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Hot Water Storage Tank vs Indirect Water Heater

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  • wklopf
    wklopf Member Posts: 44
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    I notice that for a 75,000 BTU boiler with cast iron baseboards, they specify the model 60 expansion tank. My system uses probably a bit over 50% of cast iron baseboards with most of the remainder being the old standard radiators. I'm guessing that my system should include the model 60 or equivalent.
  • HVACNUT
    HVACNUT Member Posts: 5,861
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    > @NY_Rob said:
    > ...He also told me that since this is a two story house it needs a very large expansion tank, so big that it sits on the floor. Any thoughts?
    >
    > Sounds like he bought a huge expansion tank by mistake for a previous job and is trying to offload it on his next unsuspecting customer :o
    >
    > You'll probably need a 4.4 gallon Exp Tank....
    >
    >
    > How come when I click on that chart I can meet pretty Asian and Russian women? That one girl has 2 #15's in her dress.
  • NY_Rob
    NY_Rob Member Posts: 1,370
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    wklopf said:

    I notice that for a 75,000 BTU boiler with cast iron baseboards, they specify the model 60 expansion tank. My system uses probably a bit over 50% of cast iron baseboards with most of the remainder being the old standard radiators. I'm guessing that my system should include the model 60 or equivalent.

    Really no downside to oversizing (if the #60 is oversized) the exp tank if you're in between tank sizes. When you think about it... the cast iron baseboard (Base Ray/GOV-Board) hold a lot more water than the same length of fin-tube, so they will require a larger exp tank.

  • NY_Rob
    NY_Rob Member Posts: 1,370
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    HVACNUT said:

    How come when I click on that chart I can meet pretty Asian and Russian women? That one girl has 2 #15's in her dress.

    If it was only that easy :p
    Beware the dark web!
  • wklopf
    wklopf Member Posts: 44
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    Well, after talking to six contractors, I found one who is recommending a boiler which fairly well matches my heat loss of about 58,000 BTU at four degrees outside. It is a Triangle Tube Challenger 85,000 BTU. But, it sets back only to 23,000 BTU. It appears to be a highly regarded boiler. My heat loss calculates to about 16,000 BTUs with an outdoor temp of 50 degrees. My radiators are mostly cast iron, capable of 94,500 BTUs with 180 degree water and are all on one zone, with thermostatic valves in each room. Would this system benefit from addition of a buffer tank?
  • NY_Rob
    NY_Rob Member Posts: 1,370
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    Can you ask the contractor to look in to the HTP UFT-80W which modulates down to 8K BTU's?

    If you go with a boiler like the UFT-80W and you have one big zone with all cast iron radiators you wouldn't benefit from a buffer tank.

    With that in mind, there's no boiler that can fire down low enough to stay on constantly once you hit warmer outdoor temps. You're fortunate that you have cast iron radiators which will be especially beneficial on warmer days.... your boiler may come on for half an hour to bring the rooms up to temp then it may not fire till a couple of hours later due to the latent heat in the cast iron.

    HVACNUT
  • flat_twin
    flat_twin Member Posts: 350
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    wklopf said:

    Well, after talking to six contractors, I found one who is recommending a boiler which fairly well matches my heat loss of about 58,000 BTU at four degrees outside. It is a Triangle Tube Challenger 85,000 BTU. But, it sets back only to 23,000 BTU. It appears to be a highly regarded boiler. My heat loss calculates to about 16,000 BTUs with an outdoor temp of 50 degrees. My radiators are mostly cast iron, capable of 94,500 BTUs with 180 degree water and are all on one zone, with thermostatic valves in each room. Would this system benefit from addition of a buffer tank?

    Our design day, heat loss and EDR numbers are pretty close. I've got 10 cast iron radiators, one zone fed with 2.5 and 1.25 pipes. Lots of mass. Minimum fire on our modcon is 20k btu. If I could do it again I'd select the next size smaller boiler with 14k min fire. The cycling I get in the shoulder seasons isn't bad but a little less would have given me even more efficiency.
    Our outdoor reset settings...
    max boiler temp 130 at -5 outside
    min boiler 87 temp at 60 outside





  • gennady
    gennady Member Posts: 839
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    It depends of what type dhw heat exchanger boiler incorporates. If it is plate heat exchanger with hight pressure head, then you cannot use storage tank. You will need really huge monster pump to move water trough heat exchanger.