outdoor furnaces wood burning furnace outdoor wood furnaces OUTDOOR FURNACES Empyre outdoor furnaces use a non-pressurized system that heats the water surrounding the fire chamber. The water is thermostatically controlled by setting the aqua stat to the desired water temperature. The pump then circulates the hot water from the furnace connected to a home or shop, using a 3/4" to 1 1/4" water pipe from the furnace.