Zoning w/one main trunk

[mjhar]

I have a gas heat/ac unit that has one main trunk that runs in a space between the upstairs and an unfinished walkout basement downstairs. We have two BR upstairs and a 350 sq ft living room w/15' vaulted ceiling. and will have two BR downstairs along with a 500+ sq ft. family room. Total fiished space will be 2500sq ft.

The main trunk supplies all upstairs vents(9 total)of course and has 4 branches downstairs already run to the four corners of the basement.

My questions are as follows:
1.Can an air return be installed downstairs by teeing off of the upstairs return?

2.Can I install dampers in the BR branches and have them controlled by their own thermostat?

3.Can the thermostats control their room damper only and not turn on the main hvac?(so they have some climate control, but not complete)

4.Can I do any zoning without upsizing my ductwork? This is my main concern and is the reason for the previous question.

5.Should I just suck it up and seperate the upstairs and downstairs runs into seperate trunks?

This is my first post and I appreciate all the information that I have received by reading everyone's helpful responses to other's posts.

Thanks

[Black Adder]

Question #

1. Yes

2. Yes, just use a damper, transformer and Tstat.

3. Yes

4. Yes if you don't get too carried away, 2 dampers in the bedroom runs would be no different than closing the registers off in those rooms. If you close off too many you are certainly going to have issues but I would think 2 dampers would not hurt you.

5. It would be a fair amount of work but would give you the best results, the tstats would then not only control the dampers but your A/C heating as well.

[markwolf]

You do not want to put the basement into a negative pressure .You could cause nat draft appliances to not vent properly.call a local contractor & ask about your concerns.most will have plenty of ideas for you.check into arzel zoning.