Tankless heaters are designed for domestic use. A unit that says 199,000 BTU's will not give you 199,000 BTU's for space heating. That is just a rating on what the unit needs to deliver hot water at its listed gpm. To find out how may BTU's this unit will deliver for space heating, there is a quick formula you can use. Find out what GPM the unit will deliver (e.g. 5 gpm). As a rule of thumb, at a 20 degree delta t (the difference in temperature between the supply water and the return water) you can deliver 10,000 BTU's per GPM (10,000 BTU's X 5 GPM). This unit would give you 50,000 BTU's for space heating. This works for Takagi Tankless Water Heaters, Bosch Tankless Water Heaters, and Stiebel Eltron Tankless Water Heaters.
