Bypass duct causing smelly heat?

[motohead]

I have a forced-hot-air gas furnace set up to serve two heating zones. Zone 1 works fine, but the heat in the zone 2 rooms persistently smells like hot dust (like when a heating system is first turned on for the season, only this smell never quits). After 2 or 3 days sitting in this smell we start to feel sick and have to vacate or turn off that zone. I looked at the furnace and noticed that there is a bypass duct installed between the output plenum (where heated air gets distributed to the two zones) and the input plenum (where the return ducts bring room air back to the furnace input). There is an electric flapper valve in this duct. When zone 1 runs, this bypass valve is closed thus isolating the output from the input (though it opens immediately when the thermostat turns the furnace burners off). But when zone 2 runs, the bypass valve is open, allowing some of the heated air to immediately recirculate into the furnace input for additional heating. Is this normal? I suspect that this is wrong and is allowing the air in the output plenum to get too hot and causing the heating smell in zone 2 by overheating materials in the ductwork. What is the normal purpose of this bypass? The only thing I can guess is that it prevents temperature over-shoot in the rooms being heated by cooling the furnace down using re-circulated duct air rather than delivered air. Any thoughts are appreciated.

[HayZee518]

My thoughts about this motor or solenoid operated bypass is to close when either heat zone or both are being heated. when the burner shuts down, it is my belief that the bypass will or would be open and the ducting system would function as a "gravity feed" system "without" being fan forced. sort of a way of balancing out the heating system until the next time it starts up. Uh.... there is a cold air return to the furnace isn't there? from each zone?

[paul52446m]

When you have two zones with dampers. If each zone needs 400 CFM of air to that area, then you have a blower that moves 800 CFM of air. If both zones were calling for heat then the bypass would be closed. If only one zone is heating than the bypass would have to be open enough to bypass 400 CFM of air and 400 would be going to where it is needed. Now
zones can be different sizes so your bypass might be open to a different percentage at different times to maintain the CFM of air going to a zone. If you have a low point in the bypass check and see if you are getting a dust buildup. There isn't a filter that you haven't
found yet? There is more limit controls on a zone system to control the air temp. maybe this control is getting bad. Later Paul