I had a 2 Stage-2 Zone Trane heat pump/furnace system installed about 6 months ago. Except for the main trunk to Zone 1, the duct runs from the basement to the existing upper floor duct work were reconfigured and dampers added to give us 2 zones, one for each of 2 upper floors, about 1000sf each. Overall the system works pretty well, except for the continuing problem of excessive blow-by through the main Zone 1 vent when only Zone 2 is called. There is only negligible passive air seeping through the other Zone 1 vents at the same time. To try to minimize the blow-by, the HVAC contractor installed a by-pass from the main Zone 1 trunk and replaced 6" runs with 8" runs to the Zone 2 ducts to reduce air-flow resistance. Those measures helped a bit. Now Zone 1 heats to two degrees less than the temp set for Zone 2 when only Zone 2 is called. It used to be 2-3 degrees hotter in Zone 1 when only Zone 2 was called. The tech was here last week and we measured the blow-by air blowing out the main Zone 1 vent when only Zone 2 is called. It registered as 8 cfms(?) at both heat pump stages and 15 and 20 respectively at furnace Stages 1 and 2. This is after the above measures were taken. Consequently, we are not getting the cost benefits of having 2 zones.
1) Is there an industry standard for the maximum blow-by that should come out of the "off" zone vents when only the other zone is called?
2) It has been suggested that a second damper in the large main trunk would catch the air blowing through the first damper when it is closed. Would that help?
3) Would it help if they reduced the size of that main trunk?
4) Any other suggestions?
All advice is appreciated. Thanks a million!