I am building a 30 x 36 garage. It's 2 x 6 construction with attic trusses. I end up with an 18 x 30 room upstairs. I am putting radiant tubing in the slab and also under the second floor sheeting. I bought the system from a supplier in VT. They are well known. I need a hot water heater that has an output BTU rating of 40K. That is figured by input BTU times efficiency. When I looked around the big box stores, they don't have anything. I checked Rheem and Ruud and they have one although it's a 75 gallon unit. When I called my local plumbing supply, they told me what I was doing wasn't legal. Said in Michigan you aren't allowed to do that with a water heater. This is a closed system, there is no water in the garage. It is not going to be used for anything else. They tell me I need a real boiler to do this. The company disagrees. I have a phone call into the inspector now to figure out what to do. Any thoughts out there?