Hi @Dan C. the first thing to check is if there is any abandonned duct work in the walls. If you dont find any, perhaps because the second floor was added later, your best option is probably electric heat. Menards and HD have a nice selection of different systems. The upper usually takes a little less heat anyway, because heat rises from the lower.
If you would want to ad forced air heat you need an empty building, find a central wall that you can run warm air ducts and clod air returns and then drywall arround them. Normally you would use the floor joist spaces to distribute heat. Thats an option but makes for a lot of drywall work in the lower. Plan B would be the attic, but you have to insulate very well. Either way probably cost prohibitive, I'd go with electric.
I think right now the best route is buying the property at a price which accomodates a budget for converting to forced air.
The reason I say this is the one furnance/electric in the upper configuration creates several problems that make managing this as a rental not as smooth as I want:
1) My understanding is that electric heat is more expensive. This is a blue collar neighborhood and the expense could be a problem. However since this is the upper which benefits from the heat in the lower, it could mitigate this, but their bill would still be higher than with gas.
2) As @Michael Henry pointed out above, when there is one furnace, the bill must be in the landlord's name. So for the lower I would have to advertise it as "heat included" and make the rent to include the 12-month average for the WE Energies bill. If the tenants are inefficient with their thermostat use I might have large bills and would have to get them to sign a new lease to adjust the rent. Or they may feel they have become more efficient in their usage and justified in bugging me for a rent reduction. This is just an awkward mess.
3) This situation causes confusion for prospective tenants because it is not the norm for market. I will be competing with rental units that are configured the way these renters are used to and the may shy away from renting it (I know I would).
@Dan C. Does this upper unit have an attic above it? If so, why not install the furnace in the attic and run the ducting in the attic? This is were we typically install the AC/Heat air handler down here in the south. In an attic, the installation cost would be small. I dont know how big your unit is, but around here, we can install HVAC for a 1000ft unit for around $4500.
Thanks @Mike Wood. That would definitely be feasible if this were more typical Milwaukee duplex, as they usually have a walk-up attic. And I have seen furnaces installed there. This however is a bungalow style, and the 2nd floor which was an attic is now the upper apartment - so in other words this is the attic. But its a good thought as I could find out if the furnace could actually be installed in the unit, I don't know if that is permitted here. But I'd still have to get gas up to the 2nd floor. So it will be pretty costly regardless, so we'll see if I can get it at the price that will accommodate the cost.
Any Milwaukee landlords have an idea the cost range of running furnace ductwork to an upper flat?
I ask because today I saw something I've never seen before - a duplex with only one furnace, and the upper was heated by an electric stove!
I imagine there are some out there who ripped out radiators and switched to gas/forced air would have an idea the cost for this.
The furnace in the attic is a Milwaukee thing.... here in Chi we never see that. Generally, the cost to add ducts is about $2k plus the cost of the furnace $3-4k - the closer you can get to a gas line the better, so most of our are in hall closets off the kitchen