Trying to remember where I found a gas water heater with an indirect coil in the BOTTOM of the unit. I have found ones with a space heating coil near the top of the tank, but not the bottom to be heated by a wood boiler. Can't remember if it was direct-vent either but that would be nice. It was some niche German company I believe and deadly expensive. Not sure why a unit like this isn't more widely available. Would be a boon for the solar crowd. There are solar/electric versions which makes no sense - if you are using solar, chances are electricity is at a premium/unavailable.