Those heat exchangers work good, couple of friends have them. Herman is not any much more than that, 2 steel fridge boxes one on top of the other furnace blower on back, heat exchanger in top fridge limit switch from oil furnace mounted in top fridge. Took some experimenting to find correct location. Stove choke damper. Been operating like that since about 85. Replaced blower motor once and regular cleanings. The only ***** is he’s hungry all winter long.