With all the incentives and rebates out there, it is time to switch to natural gas! I spent the last few week reading a ton of info on the internet. I live in a coastal CT town in a 1960 sq. ft wood shingled raised ranch with a timberline asphalt roof. This week I had an energy audit done. My oil boiler is a 25 year old Utica 122K BTUs which runs at 79% efficiency as per the energy audit with 3-zones but one is slaved into another so I have 2 thermostats. With the blower door test the infiltration was reduced after caulking, seals, etc. from 4470 to 3830 CFM. The SlantFin heat loss calculated at 41,200 BTUs with a 9 degree design temperature. There are 4 people living in the house and the coil in the oil boiler is inadequate and has been a pita.

What size gas boiler should I look for? Do I have to size up the boiler for hot water? A couple of contractors have suggested a Navien CH-210 Combi with the tankless HW. After reading comments I am not sure about the reliability and maintenance cost of the Navien. I am thinking of a high efficiency gas boiler AFUE 90%+ for the rebates and operating cost mated with an indirect-fired water heater. What size gas boiler and indirect tank should I get?

Does the 41,200 BTUs heat loss sound correct for my size house? Do I have to increase it for indirect hot water heating? One contractor suggested a 96K BTU ballpark whereas another proposed a 175K BTU Navien CH-210asme Combi. Am I underestimating my heat loss?

Another contractor said that the inherent problem with all combis is that the water adapter has to swivel from heat to a priority demand for tankless hot water which already has a delay to feel the HW. So on cold days the heat shuts off to accommodate the HW demand anytime someone turns on the faucet. Or if someone takes a shower, the heat shuts off and the house temps drop. He suggests the Triangle Tube Excellence over the Challenger in this regards.

Any suggestions and comments are greatly appreciated and I will be speaking to contractors this week. Thanks.