Well Son of a gun...
It's been a while since I did the original heatload on the house and not sure where it is right now. I got 120,000 from the net of the boiler.
We used about 4.5tn last year (I think), including DHW
Calculations using your link...
4.5 tn x 25,000,000 BTU/tn x .80 AFUE = 90,000,000 BTUs
90,000,000 BTUs / 4279 HDD (using 0F as design temp) /24 hrs x 72 deltaT = 63,099 BTUs heat loss
or
3000 sqft (basement not heated except boiler standby loss last year) x 20 BTU/sqft = 60,000 BTUs heat loss
I'll be heating about 1/2 basement to full conditioned temp for office this year, so add 25% =
78,874 BTUs for 2022-2023
The good news:
Not all areas will be heated to the full 72F all the time.
Although the square footage we will be insulating will be 10-20%, it will complete the upstairs and seal the envelope. We are also considering adding blow in insulation for the attic since all the ceilings will now be done.
The key basement area walls will be very well insulated. 2 walls are below ground and just block, but they will be separated by walls and a door. The floor is yet undecided (ref my other posts about infloor heating, retrofitting, geothermal heat pump, etc)
So it looks like about 80k required, plus a little cushion for the coldest days. So add what? Maybe 10k?
That would mean 90k heat source requirement, right?
you need to tighten up the house
Working on it