I have found good results converting to mini-split air-source heat-pumps when moving from central boiler systems with through wall or window air conditioning.
I recommend Mitsubishi's Citi-Multi system.
- It has a central roof top mounted condensing unit, with individual indoor units that would be installed in the bedrooms and living areas.
- You can manage the indoor units from a single networked PC.
- And it enables you to bill the calculated HVAC electricity consumption per tenant. If tenant #1 wants it hot and tenant #2 wants it cooler, this system can handle each request on a room-by-room basis.
6-16 ton units meet ENERGY STAR Commercial Requirements and will exceed the 2010 ASHRAE 90.1 efficiency ratings. This system can provide 100% heating capacity at 0? F and 84% heating capacity at -13? F.
In the summer there will be condensation produced at each indoor unit. I recommend finding a route, if not inside a wall, then through a small chase (fire-stopping at each floor/ceiling penetration) in a closet or corner, so the water can gravity feed down to a floor drain or utility sink in the basement.
I also recommend that other energy saving improvements like adding attic insulation be completed before any new system is designed, so it can be right-size d to your new thermal envelop.
Good luck with your project,
For more information:
Read Randy Potter's Q&A "I can't afford geothermal. Is an air-to-air heat pump my best option for heating?"
Also, check Richard Parker's Q&A "Can you help me select a high efficiency furnace?"