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Brazed plate heat exchanger vs indirect DHW tank

Dlauffenburger | Posted in General Questions on

I am trying to decide on installing (buying) a Indirect DHW tank or installing a Brazed Plate Heat exchanger onto an existing electric hot water heater.  I haven’t been able to find any information on the pro’s and/or cons comparing the two options.  The heat source for either will be a 125K Gas Boiler. 
  I was leaning towards adding a BPHE but after reading the specs on several sites it looks like with 125K input units only produce 1.3 GPM of hot water.  While the indirect tanks can produce substantially more water.
  A 40 gallon SS indirect tank will require the associated piping and a circulation pump.  A BPHE will require the piping, 2 circ pumps, and a aquastat which will end up being approx. 50% of the cost of an indirect setup while using an older tank and adding complexity.

  Am I misunderstanding something on the GPM capability of the BPHE?

  Can anyone provide me with the pros/cons of each setup.

Thanks for any guidance and information.
David

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Replies

  1. Yupster | | #1

    125000 btu/h can provide around 4 gpm of water at a 60°F temperature rise. So in order to use the full output of your boiler, you need a brazed plate heat exchanger sized to deliver the 125,000 btu/h. Assuming boiler supply water at 180°F and a 25°F temperature drop on that side and a 20°F temperature rise from 130°F to 150°F on the DHW side and a reasonable low pressure drop of around 9' head, a 8" x 4" 50 plate heat exchanger will do the trick for you. You can use a Taco 0015e3 ECM circ on medium speed on the DHW side and same circ on low speed on the boiler side, assuming you keep the piping nice and short both sides. You'll need 1-1/4" copper piping on the DHW side, 1" copper on the boiler side.
    4 gpm, all day long. Hope that helps!
    An even better plan might be to eliminate the storage tank altogether, do something like in the attached article. https://www.hpacmag.com/features/only-as-needed/

    1. Dlauffenburger | | #4

      Yupster,
      Thanks for the info. I am confused by the information though.
      “Assuming boiler supply water at 180°F and a 25°F temperature drop on that side and a 20°F temperature rise from 130°F to 150°F on the DHW side and a reasonable low pressure drop of around 9' head, a 8" x 4" 50 plate heat exchanger will do the trick for you.”
      From your statement it sounds like in order to heat my incoming well water from 52 F to 130 F with a 20F temperature rise it would have to cycle through the BPHX approximately 5 times. Given that the tank is 40 Gallons I assume it couldn’t keep up with a 3 GPM draw off the tank; or am I misunderstanding something.
      Thanks for any clarification.

      1. Yupster | | #9

        No no, you aren't heating your well water instantaneously with this application. You are maintaining a tank of hot water at 150°F constantly. If you wanted to get rid of the tank and essentially create your own tankless hot water heater, you would follow the procedure in the article I linked to. So if you want to keep the tank at 150°F, then as 50°F well water enters your tank and mixes with the 150°F water, cooling it down to 130°F (or whatever temperature you have it set to activate, no lower than that), the boiler kicks on and begins to circulate boiler water at 180°F through one side and your domestic hot water at 130°F circulates through the other side. The pumps and heat exchanger are sized to deliver the entire output of the boiler to the hot water tank. This would deliver enough heat to the tank to keep up with a 4 gpm hot water draw from the tank, constantly.

        Does that clear things up?

        Source: I design these systems daily :)

        1. Dlauffenburger | | #12

          Yupster,

          That clears it up. Thanks again for your help.

          David

  2. Expert Member
    Akos | | #2

    I've recently changed out an indirect for an plate HX plus pump. Our water is medium hard but over time a fair bit of scale built up on the indirect and recovery was getting pretty slow, combined with a smallish size, it just wasn't up for the job anymore.

    Although more complex the plate HX can be easily cleaned down the road, plus the cost of electric tank+HX+pump was less than a replacement indirect.

    There is no difference in the amount of hot water from a HX+tank vs Indirect. As long as the tank is sized for your largest draw, you should never run out.

    1. Dlauffenburger | | #5

      Akos,

      Thanks for the info. How old was your indirect tank? What size is your current HX?

      1. Expert Member
        Akos | | #8

        The indirect was around 10 years old. I'm using a 40 plate 3"x7" heat exchanger, with the larger tank I only need 30kbtu.

        If you have the space, I would go for a big tank with a decent sidearm. Much simpler and cheaper, plus no pumps or controls needed.

        Keep the elements in but turned low, if the house heat is running, almost all the hotwater will come from the boiler. When the boiler is off and in the summer it would just run off electric elements.

        There are a couple of install details you need to get right for a sidearm to work well, but it is pretty straight forward. Make sure you add in isolation valves so you can clean out lime scale from the sidearm down the road.

        EDIT:

        A sidearm+heat pump water heater might even be better, it would mostly need to run in the summer/shoulder season which would also help a bit with dehumidifying your basement.

        1. Yupster | | #10

          Just keep in mind the sidearm heater would actually lower the capacity of your existing tank, since the electric elements don't come on till the temperature is lower now. If the home heating isn't running, something that happens quite often, then nothing will be heating your water while you are drawing. So if one of the reasons you are connecting your boiler is to increase capacity, a sidearm heater depending on a thermosiphon while the home heating is running won't cut it. But if you are just looking to use a cheaper fuel, then it's definitely the cheapest option for integrating your boiler into your existing setup.

          1. Expert Member
            Akos | | #11

            The way I've set up sidearms with electric tank is replace the lower element with a low power one (700W) and keep the standard 3500W for the upper.

            The thermostat for the lower element is set low (can't remember the number but pretty much as low as it will go) and the upper about 15f bellow the boiler temperature.

            During heating season, with the boiler running, the lower element will turn on for short periods after a shower but doesn't run long as the sidearm catches up quick. During the winter and most of the shoulder season, the upper element never runs.

            If you go with a larger tank and a decent sidearm, about the only time the electric element would run is during a tub fill.

            In the summer with the boiler off, works as a standard electric tank.

  3. tommay | | #3

    Well you could hook up the heat exchanger to the electric tank for now. Just tap into the drain and T&P connections. Add a pump and aquastat at the higher T&P location, and wire it to a pump control as you normally would. Then in winter, the boiler will heat the water keeping the electricity off and when you turn off your boiler in the summer, you just have to run the electric. When the electric tank goes, you are all set up to either replace the electric or change over to a direct exchange, not indirect which has it's own heat exchanger inside, which may be a bit more expensive than a direct, but your boiler will still have to run year round.

    1. Dlauffenburger | | #6

      Tom,

      Thanks for the reply. That was what my original plan was but after pricing the pumps, etc I was looking at all of my options before spending the money.

  4. Expert Member
    Deleted | | #7

    Deleted

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