Much depends on weather extremes where you live and having each room thermostatically controlled is always the ultimate. In Central Texas where Summers are brutal and Winter seldom fall below 25f or stays below 25f more than 36 hours we did something similar and it worked ok. 1k sq ft house, two 144 sq ft bedrooms with 15' hall seperating them from 600 sq ft lr/dr/kitchen open concept. There was a 60k furnace and 18k btu window unit in lr when we bought house. Opening doors to bedrooms and placing a box fan on hall floor near lr cooled bedrooms for over 5 years until home was expanded and central air-heat pump was installed.

With you layout the single unit will need two box fans and can use even mor help. More help being fan forced jump vents. If you most hire jump vents installed I would lean toward heads in each room. If you have better than average finish carpentry and handyman skills I'd be delighted to give you pointers on vents. What you r-e-a-l-y need is a good hvac contractor that's in your corner and a clear understanding of what and how much you will give up to save money. Silver bullet? I have none.

Hi, dt516,

There are a number of case studies which demonstrate the viability of conditioning at least a single level of a home with a single mini-split cassette (or head.) These case studies span a number of different climate zones including cold climates like Massachusetts. I'd say that the success depends less on your local climate zone than it does on the properties of your home's building envelope (e.g., insulation, types and areas of windows and exterior doors, air sealing, etc.) Here's a link to one such case study to get you started:

https://www.energy.gov/eere/buildings/articles/building-america-whole-house-solutions-new-homes-transformations-inc-net

Since you'll be installing the system in an existing home you're more constrained than would be the case for a new home. I recommend you get a HERS rater involved in your project and have a blower door test performed before doing anything else.

"Keeping the doors open in order to become cool and warm would be a nuisance we could learn to live with."

No bedroom privacy is not a nuisance, it's awful.

One of the drawbacks of ductless systems (to include radiant systems) is how to provide air distribution for occupant comfort and health. Most of the homes that are able to be heated/cooled with a single ductless mini-split cassette on each level are very tight, well-insulated homes. In that case, they will need fresh air provided by an HRV or ERV. For either system, stale air is usually removed from bathrooms and fresh air introduced in the bedrooms. While that set up would provide some air circulation, it's clearly a compromise. It would not be my preferred choice.

We put in a 9FS Mitsubishi in the room labeled 9K on a 2C20 shared with a room downstairs. We are now consdiering whether to add single heads in the other two rooms, add 1 4 way cassette in the hallway, or a slim duct in the attic with ducting to the two rooms plus hallway and bathroom (most costly).