I have a super insulated, envelope home. I have R-88 in the ceiling and R-44 in the walls. Living in Northern WI I do have heating requirements, but nowhere near the output of the 90,000 BTU furnace that was originally installed. After living in the home a few years I decreased the burner orifices to give a 50,600 BTU output. This was better and I have used it for the last 20 years. Now my furnace is 33 years old and I am looking for the "Right" furnace to install.
I have connected my furnace to my computer and have recorded every burn cycle for the last year. I have read my gas meter to calculate the amount of gas used per minute. I then calculated that my furnace requirement is about a 25000 BTU furnace. The smallest furnace I can find on the market is a 40,000 BTU furnace. I calculate the 40,000 BTU furnace about twice as much heat as I need.
I do not want to decrease the output of my current furnace (by reducing the number of gas nozzles), as I am concerned with effecting the efficiency of the furnace.
What are my options for such a small heating requirement?
Best Answer
Low ambient mini-split heat pump is a typical approach - might want a couple-three (the 9-12K BTU/hr ones tend to be more efficient than the bigger ones, and the "1-compressor - 1 inside unit" also tend to be more efficient than the "1 compressor - multiple inside unit" versions, though the initial cost will be more.
An alternate approach for just heat, and using your ducts as is, would be to replace your furnace with a "fan coil unit" (water to air heat exchanger with a blower fan) and connect that to a highly efficient gas water heater. A circulating pump would also be required, and perhaps a heat exchanger to isolate the systems (or find a coil unit that is safe for potable water contact and use a stainless steel pump.)
On the third hand, a natural gas parlor heater (fake wood stove) in that size range could provide the heat, (or a pellet stove if you wanted to change fuel sources, but at the current time gas is probably the most economical heating fuel if you have it) and then you might still want some sort of fan on the ducts to help even out the temperatures around the house. These are available with controls that do not require outside electric power to operate, and can be quite an attractive focal point.
Ground source heat pumps are a lovely idea that suffer from often being not economically viable - A minimal GSHP setup is typically 5-10X the cost of low-ambient air-air units, though they are far more resilient in deep cold since they operate from deep groundwater temperatures that are usually stable at 52F or so.