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High temperature water heater with recirculation.

Alan (California Radiant) Forbes
Alan (California Radiant) Forbes Member Posts: 4,002
in THE MAIN WALL
I'm sure it's come up before, but I haven't found anything. When running your DHW at high temperatures with a mixing valve at the outlet and a recirculation system, how do you get past the inefficiency of superheating your hot water, diluting it with cold water and then superheating it up again when it returns via the recirculation system?
8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

Two btu per sq ft for degree difference for a slab

Comments

  • Jack
    Jack Member Posts: 1,047
    Not sure you do, Alan. Depending upon the loads, I try to break up the system so the high temp is only for the special load. The rest of the system is lower temp and safer for the public. What is the application/lay-out?
  • Alan (California Radiant) Forbes
    Alan (California Radiant) Forbes Member Posts: 4,002
    Single family home; extending the capacity of the water heater and elimination of bacteria.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • JUGHNE
    JUGHNE Member Posts: 11,060
    I've only done recir pumps for commercial setups. With the recir water connected to the cold inlet of TMV, that returning water enters the valve as "cold" water and requires less hot in the mix to maintain the temp. I have always added an aqua stat at the return to shut down the pump to prevent overheating of the mixed supply.
    Added expense of check valves and expansion tank included.
  • hot_rod
    hot_rod Member Posts: 22,120
    edited June 2016
    Another common method used in Europe is to elevate the HW for a one hour period every 24 hours. Usually a motorized mixing valve is used to allow that high temperature mode, daily.
    The HW source also need to go into a high temperature mode.

    That would solve the anti bacteria question but it would not add any extra capacity as you indicated needing.

    For public buildings a POU point of use mixing valve is required at every DHW outlet to protect when the system is in high temperature, anti legionella mode.

    You might just run 140F, and mix down via thermostatic shower valves and under the counter mini mixers.

    Certainly want to well insulate the DHW and recirc lines.
    Here is a cheat sheet graph to compare heat loss differences between bare copper and 1/2" foam insulated.

    Idronics 11 has some good info on DHW recirc






    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Alan (California Radiant) Forbes
    Alan (California Radiant) Forbes Member Posts: 4,002
    Thanks, guys. It looks like I can either have either one or the other, but not both unless I install mixing valves at each fixture, which in this case is not practical.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • Firecontrol933
    Firecontrol933 Member Posts: 73
    edited June 2016
    The recirc loop will loose X amount of heat/energy no mater the source temperature ahead of the mixing valve. The amount of energy loss is dependent on the temperature of the water supply being mixed by the valve, the insulation value on the loop and the temperature of the space the pipe travels through. By returning the loop to the cold inlet of the mixing valve nothing changes.

    The cold to the mixing valve needs to be the same cold feed to the tank, because in some instances the mixing valve will be wide open to the hot side only.
  • Jamie Hall
    Jamie Hall Member Posts: 23,276
    You can do it with heat exchangers. It's not worth it.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • JUGHNE
    JUGHNE Member Posts: 11,060
    Hot Rod, what will the electronic mixing valve do in the event of a power failure?
  • hot_rod
    hot_rod Member Posts: 22,120
    JUGHNE said:

    Hot Rod, what will the electronic mixing valve do in the event of a power failure?

    Excellent question, I'll find out.

    It may be a thermostatic valve with an operator that puts it into high temperature mode. In the event of a power failure it would go to the thermostatically mixed temperature?

    I know there are some motorized mixers out there, not sure what standard they are listed to? I would guess they have an ASME category, if so a fail safe provision.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • JUGHNE
    JUGHNE Member Posts: 11,060
    So maybe the electronic part is just for the "bug killer" mode and otherwise a standard mixer with no power needed?

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