Ultimate water heater?

[flameretardant]

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.

 

[GT26]

(broken link removed to http://www.htproducts.com/versa-hydro-solar.html)
This co has other products like you want

 

[in hot water]

There are some other brands hitting the market, less expensive options. They are glass lined steel tanks with a conventional gas burner and a power vent on top. Schuco has one, Heliodyne does and a few others. I believe they are all US built with private labels on then.

We have been data logging one for about 6 months. The burner is at the bottom but the control sensor is mid tank. The concept it to heat just the upper portion of the tank leaving the bottom 1/3 cool for the solar coil to warm. It does tend to heat more of the tank than we expected so it does limit the solar fraction. A bit of a compromise tank, like an SUV

It is intended for customers that do not have room for a dual tank solar DHW and lets them use gas or LP instead of an electrical element at the top which is typical of most solar tanks.

I have an HTP Solar Phoenix tank which has solar coil in the bottom and a condensing burner and HX inside and the burner and control is mid way up the tank. It modulates and condenses and if very fuel efficient. It's a stainless steel tank and fairly $$.

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[in hot water]

Here is the HTP Solar Phoenix tank.