One zone is the garage, and one is spare rooms over the garage. Both of those are rarely heated over 40F. Is it crazy to size a new boiler based on measured heat usage, even though it's significantly lower than the design load implied by the size of the radiators? FWIW, the current boiler is 413 MBH DOE. I'm pretty sure that's too big in any case!
About 10 years ago the boiler had all but disappeared from the residential sector. Today, they are still considered the premium system and a new design of compact, high efficiency boilers are seeing a market resurgence. Boilers are much more expensive to purchase and install than forced air systems.
a steam boiler is in the design of the heat ... All new boilers smaller than 300,000 Btu/hr come with an efficiency rating called the ... require little maintenance ...
SIZING THE REPLACEMENT BOILER Proper sizing of the new boiler plays a major role in the success of your replacement job. It is very important to ensure that the new steam boiler has the same capacity in producing steam as the system you are connecting it to has in condensing the steam. This is the one fact that trips up many contractors.
8'x8' would be very tight. The inside dimensions of a concrete block building would be 6' 8"x 6'8". Do a scale drawing of the room allowing for all necessary servicing clearances for the boiler, circulators, etc and the swing of the door and if it looks like there is room, draw it to actual size with chalk on the driveway and see if it works!
Knowing how much water, or water as steam, the boiler can put out is good, but don't pick a pump size yet. ASHRAE recommends sizing your return pump for two to three times the boiler's evaporation rate—so the return unit can return condensate faster than the boiler can put it out.
STEP 3: SELECT THE BOILER The size of the replacement boiler will be based on the Total Heat Loss of the building calculated in Step 2. The boiler should be selected based on its AHRI NET Rating in BTU/Hr. For example, if the Total Heat Loss of the house is 85,000 BTU/ Hr. then the AHRI NET Rating of the replacement boiler must be at least 85,000.
The outdoor design temperature for my area is 12°F and the outside temperature was actually -9°F. Let's evaluate these answers: Because the boiler was not running continuously, it's not too small. When the boiler cycles on and off, it's producing more BTUs than the radiation can get rid of, which causes the boiler to cycle on high limit.
LxWxHx 4,5,or6 depending on insulation and or window/slider size of each room gives you BTU's of that room. Add them up you get boiler size in BTU's. Divide each room BTU's by 610 you get ft. of baseboard. It looks like your boiler might be a little undersized, this depends on if all 180 ft. Of base is actually filled with element and not dummy.