150000 input, 120,000 output running nat.gas. I just removed a Lennox GWB8-100E-2 boiler that

is dated to 2013 for a friend. We gutted his house for a remodel and to ditch the knob and tube.

He will be switching over to forced air to get some AC as well, now that all the wall cavities

are exposed. The Lennox is rated at 100000 input and 83000 output. I know they used to

oversize boilers so I was doing some calculations (minus a true heat loss calculation) for my

existing demands. I have 2 existing zones. Zone 1 has 30' of baseboard fin, and zone 2 is

running 7 ancient cast iron radiators. House was built in 1840 - unsure when they retrofitted

a boiler for heat. Anyway, I calculated the fin at 550 BTU/ft and came out with 16,500 BTU on

that zone. I measured the dimensions, height and number of fins on the radiators to get the

square footage and calculated the BTU's at 180 degree average water temp (I know that's a

bit high for cast radiators, but that's what I'll be setting the Aquastat to.) For the Radiators I

came out with 62,500 BTU's, for a total of 79000 BTU's. The house has always been

comfortable with this setup, and given the "new" boiler is rated for 83000 BTU output, I would

think this would be sufficient for 99% of the time (minus the odd polar vortex). Am I wrong in

my thinking figuring it this way, rather than doing a true heat loss calculation?

Thanks in advance, hydronics ain't my bag..