Some space heaters use propane, kerosene, or natural gas, but these require special ventilation and pose a carbon monoxide risk. Catalytic wood or pellet stoves are designed to heat large spaces.
Typically, a 9 or 10 GPM propane tankless water heater might deliver anything from 25,000 to 200,000 BTUs of heat, which many consider sufficient for a 3-bathroom home. A high-efficiency water ...