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Preheating dhw with a pressurized storage tank

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by Ridgewalker, Sep 24, 2013.

  1. Ridgewalker

    Ridgewalker New Member

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    I'm replacing my 8 year old unpressurized storage with a pressurized tank. In the old system I preheated the dhw with a hx in the tank. How can I preheat the dhw using a pressurized tank? My boiler and storage are in the workshop 100 feet from the buildings they heat. So it is a one way insulated pipe to each building's hot water tank.
    Really have appreciated the info on this site over the years!

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  2. goosegunner

    goosegunner Minister of Fire

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    That is the same dilemma that I have. I have not hooked up my DWH yet. From things I have read here it seems as though a buffer tank near the DHW would be the best way to keep the system ready for heating the Domestic side.

    Search for DHW and buffer tank.

    gg
  3. ewdudley

    ewdudley Minister of Fire

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    Storage that is remote from DHW distribution points is not ideal for DHW purposes. I use a buffer as goosegunner suggests and it works pretty well. In effect you have a small storage tank close to the DHW distribution point that is filled in 'batch mode' and the DHW load draws from it.

    If I had it to do again I think I might put the DHW heat exchanger in the top of storage and then draw DHW all the way from the storage area with a separate DHW line pair. It would probably require a non-ferrous circulator to keep hot water available at the DHW distribution point. In the OP's situation where there are multiple DHW distribution points remote from storage it might work well to have a single DHW HX with anti-scalding mixer in the storage area and then use multiple non-ferrous circulators to keep DHW available at the DHW distribution points.

    [Edit:]
    Reading the OP more closely the above may not be very helpful.

    To answer the question, a DHW HX can be installed in the top of storage and pre-heated water can be drawn from it one-way to the hot water tanks, same as it is with the non-pressurized storage. If DHW use is in dribs and drabs then each draw will leave hot water in the lines that will cool somewhat before the next use, but that's just the cost of doing business in that situation and it should work quite well nonetheless.
    Last edited: Sep 25, 2013
  4. hobbyheater

    hobbyheater Minister of Fire

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    When I incorporated storage into our heating system, it required building a new boiler room at one end of the house. From the boiler room where there is a coil in the storage to where the electric hot water tank is located in the middle of the house is a run of thirty feet. Just before the electric hot water tank, I have installed an anti-scald valve set at 160 F. The electric tank stays at a pretty consistent 130 to 140 F. The elements in the electric hot water tank are set to come on if the water should drop below 120 F. Storage is maintained between 150/195 F.
  5. woodsmaster

    woodsmaster Minister of Fire

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    Why only preheat it ? Put an hx at your hot water tanks with a mixing valve on the tank. I haven't used any elec from my water heater for a couple of years now. Goosegunners suggestion of a buffer tank is good to keep storage from mixing, but involves more space, money, and is not a necessity.
  6. woodsmaster

    woodsmaster Minister of Fire

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    A coil in the storage tank would be ideal if the storage is close to your faucets
  7. hobbyheater

    hobbyheater Minister of Fire

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    Preheat? With storage in operating range, the elements in the electric tank never come on. The mixing valve between storage and electric tank prevents the the need for resetting the overheat breaker on the electric tank where the temps from storage can easily exceed the 170 F limit for that breaker. There is a mixing valve on the outside of the electric tank set at 120F for water at the taps.

    Originally our DHW came directly from the coil through a mixing valve but our home is at the top of the town water system and has very poor water pressure.
    The electric tank was used as a buffer to cut down fluctuating water temperatures at the taps because balancing type faucets would not work properly. The electric tank also now serves as back up should storage temperatures drop below 120 F.

    If storage is 100 feet from the house and you only want to produce DHW water during the heating season then a HX in the house would really make sense.
    To produce your DHW from storage year round, in periods when there is no demand for heat, which will create a bigger heat loss?
    One 3/4" well insulated line to the house through a domestic buffer tank at the house for DHW?
    One 1" to 1 1/2" of the same distance to and from the storage to hx at the house for DHW?

    IMGP4211.JPG
    From the top of town we have a nice view ! But poor water and flow pressure .
  8. BoilerMan

    BoilerMan Minister of Fire

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    You just need a coil to go into the tank through a fitting like a domestic coil on an oil boiler, but threaded into the tank at the highest location available/possible. With hot water useage that far away, how did it perform before? A pressureized tank should act no different than the old non-pressureized.

    If I had multiple buildings that were fed from one central water heater location (boiler barn) I'd consider the cost of small electric tanks at each building. Located as close to the point of use as you could. As long as they were sized to store a bit more water than is in the underground lines (from boiler storage) then the electric consumption would be very minimal as it would only need to overcome standby losses. Losses from a small electric water heater are small as there is little surface area and no flue.

    Also as mentioned before, anti scald valve is a must I'd put one at each location after the water heaters. This way the electric WH gets really hot water which would overcome the cold water that just got pushed into it by a draw from a long underground line. If I understand it correctly this is what Allan has been doing for years.

    TS
  9. Ridgewalker

    Ridgewalker New Member

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    Thanks all for the feedback. Two of the electric dhw heaters are small, 10 gallons each. They only put out dribs and drabs for two kitchens. The 40 gallon tank serves two bathrooms. Nice idea doing a buffer tank with hx next to the dhw tank, and maybe I'll do that in the future. For now think I'll go with hx at the storage tank, and an anti scald valve after the 40 gallon tank. Thanks for the ideas.
    Rob
  10. ewdudley

    ewdudley Minister of Fire

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    We have a small electric hot water heater under the kitchen sink. I think I made a mistake by feeding it from the hot water line. We use so little at a time that nearly all the water is heated twice. It leaves the main heater and sits in the pipe and cools, then enters the little heater and is re-heated for use in the kitchen. Need to re-plumb it but it's just one of those forty-five minute jobs that never gets done.
  11. maple1

    maple1 Minister of Fire

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    I'm surrounded by those, everywhere I look....
    woodsmaster likes this.

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