My house has been improved in some way every year I've lived in it. I believe the boiler was oversized when I started caring for this house. Since then, I've insulated and sealed the snot out of it and also finished off the attic adding 400 sq ft to the living area. I think my boiler is at least 50% larger than needed. Adding up my baseboard, the bottom floor has 45K btu/hr that can be put out from the amount of baseboards I have at 180F. Upstairs has 19K and change. Neither zone is really stressed so actual heating requirements are much lower. I heat my water with my boiler now, and the standby losses are pretty annoying. Is there any reason why a tankless water heater, or even 2 of them, couldn't be used to heat the house. It seems like the downfalls of a tankless system would be removed since the output would be connected to the heating system. it really wouldn't matter if the first 30 seconds of water were the input temperature. Since my heating needs are fairly low, 120 degree water, or less, wouldn't be a deal killer going through my pipes. Replacement costs for tankless units are much lower than a new boiler. What crucial piece of information am I missing? I'm not worried about gas lines. They are easy to run.