I have a 100 gallon commercial water heater (A.O. Smith) in my home that serves my radiant floor heating system (7,100 sq. ft.) plus hot water for two guest bedrooms and our kitchen. The water heater is now leaking (after only five years but, of course, out of warranty) and the radiant heating system installer has recommended against my replacing it in kind. He's suggested instead that I get either a 75 gallon water heater or a tankless water heater for the DHW and that I get either a high-efficiency boiler or a 75 gallon water heater for the radiant system. Now I'm trying to decide how to make the best decision based on economics.

My average annual gas bill is about $2,700 and it looks like about $1,700 of that is for the radiant heating expense, leaving $1,000 spent on DHW. The radiant guy says I'll be able to install everything but the boiler with fairly nominal costs for re-plumbing but that the boiler will require a PVC stack and, while the chase that currently encloses the water heater's stack could probably accommodate it, the piping would have to go up through through two floors and the attic and that would add to the cost.

And one last complication...I have an electric pump that circulates the hot water in the pipes to eliminate the long wait for DHW. Can that system work with a tankless hot water heater?

Based on this, can you help me decide:

a) tankless or hot water heater for DHW?
b) hot water heater or high-efficiency boiler for the radiant system?

I plan to stay in the house for another 20 years (if I live that long!).

Thanks for any advice you can offer.