the mixed output temperature to perform accurate temperature regulation.

    Looks a bit like a tank style water heater you are building :)
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Tom_133
    Tom_133 Member Posts: 772
    cmon Hot Rod, tell us more...
    Tom
    Montpelier Vt
  • hot_rod
    hot_rod Member Posts: 15,019
    edited July 2018
    Tom said:

    cmon Hot Rod, tell us more...

    Here is a good explanation of the ASSE 1082, 1084, & 1085 standard in development.


    https://www.workingpressuremag.com/water-heaters-used-for-temperature-control/
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Ed_32
    Ed_32 Member Posts: 32
    If you use a recirculating pump with a thermostatic mixing valve you wil need a thermostatic control to shut off the pump before the returning water reaches the mixing valve’s setpoint or it will cause problems with the valve due to it having the same temperature on the cold side (recirc return) and a hotter temp on the hot side. I use a Honeywell strap on aqua stat so I can adjust the shutoff temp, or a pump with a built in thermostat
  • hot_rod
    hot_rod Member Posts: 15,019
    A couple things can happen if a record is not properly piped to a thermostatic mix valve. The loop temperature can droop, due to lack of hw being added to the loop, or the loop temperature can drift up to the tank temperature if the valve cannot see a H&C cold differential to mix properly.

    The correct piping has a balance or globe valve to adjust the correct amount of recirc flow to offset the heat loss around the loop.

    A sensor at the return to the recirculation is a good idea, it may not guarantee the most distant fixture has adequate temperature on a long loop, like a large hotel for example. Better to run the pump and modulate flow, or pulse it to assure the entire loop is being maintained.

    I doubt you would ever see the return the same as the supply? Possibly a very short, well insulated loop? But then you may not need recirc? if there is little heat loss from the HW piping.

    I think the newest plumbing or energy codes require recirc on any distance of over 50 feet from HW source to distant fixture.

    Idronics 11 and 21 cover the topic of DHW recirculation.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream