Best Of

"More Modern" is not necessarily an "Upgrade". Note Brother Hall's post above. Heat pumps are NOT "maintenance free"

Keep the steam as your primary heat source. Over the long haul, it costs no more to maintain and keep in good repair than any other system, and probably less (and less hassle) than heat pumps — as well as greater comfort.

Now what you might want to do, if you have the extra cash on hand, is install heat pumps for the shoulder seasons and for summer air conditioning. They cost a good bit to install — a lot more than a boiler, for instance — and they have ongoing maintenance issues (even the best of them) and a short — 10 to 15 year — reliable life — but they are "modern" and "green" and both of those a selling points.

You want a thermostat in each unit. OK. What I would do is keep the steam as the primary heat, set at a fixed, low temperature — say 60 to 65 F — where the occupants can't get at the thermostat. Then a heat pump for each unit, with its own thermostat, and powered by that unit's electric meter (I presume they will be separate?). That way the occupants can set their own temperature on their own dime and be happy, except on the super cold days when the heat pumps can't keep up.

KEEP the steam heat, if you have a brown out, power surge or a nearby lightning strike the electronics on your mini splits will be fried, and then where will you and your tenants be????

Heat pumps are fine for the shoulder seasons. Once the NE wind begins blowing and the thermometer begins falling, you're going to miss the steam.

That boiler is probably 30 years old. One time investment with simple easy maintenance. Heat pumps are highly computerized, maybe good for 10, 12 years then throw them away. Few techs are properly trained, and parts can be scarce. Not a good combination for your primary heat source.

@Jamie Hall "heat pumps can't keep up" My design temp is 2F, at the point the heat pumps are not loosing any output. Heat pumps only working in shoulder season was the case for units 20 years ago.

Any combo heat only makes sense is if you already have something that works and costs you zero dollars to keep.

Tenants pay exactly $0 extra for comfort, will definitely pay more for AC. So the decision comes down to parts/labor to install and long term maintaince cost. New filters costs me $8 each in bulk and takes 15min to replace all which you have to do on an AC only setup as well.