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Using an indirect HW with smaller boiler

rlugg88 Member Posts: 3
Looking at specs for indirect tanks, they all have larger boiler sizes listed as being optimal. I am wondering what the result would be if it were used with a smaller boiler. In my case I am installing 2 62000(input) btu conventional boilers and wanted to use 30 gal indirects with them. Most of the tanks I have looked at show a much higher BTU spec. and while I understand I can use them with any boiler, I am wondering if there is a downside to using them with a smaller boiler. 


  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    I am just a homeowner.

    I have a mod-con and an indirect. Mine boiler happens to be the size recommended by the indirect manufacturer.

    As far as I can tell, the only problem with too small a boiler is that the recovery rate will be less. For mine, they specify that I should set the boiler temperature to 190F when satisfying a call from the indirect. I have a 36 gallon indirect and a 80,000 BTU/hour (input) boiler. My hot water needs are small. I have actually reduced the boiler temperature to 170F when answering a call from the indirect and notice no lack of hot water. YMMV, of course.
  • bob eck
    bob eck Member Posts: 930
    Indirect water heater

    Check with the manufacture of the indirect you are going to be using and what BTU input boiler and they can tell you the GPH you will get. Look at storing the water in the indirect at a higher temp like 160*F and use a temp valve to reduct temp going to faucets to 120*F. You can also give the indirect priority over heating zones to make sure you get the hot water needed. You can also use Delta 52655 1.5 GPM shower head or their 52657 2.0 / 1.5 GPM adjustable shower head.
  • Zman
    Zman Member Posts: 7,542
    Boiler will dictate

    When the tank has more exchange capacity than the boiler the boiler will dictate the performance.

    Care should be used when piping conventional boilers so the large exchanger will not cause the boiler to condense.

    The production of the system will be (boiler inputXefficiency) /  (500) / (delta T)=gpm
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Aaron_in_Maine
    Aaron_in_Maine Member Posts: 315

    Make sure you make the indirect priority. And do what bob said store your tank at 160 and install a mixing valve to get down to 120 and you won't have a problem.
    Aaron Hamilton Heating
    ahheating@ yahoo.com
  • gennady
    gennady Member Posts: 839
    boiler sizing

    In my opinion, indirect tank gives an excellent option to size boiler to heat load not to hot water. then tank sizing depends on domestic hot water requirements and nessesary recovery rate. I tend to increase tank size , not the boiler size if there higher hot water demand. It is especially actual when house is attached on both sides. Then heating load is much less than hot water load, and increasing boiler size will decrease overall system efficiency.
  • Ross_9
    Ross_9 Member Posts: 12
    similar question.

    I live near lake superior so winners are pretty harsh.

    When I built about 15 years ago I thought I would never have to use my chainsaw again with oil as my only heat source $.75 a gallon.

    But 9 years later when oil got to $3.00 a gallon and having wood on site o bought an AHS wood gun. Great in the winter months. But I have a weil McClain

    Oil boiler and a indirect water heater for summer months.

    My thought is if I bought a tankless electric heater to put on my primary loop I could use that to heat my domestic hw priority zoned for the indirect heater and occasional radiant stapled up in the main living area (we have had 42 degree raining 4th of July)

    Our electric rate is. $.07 a kilowatt so achieving to the department of energy this its a pretty good way to go.
  • SWEI
    SWEI Member Posts: 7,356
    have you considered

    A heat pump water heater?  Assuming you're using electricity as your energy source and that you're running it mostly in the warmer months, the math could be rather favorable, plus you'll get some free cooling of the space where the HPWH lives (might even be worth moving it to the laundry room or other semi-occupied space.
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