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Adding dormer and master bedroom, what to do about heat and AC?

OK, here's the situation. We live in a classic 1918 two family home in Arlington MA, and are adding a double shed dormer to our attic for a master bed, bath, and walk in closet. This will increase our square footage from about 1100 to 1600. We have steam heat, single pipe, with a newer (8 years old) natural gas boiler. Our winter gas bills are actually rather small. There is only one small radiator in the attic where the new construction will be. New construction will be insulated with spray foam, most likely closed cell. The three new rooms (master bed, bath, walk in closet) will need some form of heat and AC.

Dilemma: What should we do about heating and air conditioning?

I have researched many many options, but the variables are starting to drive me a little bonkers. I have a big spreadsheet, but will try to share some of the details here. Options are (but not limited to):

1. Keep the steam heat and just go with mini-splits, maybe add electric radiant to new rooms in attic. Problem is we hate the way mini-splits look on the walls, and electric heat would most likely be expensive up there. Then there is the issue of dealing with something like 5 different thermostats. We really don't want to do this.
2. Can the steam system be extended? Unfortunately pipes aren't really in the right places, and I don't know about tapping into a 100 year old system. Then do something separate for the AC, like high velocity?
3. We have priced high velocity for both heat and AC. Seems like this system was really just meant for AC, and heat is an afterthought. Gets expensive. We only get a single zone for both floors. Also, why do high velocity when we have access to the joists in the attic (which will be the new floor in the dormered area). Could just do ducts...
4. What about a whole new forced air system? We have access to the joists in the attic right now, and there is space to run 7" ducts. A high efficiency furnace and ac with all the bells and whistles. All very expensive, but would do just about everything. Might be the way to go, but I can't help thinking I'm crazy when we already heat 2/3'rds of the place just fine, but we still want to put in air conditioning. Also very wary of the humidity loss or "scorched air" everyone talks about with these systems. We would do an add on humidifier, but would it be enough. Then there is ongoing maintenance, which is almost non-existent for the steam boiler.

The only other thing to consider is that we have enjoyed the steam heat for the fact that there are no huge relative humidity swings when using it. That said, it takes 45 minutes to even warm up, and we are the kind of people that turn our thermostat down at night, and then up again when we come home (which isn't easy on a steam system).

OK, so this is what is driving me insane at the moment. I feel like I'm having trouble getting real opinions that aren't just from a salesman. Any thoughts, insight, or help would be greatly appreciated.


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