I'm getting estimates for switching over to natural gas. I have had 3 plumbers recommend 3 different types of setups. First is a combi boiler, second is a condensing boiler with a condensing gas water heater, third is a cast iron boiler with a conventional gas hot water heater. My house is 1500 sq.ft, one bathroom, r-45 in the attic, energy star rated windows, no wall insulation, built i believe in the 1930's. My oil fired burner gives us endless hot water, only have to wait a few seconds to get it as well. I would like to continue that kind of performance. I have been reading on the internet combi boilers can be very troublesome, where the cast iron boilers last forever. Any experienced plumbers around? Which makes of boilers are the most reliable.