To calculate the radiator output we also need the number of tubes per section, and the number of sections. You've only provided height & width of the individual sections (?). The model number or wattage of the toe-kick and bathroom wall heaters may matter too. These are all important factors for sizing the boiler correctly, whether cast iron or modulating-condensing. With actual sizing of the rads it's also possible to determine how much water volume and total thermal mass we're working with. If there is sufficient thermal mass in the system a combi boiler just might work well enough to at least think about that as a solution.l
A right sized cast iron boiler is likely to be less than half the burner size of what's there, in which case it would need a narrowing flue liner to avoid flue condensation or backdrafting risk. Those liners aren't free. Condensing boiler use plastic venting (and not much of it if it's vented out the side). Plastic venting is also no free, but it's pretty cheap. Oversized chimneys can be used as a chase for housing the intake & exhaust venting for a mod-con, but there are usually cheaper/better ways. A chimney on an atmospheric drafted boiler draws outdoor air infiltration 24/365- it's a parasitic load. A power-drafted sealed-combustion appliance such as a modulating condensing boiler allows you to seal up the chimney ports, which removes the parasitic load.
Most gas utilities in New England have the monthly fuel over the past year listed in some form on each month's bill. What were your two highest-used months, and the amount of fuel used (in therms or ccf, not dollars)?
Yes, when the power goes out a combi-boiler leaves you without hot water, whereas any tank-type water heater (including an indirect).
