- The Open Direct System uses the same domestic hot water heater that makes hot water for bathing and laundry to provide space heating, too. Everything in the system is made of materials that are fit to drink from, and a plumbing detail prevents stagnation of the water in heat tubes during the summer when space heating is not required. For details of this system, see Open Direct System. By reason of its efficiency, cost, and simplicity, this is our preferred system.
