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boiler choice for small house
I want to replace my 34 year old gas boiler and take advantage of good rebates currently available. My house has about 1000 sq feet of living space to heat. It is a split entry, living area on 2nd story, lower story is unheated slab for laundry, storage, garage.
I did my own two heat loss calculations. One followed steps specified by Chester on Feb 9 on this forum, using two months of utility bills, heating degree days for my town found online, and outdoor design temp for my locale (also found online). I assumed that my current boiler is 80% efficient. The other used the calculator available at the US Boiler web site. I estimated that my 34-year-old house is neither extremely drafty nor tight. My results were: 26,460 MBH and 29,869 MBH respectively. These are in line with the "rule of thumb" I saw somewhere on this forum of 30 MBH per 1000 sq feet.
I got proposals from three contractors for a total of 5 different boilers, each also with an indirect DHW. Here are the boilers and their MBH ratings:
1. modcon Burnham Alpine 080W : input 16-80 MBH
2. modcon Viessmann 100 : 37-91 MBH
3. modcon Buderus GB 142/24: output 22.7 - 75.2 MBH
4. conventional (not modcon) Weil-MCLain GV90-3: Input 70 MBH, Heating Cap 65 MBH, Net AHRI 56 MBH
5. Bosch Greenstar Zwb28 (I don't know if this is similar to the first 3 or not. Online specs say it has a modulating fan...): Input 101 MBH, Heat capacity 91 MBH, Net Rating Water 79 MBH
I don't know how to compare the specs for the Weil-McLain and Greenstar. This is what the web sites showed.
I am most interested in one of the first 3. BUT - I am concerned that even though each is the smallest of its model series, they may be oversized. That is, should I look for a boiler where the low number is lower, e.g. around 10 MBH? How do I compare the numbers when one manufacturer gives the input, another gives an output? I don't know which it is for the Viessmann numbers. I am considering putting an addition of approx 400 sq feet to the living space (and an unheated garage space underneath) but that is down the road.
Should I remove any of these choices due to size alone? Can anyone recommend a more appropriate boiler, esp. regarding size?
Thanks for any ideas! I know next to nothing about plumbing except for my recent research online.