I am replacing two heating appliances for two micro apts. in a 4 story brownstone. The current boilers are 70K BTU, hot water, gas fired heating only. The apts are 400 SF and are well insulated because there are floors above and below. I did a heat loss calculation and the space would only need less than 50K BTU. I assume the 70K was the smallest unit at the time. I would love to replace with condensing combi units but these boilers are in the cellar 46 feet below the roofs chimney top so cool exhaust wont rise up that distance. Also there are three other water heaters and another newer boiler sharing the same 7" SS flue liner in that 46' tall chimney.
So the question is who makes a micro sized gas fired, non condensing boiler? It would be nice if I can wall mount it to save space. It dose not have to be cast iron. I'm open to any appliance that makes hot water for the existing fin tube base board. This is singe zone heating.