Benjamin Dinkel wrote:Hi guillaume and welcome to Permies!
In theory your plan works and I’m sure you could figure it out somehow, depending on your plumbing abilities. But water heaters involve a bunch of technology and I’m not sure if it’s an easy or cheap way to heat your garage. Or necessarily the safest.
Have you considered building a rocket mass heater in your garage?
Why do you want to heat a garage in the first place?
Are your measurements feet?