Separating Heating Zones in a Multi-Unit

[Valley of Plenty]

I recently acquired a triplex (See thread: The $55,000 Triplex Saga under the real estate section of the forum) that was originally built as a duplex. Trouble is, whoever did the conversion never separated the two units in terms of heating zones. As such, both are currently heated by the same furnace, with a shared thermostat that is located in the downstairs unit. The furnace is an almost brand new forced air natural gas furnace.

The previous owner swears that in the several years he owned the property as a rental, he never had an issue with tenants in the two units disagreeing about what temperature the thermostat was set at, but I'd imagine he's either lying or got very lucky. I currently have just the upstairs unit rented, so I've given that tenant control over what the thermostat is set at (since he's paying the gas bill anyways). I really would like to find a way to give each unit independent temperature control, but I'm not sure what's the most efficient and cost effective way to go about this. Far as I know there's no way to create two separate climate zones running off the single furnace, at least not with forced air. Maybe if it was radiant it would be different.

The other side of the building is still using oil heat, and the furnace over there is getting quite old, so I'm considering switching that side to natural gas and just stealing the furnace from the other side, then installing mini splits in the two units that currently share the furnace. The side with the oil furnace has radiators; I don't know if that matters for what type of furnace it needs. Can a furnace be used for both forced air and radiant, or are they built to only be used with a specific type of heat delivery?

Any HVAC savvy Mustachians able to weigh in on what my options are here?

[BudgetSlasher]

You might be able to zone it using zone dampers in the duct work (assuming furnace means forced hot air and not just generic terms for heating device). I briefly flirted with zoning our upstairs and downstairs using this method, but ultimately decided against it for several reasons. My general idea was to place a series of normally closed electrical dampers such that with all damper closed all vents were blocked off, then when a heat call came from a zone it would also open the damper associated with that zone.

There is probably a similar set of valves that can be used with a boiler to handle radiant heat, but I had no need to look into that.

In a rental situation with such a system I am not sure how you would have the tenant pay the gas bill (one meter split between two units). Might have to just include it in the rental price, but it would be any better than the current tenant paying to heat both zones even though he only has access to one of them.

[Valley of Plenty]

You might be able to zone it using zone dampers in the duct work (assuming furnace means forced hot air and not just generic terms for heating device). I briefly flirted with zoning our upstairs and downstairs using this method, but ultimately decided against it for several reasons. My general idea was to place a series of normally closed electrical dampers such that with all damper closed all vents were blocked off, then when a heat call came from a zone it would also open the damper associated with that zone.

There is probably a similar set of valves that can be used with a boiler to handle radiant heat, but I had no need to look into that.

In a rental situation with such a system I am not sure how you would have the tenant pay the gas bill (one meter split between two units). Might have to just include it in the rental price, but it would be any better than the current tenant paying to heat both zones even though he only has access to one of them.

It is a forced air setup.

And yeah, I guess even if I can find a way to split the output, I can't separately meter the usage.