Slab heat propane water heater

What you are talking about is a hydronic heating boiler. Weil-McLain makes good units of this type,and I am sure there are others as good. The BTU rating will depend on the size and type of the building, climatic conditions, nature of use (i.e. a building with large overhead doors that are opened frequently in cold weather may need supplemental forced air heat to bring the interior air temp back up after a door operation since floor heat is relatively slow to respond to interior air temp changes). As a guide, in Northern NY climate, our 5200 sq. ft insulated metal shop building with split level 16 ft and 30 ft ceiling uses a 218,000 BTU Weil McLain boiler to serve ~4500 LF of 5/8" tubing in four zones, with a surge tank of around 10 gallons. A good vendor can help in system design for your specific conditions. good luck