We are installing Weil McLain GV90+ boiler and I am trying to figure out the size we need - our plumber wants us to go with larger model (BTU 140,000; DOE 130,000; IBR 113,000) but we want to go with a smaller model (BTU 105,000, DOE 97,000, IBR 84,000). The boiler has 91.2% efficiency and we are on natural gas. The boiler will only be used for space heating - we are installing Runtals throughout the house (total BTU output 75,000). We will also have a separate AO Smith direct vent water heater, so our potable water is not connected to the boiler. The house is approximately 2,500 sq. ft. and we are doing some renovations on it - which included insulation, new energy-efficient windows and doors, insulated piping and duct work, etc. The old 30-year cast-iron boiler (used for heating only) was 120,000 BTU but the house had no insulation, etc. My contractor did not do heat loss calculation, but I did some preliminary calculation and came up with 77,000-78,000 for heat loss. We plan to finish our basement (approximately 600 sq.ft. - lets say it may need another 25,000 BTU) later, but will the smaller boiler have the capacity to heat our basement? Our boiler is also located in the basement. I tried to do my heat loss calculation based on the assumption that the basement is finished, not sure if this was correct.
Based on the above, would you go with a smaller or larger boiler?
Thank you everyone!