sizing a new gas boiler and indirect water heater
I live in Boston, MA and am finally getting around to replacing a very old oil boiler (perhaps converted from coal and original to our 1926-built house). The house has about 1200 sf of heated space over two floors in a cube-like shape. One heating zone. Windows are original to the house, but storms were added. Someone blew insulation into the walls maybe twenty years ago. (I had a thorough energy audit last year and they could not recommend additional insulation.)
The existing old system works just fine, but the oil tank is ancient and having oil heat in the house will be a liability when and if we decide to sell.
I'd like to go with as efficient a system as possible for both bill savings and environmental reasons. The utilities here in MA offer deep discounts and rebates on Burnham Alpine boilers, which they show as available in "input" ratings of 80,000; 105,000; and 150,000. I've had three contractors in to get bids and they each have recommended the Alpine 105. One recommended the 150, but when I made some noises about over-sizing concerns and the product cycling on and off, he backed off to 105. The one who started with the 150 made his recommendation with having never left the basement; another walked around the house and counted the radiators; a third measured each radiator and counted the tubes. They are big, old cast-iron radiators from the 1920s. But they all ended up recommending the Alpine 105.
Should I still be concerned that the 105 might be oversized? If I instead instruct them to put in the Alpine 80, is there a risk of undersizing?
I'd hate to install a 95% AFUE boiler that actually performs like a much less efficient product.
Should I be paying someone to do a manual J?
I'm also planning on putting a SuperStor indirect water heater on as a second zone off the boiler. We've had a 40g conventional gas-fired water heater, which has been fine for my family of four. My instinct is to stick with a similarly-sized water heater, but should I consider going with a smaller Super Stor? Based on the Super stor spec sheet, the 20 gallon Super Stor has a first hour rating of 168 gallons of 115 degree F water, which sounds like plenty. Feels risky though to only have 20 gallons hot at a time. Next size up is 30 gallons with First hour rating of 212 gallons; and the 45 gallon tank (next size up after 30) delivers 292 gallons in the first hour, which sounds like an absurd amount of hot water for a 4 person household. However, some other posts have said the standby losses are so low on the Superstor that one shouldn't worry much about heating too much water.
Any help with these questions would be very much appreciated!