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Hot water temp stacking

cnmdesign Member Posts: 102
edited April 2018 in Domestic Hot Water
This will stir the pot :smile:

Thinking of putting in a 1/25hp circ pump between hot and cold on my indirect DHW tank, connecting it to a timer to come on an hour before we all get up the morning and run for a 1/2 hour to insure the whole 50 gal is heated to max temp. Does this sound feasible?

Combat Veteran owned, Final Salute LLC on FaceBook & Twitter.


    HVACNUT Member Posts: 4,593
    How many is "we all"?
    If the water heater is sized, piped and installed correctly, you shouldn't have trouble keeping up with denand.
    I don't see a benefit for what you want to do.
    You can however install a thermostatic mixing valve which allows you to set the tank temperature higher and temper down (usually 120 degrees) for the domestic hot.
    It will give you more hot water as you're not completely depleting the tank.
    cnmdesignkcoppdelta T
  • Jamie Hall
    Jamie Hall Member Posts: 18,934
    The heater should circulate naturally inside itself. The pump won't hurt anything -- but I doubt that it will help, either.

    That said, if "we all" is enough people, and they all take long showers... it may not be enough to keep up with demand, no matter what you do.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    cnmdesigndelta T
  • STEVEusaPA
    STEVEusaPA Member Posts: 5,689
    I think you would be better off with a flow switch that turns on the boiler when the shower starts so it can start replenishing the indirect, rather than wait for the indirect to call, then not keep up. Even a button to push before you turn on the shower-just for that bathroom. They incorporate them into some recirculation systems for DHW.
  • cnmdesign
    cnmdesign Member Posts: 102
    Thanks Everyone

    It does have a mixing valve. Just thought it would be better to start with all 50 gal at 160 or 70.
    Combat Veteran owned, Final Salute LLC on FaceBook & Twitter.
    JUGHNE Member Posts: 9,760
    I have a gravity flow recir line connected into the bottom of my 120 gal tank. No pump needed. Immediate hot water to the far end of the house. Yes, it is a heating loop but is throttled down to just maintain a trickle flow.
    This helps to get the entire tank hot.
  • SuperJ
    SuperJ Member Posts: 605
    edited April 2018
    You will be giving up exergy (usable energy) by blending the tank.
    What you may not realize is the water at the top of the tank is likely above setpoint, similar to how the water at the bottom is below. If you blend it you will warm up the cold, but will also cool of the top. So the net effect won't give you more hot water. Since the aquastat is likely in the lower half of the tank, better to just to raise the setpoint a bit if you want more tank capacity. Let the mixing valve mix it down.
    The indirect coil is generally in the bottom half, so it can most effectively heat the cold water as it comes out of the diptube. It's easier (more efficient) to move btu's from the coil into the cold makeup water, than into hot blended water. As the water is heated it rises, stacks up in the top where it is most useful.

    You want the hot water to stay at the top so it isn't diluted with cold inlet water as you shower. Not blended so it cools off as soon as new water enters the tank.
  • delta T
    delta T Member Posts: 870
    If you decide to do this, don't forget the check valves! I don't think this will change the amount of hot water you have in any usefull way. IF it does increase your total hot water availability, it will be by a couple minutes of shower time. I think you would be far better off raising the tank temp setting and using the mixing valve to temper.

    What size is your boiler? is the IDWH prioritized? What size and how long is the piping connecting it to the boiler? What pump do you have for the water heater (or the whole system if it is piped as a zone)? These factors will make a much bigger difference in your hot water delivery than will a blended tank vs. a stratified tank.
  • Gordy
    Gordy Member Posts: 9,514
    Hopefully you have a tempering valve installed. Like @SuperJ said. In the end you are mixing down. The average tank temp will be the same. Careful on that storage temp. Some indirects will fail premature when using storage temps 160 or higher. Usually calls it out in the manual.
  • Zman
    Zman Member Posts: 7,330
    It sounds like you have found a solution that is now desperately looking for a problem :)
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • hot_rod
    hot_rod Member Posts: 16,304
    A stratified or stacked tank will provide more useful energy than a blended tank.

    While both these tanks contain the same amount of heat energy, the stratified tank with it's higher temperature up top would have more useable energy available.

    It is not unusual to see a 20° or more stacking in an insulated tank. tall talks will stack better, also.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    delta TSTEVEusaPAcnmdesign