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Boiler sizing on a gas conversion
I'm in the process of converting two systems from oil to gas (hot water baseboard) in my two-family. I've had a few different quotes and have decided to move forward with a someone who has been in the business for a while and has good reputation. Planning on installing a Burnham ES2 (105 btu) with SuperStor indirect hot water for each unit. I'm happy with the equipment choice and he'll also be putting in a liner for the chimney. Each apartment is about 1100-1200 square feet. The house is 100 years old with blown-in cellulose, newer windows, and no insulation in the flat roof.
My wife and I live on the first floor and the place is pretty cozy. It's a one zone system, but we will likely be adding an additional piece of baseboard in a bedroom that currently has no heat.
The second floor unit has the problem of having no insulation in the roof and the back half of the apartment is under-radiated(?). It is also currently one zone of baseboard, but we are adding baseboard in 3 rooms.
My question is about the sizing. My heating guy did not do a heat calc but had me measure the rooms and baseboards and had me send this information along to someone who ran the heat calc off those numbers. I've attached a page to this post (Zone 1 is actually apartment 1. Zone 2 is actually apartment 2). The heat guy says the 105s are the way to go and that a smaller boiler (70btus) would be too small for heat and hot water. In looking at the heat calc, my untrained eye wants to say the 105s would be oversized for, at the very least, the first floor apartment. All three quotes I got suggested 105s. I'm assuming I must be missing something. I've read this board enough to know that heat calculation as done isn't the most precise. I just want to know that the boilers will be running as efficiently as they should. Any opinions? Thanks for taking the time.