Houston TX - Bypass Duct Question

JRMarks

-Just purchased a 2019 home in Spring, TX / Houston area
-One outdoor AC unit
-One attic air handler
-Two thermostats/zoned system: upstairs and downstairs
-Honeywell zone board/dampers
-Bypass duct from supply plenum to return plenum is manually closed with a damper.

Question: Should bypass duct be re-enabled with a barometric/spring bypass damper? If it was closed when we bought the house, and may have been closed for the last 7 years, was there a reason? How do you decide whether this bypass should be open or not?

EdTheHeaterMan

The concept of a zoned system is that the air conditioner must be large enough to cool the home when all zones are calling. That means that when one or more zones are satisfied, the zone that is still calling is now much too small for the equipment. So, how do you solve this oversizing problem?

1. You duty-cycle the compressor in order to match the required output. For example, if you have three zones and each one requires one ton of cooling, then the three-ton compressor may only need to operate about one-third of the time when only one zone is calling. This is accomplished with a temperature sensor on the coil that turns off the compressor when the evaporator coil temperature approaches the point where condensation will begin to freeze. When the temperature rises sufficiently, the compressor starts again and continues cooling the one zone. The part is referred to in the equipment literature and wiring diagrams as the DATS, which stands for Duct Air Temperature Sensor.
2. The second problem is that the ductwork for one zone is also undersized for the amount of air the blower needs to move in order for the system to operate properly. To solve that problem, some of the conditioned air must be allowed to go somewhere else. That is where the barometric damper in the bypass duct comes into play. When properly calibrated, the airflow to the active zone is sufficient to cool that zone, and the excess air pushes open the bypass damper as needed to keep the equipment operating properly.

The fact that the bypass damper allows cooled air into the return is the reason the evaporator coil wants to get so cold that it freezes the condensation. Therefore, the compressor must be cycled properly for the system to work as intended.

My guess is that the DATS was not operational at some point, causing the coil to freeze regularly. The band-aid fix is to keep the bypass damper closed so the return air remains warmer and the coil does not ice up.

You need to make sure both the DATS and the bypass damper are working in harmony in order for the system to operate properly.

HVACNUT

https://forum.heatinghelp.com/discussion/204498/houston-tx-bypass-duct-question

It needs to be sized correctly. And it helps if the air handler has a constant torture motor.

mattmia2

it seems like all the parts needed to make it a 2 zone system cost as much as just buying 2 smaller systems?