I've had a couple installers out to look at my house, and both thought that zoning wasn't practical. I'd like to see whether it's worth the effort to seek other opinions.
I have a two-story plus finished basement house in Virginia built in 1947. An addition was added to the south and west sides of the building in 1984 that absorbs most of the mid-day and afternoon sun. The ground floor of the addition is a family room that is cold in the winter, and the second floor master bedroom is hot in the summer. The second floor master bedroom is half in the addition and half in the original house. The remainder of the original part of the house is well-balanced, and does not have a noticeable temperature variation between floors in either summer or winter.
The main supply duct splits into two branches:
1) The original house supply ducts branches off from a duct that goes down the center of the basement, and branches go straight up to the rooms quasi-randomly--a branch will go to the second floor, the next couple branches to the first floor, etc. This makes it quite difficult to zone as there is no neat divide between first and second floor.
2) The other main supply branch goes to the addition. Once the duct goes through the basement and reaches the crawlspace of the addition, several ducts feed the first-floor family room, and then the remainder goes up to supply the second floor master bedroom.
The contractors I've talked to seem to think a zoned system is only done between first and second floors. I'm thinking a three zone system could work here: one zone for the original house, a zone for the family room addition, and a zone for the master bedroom addition. A damper would be installed at the furnace to divide between the old part of the house and the new, and another damper would be located in the crawlspace to divide between first and second floors of the addition.
The other complicating factor is that one of the small branches from the original part of the house also feeds into the master bedroom suite. (In other words, air reaches that room from one vent of the original ducting, and two vents from the supply to the addition, which would be in different zones.) As I see it, there are two options here.
1) Ignore it as there are two other supply vents in the room that would be zoned. This wouldn't be ideal as there would be supply in the room not controlled by the same thermostat, but I don't think it would substantially affect the temperature variation as the other two supply ducts would be zoned and I wouldn't ask the thermostat for much temperature variation between zones anyway. (The point of the zoning would be to send additional cool air to the master bedroom in summer.) It seems as though this isn't much different than having a door open that allows cool air to travel between zones controlled by different thermostats.
2) Put a damper on this original duct into the master bedroom that is controlled at the same time as the damper from the supply that feeds the rest of the addition. Is it possible for a zone controller to control two dampers to supply one zone? This would allow two of the master bedroom ducts to supply the room when only the master bedroom thermostat was calling for A/C, and all three would supply when both the main house and the master bedroom were calling for A/C. When just the main house was calling for A/C (probably a rare occurrence), some cold air would also flow into the master bedroom.
I plan to use a variable speed two-stage furnace with two-stage A/C. There are large returns located in each proposed zone.
I apologize for the length of the post, but I'd appreciate comments. I think the current problems are rooted in inadequate airflow to the addition, especially to the second floor in summer. I don't see an easy way to increase the size of the ducts, so a zoned system seems to be the best approach. Should I keep looking at other installers?