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Stuck in the middle of NCB-240E and NFC-175

After exhaustive work to determine heat loss, I've settled on 133k Btu's for my 2,600sf house, a heat load that includes basement, garage, and sunporch. Main living area is 1/2 that load, and the latter 3 areas are about 1/2 that load. Looking for opinions about whether I should go with the smaller boiler or the oversized boiler? NCB-240E is 18k-120k and the NFC-175 is 18k-175k. Emitters are low-ish temp... 140 degrees, and I'm shooting for efficiency that'll take full advantage of the condensing capacity. Is it worth going with the larger unit for those few heating degree days a year that the entire house will be heated at the outdoor design temp? Do I cut myself and my wallet a break on the coldest days, and stay out of the garage workshop (35k btu/h) on those days? I don't mind making a few compromises if it's logical to do so, but if I'm being shortsighted by "undersizing" the boiler I'd like to hear experienced opinions. Up front cost is not a huge deal (within reason)... I'm a craftsman and I want the job done right. Are there inefficiencies inherent in going with a 175k boiler if heating the majority of my house requires only 75k btu's... just to be guaranteed that I'll have the oomph in the system if I want every last inch nicely heated on the coldest days?

Comments

  • Alan (California Radiant) Forbes
    Alan (California Radiant) Forbes Member Posts: 3,933
    edited October 2020
    Fifty BTU's per square foot is a high number. You must have limited insulation, 12 foot ceilings, lots of single-glazed windows and live in Fargo.
    Do you really need to keep your garage at 68F when you want it heated? Lowering that temperature will bring your heat load down.
    If you have multiple zones in your living and sleeping areas, the chance of all the zones being on at the same time is less than 100%.
    How many people in the house? If you have lots of family or tenants and don't want to worry about ever being cold, go with the larger one. I don't know the boiler, but you may be able to de-rate it so it won't short-cycle.
    I like to live dangerously, so I'd probably go with the smaller one.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • jmatthewwest
    jmatthewwest Member Posts: 6
    The garage (with uninsulated slab) and basement aren't in the 2600 sf number. 5 zones. Figuring the workshop at 60 degrees. House is 1950's, currently electric heat, fairly well insulated and I've sealed well. Upgrading the insulation is an ongoing process. Just my family of 4.

    Thanks, all.
  • hot_rod
    hot_rod Member Posts: 21,841
    If you plan on more energy upgrades, and considering some fudge to the high side in load calcs, I too would go smaller.

    Where are you located? You could look up past weather data to see how often you are at or near design.
    An example for upstate, NY.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • jmatthewwest
    jmatthewwest Member Posts: 6
    I used John Siegenthaler's methodology from Modern Hydronic Heating to determine the heat loads... hence the over the top comment about "exhaustive"! Was like going back to school. I'm in Zone 6b in Western CT... local design temp is 9 degrees. We see that maybe 7 times a year? I tweaked the heat calc every which way to see how I could improve things. Being realistic, I think the 133k number is pretty accurate. I have a hard time overcoming the losses to glazing in my sunporch and the uninsulated slab in my garage. And realistically, I'm not going to improve on either of those in the near future given a number of factors. In 5 years? Maybe...

    I appreciate the opinions!
  • hot_rod
    hot_rod Member Posts: 21,841
    A little bit of window covering goes a long way to reducing a large glass load.
    Pretty much all load calcs are based on Manual J. As such, figure a 10% CYA factor :)
    But, as you discovered the data input is the key, infiltration being the hardest to "guesstimate.

    Check with your utility, some offer cheap or free blower door test. That can really be an eye opener in an older home. A few cans of spray foam can really change infiltration.

    www.dsireusa.org will lead you to any programs available for winterizing and upgrades.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • pecmsg
    pecmsg Member Posts: 4,687

    I used John Siegenthaler's methodology from Modern Hydronic Heating to determine the heat loads... hence the over the top comment about "exhaustive"! Was like going back to school. I'm in Zone 6b in Western CT... local design temp is 9 degrees. We see that maybe 7 times a year? I tweaked the heat calc every which way to see how I could improve things. Being realistic, I think the 133k number is pretty accurate. I have a hard time overcoming the losses to glazing in my sunporch and the uninsulated slab in my garage. And realistically, I'm not going to improve on either of those in the near future given a number of factors. In 5 years? Maybe...

    I appreciate the opinions!

    The problem with Mr Siegenthaler's methodology is no 2 houses are the same. In reality every system we install is a Custom System!

    Try a manual "J" Heat Load / Loss calculation. I think your actual #'s will come in much lower!

    https://www.loadcalc.net/