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heat load

Jim Pompetti
Jim Pompetti Member Posts: 552
Sorry dan, yesterday I poised a question , how to do a correct heat for hydronic heat ,as if I was a homeowner . The question was, the RIGHT way to do one . Either connected radiation or measure the house ,height of room ,windows and insulation.(This is for hot water not steam) I also check the input of the nozzle to see what is actually doing the job , And have found that the nozzle size is less then the load . My guess, is that the fudge factors  are way more then needed. Any help guys.


  • STEVEusaPA
    STEVEusaPA Member Posts: 5,688
    edited May 2012
    i think you should

    measure the entire house, all windows, doors, and other factors, etc. I wouldnt put too much faith in the nozzle, because it could be the wrong one. Also, who knows how the last boiler was sized...could be over or undersized.

    Slant fin has a free program for download


    or you could do it manually.

    Weil McClain also has some literature for sizing. I actually compared it to some of the heat losses I perfomed and it was pretty close. Dont have a link, but go their website.
  • Steamhead
    Steamhead Member Posts: 15,330
    I was wondering about that ;-)

    but didn't want to say anything at that moment............ Yes, definitely start from scratch. The Slant/Fin program is as good as any, and it's free. 
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    W-M heat loss vs. Slant/Fin program.

    I am a homeowner, not a contractor. But my contractor did not do a heat loss, and that offended me, so I did mine three different ways.

    1.) I had an oil burner with a 1/2 gallon per hour nozzle and it always provided enough heat, so my first heat loss calculation was 70,000 BTU/hour maximum.

    2.) Since I was getitng a W-M Ultra 3 to replace the oil burner with gas, I decided to calculate what I needed with the W-M worksheets. These:


    When I was done it came up with 36,250 BTU/hour or 41,450 BTU/hour depending on the heat loss downwards from the radiant slab at grade or not. At the time I did not know if that is counted or not with a radiant slab. I am still not sure, but it did not matter.

    3.) Then I found the Slant/Fin program and calculated it with that and got 29,234 BTU/hour. That confirmed to me that the smallest W-M Ultra 3 would be the one I got. The installing contractor said they recommended the larger boiler for a margin of safety. I have since figured out that he did not know anything about mod-cons even though he suggested using one.

    So for my house, it shows how the methods compare.


    1.) After the fact, I could have known method 1 was oversized because the boiler would run 45 seconds, shut off on upper limit for 120 to 150 seconds, and repeat until the thermostat was satisfied. I know I found the rapid cycling annoying because I could hear the boiler run through the wall between my attached garage where the boiler was (and is), and inside the house. Made sleeping difficult.

    2.) Method 2 turned out to be good enough because W-M do not make a small enough mod-con. It was enough that I thought about refusing the 105,000 BTU/hour recommended by the installing contractor.

    3.) I think this is the most accurate of the three calculation methods. It wanted to know a lot more about the house, like the number and size of the windows, more choices as to the insulation, wall construction, cold and not so cold walls, etc.

    My guess is that methods 2 & 3 are also a little too high as well. The cycling of the large radiant zone is acceptable, and the small baseboard is too fast, but much better now that I fiddled around with the controls (reset curve, upper limit on boiler firing rate when heating the baseboard zone, etc.). But basically, since it takes only 6500 BTU/hour when it is 0F and design temperature around here is 14F, there is not much that can be done. More baseboard might be good, so it could heat up the small zone faster (though it does not need to heat faster) and shut the zone off by the thermostat instead of the reset limit. That zone is the entire upstairs. Had I zoned each room separately up there, it would be an improvement to combine the zones.

    Should I put snow melting in my driveway and make it part of the upstairs zone? Just kidding.
  • icesailor
    icesailor Member Posts: 7,265
    edited May 2012
    And then there is DHW:

    And then there was domestic hot water. That 6500 BTU number is about half of what you get with a 4500 watt element in an electric hot water heater. Put that in a 6 gallon electric hot water heater and try to take a hot shower.

    The heating load "sees" design day a few times a year. Domestic potable hot water "sees" design day, every day unless you don't use hot water for a day.

    I know that if I ignore DHW heating loads, I will be doing so at my own peril.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    I know that if I ignore DHW heating loads, I will be doing so at my own peril.

    Yes, but you are a qualified professional and I am only a self-educated homeowner. When I did the heat loss on my house, I found that the smallest boiler I could get in the product line my contractor offered was about twice the size I needed to heat the house. And my hot water needs were modest. So I ignored its needs except to check with the requirements specified by the indirect manufacturer. I got a 36-gallon indirect, and I probably could have gotten away with an even smaller one.

    I doubt you would have done any different with my house (you might have picked a different boiler manufacturer, though. You might have installed a buffer tank for the tiny upstairs zone if you could have found a place for it).  Because you would have interviewed me about my consumption of hot water, examined my small front-loading washing machine (that I run cold most of the year and sometimes warm in winter; never hot), looked at the dishwasher, the low-flow shower head, and so on.
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