The house was built in 1925. It's a two stories row house with flat roof. Living area is approx. 1,350 sq ft (excl. basement).
First floor has two front big windows which are clear glass single-pane (which is an enclosed porch with 2 windows and a door separating from living room), 2 smaller back windows that are clear glass double-pane, and 1 small clear glass double-pane on the side. Since it's a row house, most part of the sides of the house share walls with houses next door. Second floor has 1 master bedroom with 3 front clear glass double-pane + storm windows. 1 smaller bedroom in the back with 2 clear glass double-pane + storm windows, 1 small bedroom in the middle with 1 clear glass double pane + storm window, and 1 small bedroom with skylight. Each room of the second floor has radiator but the hallway itself (total 4 radiators which are varies in sizes).
First floor only has 3 radiator (1 in the enclosed porch, 1 in living room, and 1 in dining room. No radiator in the kitchen).
I'd say overall the insulation is fair at best.
Front wall has aluminum siding, back wall and side walls are brick wall.
Basement is not insulated.
First floor ceiling is 8 ft 8 in high. Second floor ceiling is 8 ft 3 in high.
Maybe in the next couple of week I'll have to call someone to do this since winter is coming soon.
I thank you very much for your help in this matter, Dana. Have a nice day.
You are correct that only the front & back walls (including the front & back basement walls) & roof of a row house count toward the heat load.
But the wall construction and actual R values matter. Is this 3- wythe brick, with no cavity, 2x4 cavity wall with brick siding, or something else? Is the foundation cinder-block with filled cores, hollow cores, or poured concrete?
The biggest heat loss factor is likely to be your windows. Wood-sashed single-panes + storms have a U-factor of about 0.5 BTU/hr per square foot per degree, which is about the same as clear glass double-panes, which is also about the same as a solid wood exterior door (without a storm door). The temperature difference between a 70F indoors and your +5F 99% outside design temp is 65F, so add up the total square footage (the skylight too), and do the math:
U0.5 x 65F x (square footage)= xxxx BTU/hr.
If you give me the rest of the info we can cook up reasonable U-factors for the walls, basement walls, and ceiling.
By the time you add it all up I'd be truly shocked if your heat load is even 25,000 BTU/hr let alone 88,000 BTU/hr, but it's worth running the exercise anyway, since that, plus your radiation determines the max water temperature requirements out of the boiler, as well as the burner sizing.
Assuming it's all controlled as one zone, measure up the radiators and come up with the square feet of "equivalent direct radiation" using
this document as a guide.
If it turns out that the place can be heated with 120-130F water with you radiators it might make more sense to heat ht place with a tank type hot water heater than a boiler. There are trade-offs to the approach, but let's continue running the napkin-math to see what ballpark you are in.
It might also make more sense to heat & cool with ductless mini-split heat pumps (or a mini-ducted variable speed heat pump) depending on where the room-by-room and whole-house numbers fall. The cost can be comparable to, often less than a condensing boiler solution, but it also comes with high efficiency air-conditioning. There are trade offs to that approach too, but it's easier to size the equipment correctly to your probably fairly low loads than with boilers.