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Million dollar question what size modcon gas boiler

Ktfinch2009
Ktfinch2009 Member Posts: 22
edited November 2018 in Gas Heating
Hi everyone I'm a homeowner in the market to replace my old cast iron boiler. I'm looking at going to a 95+ afue modcon boiler to take advantage of the rebates offered by massave. I live in Seekonk Massachusetts. My home is 1092 sqft ranch built in 1963. I've had blown insulation added to exterior walls, new ext. doors and replacment windows and vinyl siding with foam insulation. Attic is insulated with 8" of insulation. Basement is finished and heated with a minisplit heat pump. I currently use a 41 gallon indirect for dhw. I have 84 ft of baseboard on 1 zone and indirect on a second. I will be adding a 24x24 bedroom above a new attached garage at some point which will add another 576 sqft so 1668sqft total. I'm concerned about what size boiler to buy. We have 5 people total living in house which includes 3 teenage girls who love to take showers! House has 1.5 bath but will be adding 1 more full bath with addition. I dont want to oversize the boiler to avoid short cycling and not condensing enough. Based on some calculations I've done I would need a boiler that would mod down to about 17k btu. Does this sound correct? I will be increasing the indirect to a 50 gallon with the new install but was hoping for some help from the pros with sizing the boiler for my application. Looking into a Lochnivar knight boiler or Navien. Thanks in advance.

Comments

  • hot_rod
    hot_rod Member Posts: 16,278
    Did you do a room by room load calc? I'd guess you will be in the 35K load range for the total space. I'll bet the KHN 55 Lochinvar will be about right. Is the 41 gallon indirect you have now adequate?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Ktfinch2009
    Ktfinch2009 Member Posts: 22
    edited November 2018
    I didnt do a load calculation. I was being told around 150k btu due too hot water demand and upcoming addition. We really only have a full house every other weekend with my kids staying over. The boilermate has held up well with usage demands. Contractor figured once we have another shower going with addition we def will be using more hot water at once. I forgot to mention we have 2 woodburning fireplaces that help supplement the heat in basement and 1st floor but I'm sure they let out quite a bit of heat when not in use. So 35k would be sufficient? He recommended I up tank to 50 gallon. Thoughts? Navien nfb175 is 155k btu 17.5k btu min. Lochinvar is a 155k btu 15.5k btu min. Sounds way oversized then even with addition.
  • Zman
    Zman Member Posts: 7,328
    What is the size of the existing boiler?
    The Boilermate that is most common is only rated 40,000. Buying a boiler sized larger than the heat loss and DHW does not make much sense.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Ktfinch2009
    Ktfinch2009 Member Posts: 22
    I'll be swapping tank for a 50 gallon stainless h2o with new boiler. The existing boiler was installed used in a pinch by a friend as ours died 3 months into owning the house and we couldn't swing the cost of new. Current boiler is a crown 94k btu. Keeps the house toasty but does run quite a bit between hot water and cold temps in winter.
  • hot_rod
    hot_rod Member Posts: 16,278
    just a WAG, 1668 X 20 btu/ sq ft= 33,360 btu/hr. A 150K would be way oversized, if my numbers are close. But run some numbers to get a closer load number.

    A larger tank, say a 60 or 80 running 140F with a mixing valve should give you plenty of DHW if you have occasional high useage.

    Here is a fairly easy to use free ap for load calcs.

    https://www.slantfin.com/products/virtual-heat-loss-calculator/
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Ktfinch2009ZmanJean-David Beyer
  • Jamie Hall
    Jamie Hall Member Posts: 18,907
    Based on the leaky old (1780) sieve which I care for, your boiler estimated size is way over. The main place I care for is seven times as large, uninsulated, in a colder climate -- and uses only 400 K BTUh. Which, for your building, would work out to around 60 K, max. Do NOT oversize your new boiler. If you need more hot water, look into an indirect or a free standing unit.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    Ktfinch2009Jean-David Beyer
  • Gordy
    Gordy Member Posts: 9,514
    Agree with @hot rod also think about when the extra bath gets added. The girls will not be waiting in line anymore :) . Larger DHW storage capacity with higher storage temps as mentioned is a perfect work around to smaller boilers for lower heating demands. A 40 gallon conventional gas water heater is 38k input.
  • Ktfinch2009
    Ktfinch2009 Member Posts: 22
    edited November 2018
    The indirect I was quoted is an h2o 50 gallon stainless. Brochure is saying boiler output needs to be 133k btu. Will an 80k btu work with this tank?
  • Zman
    Zman Member Posts: 7,328
    edited November 2018
    If you need to transfer 133k, then you need the bigger boiler. If you use an 80k boiler with that tank you will get 80k.

    Don't size the boiler for DHW. Size it for heat and upsize the tank storage if you have high demand periods.

    I am sure by now you have noticed that having two threads about the same subject does not work very well. :s

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    Ktfinch2009
  • Ktfinch2009
    Ktfinch2009 Member Posts: 22
    If I upsized tank doesnt it require more btu to heat it? I posted in gas heat catagory first. Sorry 1st time using site lol Thanks
  • NY_Rob
    NY_Rob Member Posts: 1,370
    So.... who gets the million $$ at this point? :)
  • Ktfinch2009
    Ktfinch2009 Member Posts: 22
    Have to split it I guess lots of great info from you guys lol!
    NY_Rob
  • Zman
    Zman Member Posts: 7,328
    Think of it this way, heat transfer between the boiler water and domestic water is determined primarily the amount of surface area between the 2 sides as well as the temp difference between the 2 fluids. The larger tank has a larger heat exchanger. It would transfer more heat if you had a bigger boiler.

    If you use a smaller condensing boiler with a larger heat exchanger, you actually gain some efficiency because the boiler can run at lower temps to satisfy the same load. A big tank and exchanger with a small condensing boiler is a beautiful thing. :)
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    Jean-David Beyer
  • Ktfinch2009
    Ktfinch2009 Member Posts: 22
    Gotcha so a 60 or 80 gallon would work better with 80k BTU boiler. Thank you sir!
  • Zman
    Zman Member Posts: 7,328
    If you need the extra storage (it sounds like you do) you should absolutely go that way.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    Ktfinch2009