I have a Climatemaster Tranquility 27-064 geothermal furnace. When i had my house built in 2004, I had my contractor install the ductwork so that it could be zoned upstairs and downstairs, but I didn't have the zoning electronics installed. I'm now looking to add the electronics and my contractor is out of business. I'm looking at using the Honeywell 8835 zone control primarily because I already added a VisionPRO IAQ Thermostat and have the outdoor temp sensor installed (I did this to control the excess humidity that the humidifier was adding in the winter, and also to lock the emergency heat out when recovering on a vacation schedule and the outdoor temp is reasonable), and I have the second floor prewired for the zone tstat. I have a 10x20 (downstairs) and 10x14 (upstairs) rectagular supply duct, so it looks like I need to bypass 1060 cfm. I'd like to use round duct for the bypass, so I have to choose between a 10" and 12" bypass - one is undersized, one is oversized. Is it better to err on the side of oversized or understized? I've read differing opionions on this. Also, I've read that many people prefer a weighted bypass control vs. a modulating control with VS blowers, so it sounds like weighted damper is the way to go. Am I on the right track?
Thanks in advance for any advice.