Is it efficient to get a tankless water heater when you have radiant heating that runs off the hot water tank?

Alvarz. I know it seems strange, but there are some systems out there that run potable water for heat. I've installed a few of them, but what I've seen was potable hot water run through a coil in the air handler. As I remember, it was some sort of promotion from the electric co. In any case, this was probably 10+ yrs ago, but the system did work.

Back to the question: It would not be efficient to run two heaters. If your radiant heat runs potable water, I'd leave it alone. If your radiant heat does not use potable water, but rather uses a water heater as a sort of a cheap boiler, then the tankless heater would work, although I still prefer the old water heaters, they're cheaper, the electric ones are reliable and cheap to fix when they do break, and although they hold hot water, which can add a smidge to the electric bill in the summer, the lost heat goes into your home in the winter, so it's not lost. The tankless heaters are smaller, give instant heat, but the power spikes when they turn on can be pricey when you get the electric bill.

Your household hot water tank definitely does not circulate thru your house to provide heat. That is a separate unit and has nothing to do with the hot water that you shower and wash with.,and cook with.

A tankless hot water heater cost more initially but retrieves this money in a year because of lower operating costs.