I looked into the electrical ones and they use alot of power. Rinnai makes a tankless outdoor unit that has a direct vent on the unit itself and is designed to mount on an outside wall and then run a hot water line into the building. Conceivably you could mount it anywhere outside the building and run your hot line in. The clearance required is minimal and you can even remote the thermostat into your brewery so you can control the temp without going outside to the unit. Here is the link.