Heating duplex without ductwork.

[Aaron Carter]

I own a duplex which currently has electric baseboard heat that was installed by the previous owner. It is an older house and electric heat is really high (no off peak as it is the only source). I live in northern Minnesota... regularly 20-30 below zero.

The house used to be heated with a natural gas hydronic boiler, the former owner was an electrician and I am thinking the boiler malfunctioned and instead of replacing it he installed electric baseboard heat instead.

The pipes for the radiant heat are not hooked up in some areas. I had a plumber look at it and he recommended not to hook up the radiant heat again due to cost of fixing what is there.

both of my units are two bed two bath. I am adding two more bedrooms and one bath in the basement which will be connected to the unit on the main floor. I am planning on putting in duct work for a natural gas unit to heat the main floor and the new basement remodel (which will all be one apartment).

My question is what should I do with the upstairs apartment? I cannot feed the duct work to the upstairs apartment because I have no way to split the utility bill between the two units even if it were zoned. I had a natural gas line fed up to the upstairs unit because I was initially going to put in a wall furnace in to act as a large space heater. The unit is approximately 800 square ft with an open floor plan so I think that a central unit would work fine. However, the wall furnaces that I have been looking at are only rated at around 75% efficient compared to the regular central air units that are around 95.5% efficient.

Any suggestions on heating a 800 square ft apartment without duct work using natural gas efficiently? Maybe a mobile home furnace?