Following up on what George2 was saying, you need to first understand that when any blower/fan comes on, there is a certain amount of resistance to airflow. In a ducted system, we refer to all of the resistance outside of the equipment jacket as External Static Pressure (ESP). In a single zone system, the ESP is what it is based on the design and installation. Ideally it would be, in a residential application, about .5 IWC (inches water column).Anything above 1.0 IWC is huge and a definite no-no. The ways to reduce ESP are to keep all filter and coils clean, as well as a properly designed and installed duct system. If the duct system is too small, the ESP goes up quickly.
Now to the zoned system. Clearly if the system has an ESP of .5 IWC with all zones open, then closing one or more zone will serve to decrease the overall size of the duct system and the ESP goes up. Recognizing this issue, the designer of the zone control system must design in some sort of 'relief strategy' to eliminate the excess static pressure. A common solution for basic zoned systems is to install a barometric by-pass damper that responds to high static by opening and therefore limiting the ESP to some tolerable value. However, a modulating zone system can accomplish the same thing but without the damper. This is done by some sort of 'intelligent' controller that controls ESP by opening zone dampers as needed to keep ESP below a specific value. In the case of modulating zone systems, a by-pass damper is not used. Likewise, with the modulating system, the variable speed blower motor is directly regulated by the system controller and also aids in keeping ESP under control.
Now come the complication of a by-pass damper. Since a variable speed blower is regulated primarily based on ESP (the resistance to airflow is reflected in motor RPM and amps) a by-pass damper will first open, reducing ESP, causing the ECM motor to increase in speed to keep ESP within set parameters but some of the increased airflow will travel through the by-pass and into the return air, thus reducing the return static, causing the ECM motor to slow down, which causes the by-pass to close, increases the ESP, increases the return air static, which causes the ECM to increase. This is a never ending 'hunt' for the variable speed (ECM) motor. This is also the reason that ECM motors are NOT allowed on 'twinned' systems, where more than one blower motor operates in the same static environment as another. Hope this helps. SO
If YOU want change, YOU have to first change.
If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!