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Need help with mod/con boiler sizing!!

fiddlermd Member Posts: 59
Hello everyone, I'm very new to this whole thing so forgive my ignorance.
I recently bought a home in northern NJ and am renovating. part of the renovation is getting rid of old boiler and replacing with something a lot more efficient without a chimney.
So far I got far enough to know that i'd like a mod/con boiler with an indirect water heater tank.
Now i'm researching brands/types/etc.. and came to various discussions of how a mod/con can and should be sized properly... and that following generic calculators can lead to efficiency loss.. so with that said.. here are the specs:
the house is about 3700 sf. Built in 1972.
There will be a total of 7 heating zones. 2 of them will be radiant, the rest will be baseboards. radiant will be in the kitchen ( about 400 sf) and master bath.
House has 5 bedrooms (4 upstairs, 1 down) and 3 total baths

Insulation in most of the house will be new and older parts seem fairly well insulated.
What else do I need to explain to get some idea of how to size it properly?

Thank you all!


  • Snowmelt
    Snowmelt Member Posts: 1,418
    I just wrote on your other post, but you need a room by room heat loss for all them zones
  • RomanP
    RomanP Member Posts: 102
    As @Snowmelt posted above, room by room, then add it all up. If pretty well insulated, shouldn’t be over 120000 btus. Another key is figuring out your hot water demand for household. If the family is big and House has a lot of bathrooms, then required btu input for an indirect HWH might exceed your heating load and you might have to size your boiler by that.
  • fiddlermd
    fiddlermd Member Posts: 59
    the plumber is suggesting a 75 gallon indirect tank .. which should suffice I believe.. is there an easier way to ballpark the calculations? can someone suggest a good calculator tool that you don't need a masters in HVAC to understand? :)
  • Jamie Hall
    Jamie Hall Member Posts: 23,652
    In general, the various heat loss calculators -- SlantFin has a good one, and there are others -- don't take much more than good common sense -- and a good knowledge of the characteristics of the structure. It's well worth the time and effort to either get to know one and use it one's self, or hire someone who does and will. Ballpark estimates rarely work out well in the end.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • NY_Rob
    NY_Rob Member Posts: 1,370
    What is the length of the actual fin-tube element in each zone?
    It's common to have the baseboard covers run from wall to wall, but the length of the radiating element inside the enclosure is what counts.
  • Tinman
    Tinman Member Posts: 2,808
    edited August 2018
    In addition to the room by room load calculation, I would highly recommend you consider a buffer tank unless you go with a mod con that has a high water volume like Viessmann's CU3A or HTPs. Otherwise, your new boiler is likely to short cycle which is never good. I base that on your 400sf radiant zone. More than likely that zone will only require 8000 BTU/h or less.

    You have to start with the load calc. though. Once the boiler size is accurately determined, then your designer can compare the lowest turndown ratio to the smallest zone. There are calculators online for that as well. I know Lochinvar has one. It always helps if the installing contractor is also well versed in designing systems. It beats ballparking any day.
    Steve Minnich
  • Rich_49
    Rich_49 Member Posts: 2,766
    You most likely do not need a 75 gallon tank . You would be well served with a smaller tank with an elevated storage temp and a mixing valve .

    The 120,000 BTUh mentioned above is certainly going to be a number that your house will not be above requirement wise . My WAG is more like 75,000 or less .

    You would be well served to contact someone in North Jersey that has a real clue about things like this that can assist you without the frustration and back and forth that you'll surely encounter trying to figure this out yourself . My recommendation would be Ezzy T , a regular here , 1-347-436-6593 .
    You didn't get what you didn't pay for and it will never be what you thought it would .
    Langans Plumbing & Heating LLC
    Serving most of New Jersey, Eastern Pa .
    Consultation, Design & Installation anywhere
    Rich McGrath 732-581-3833