Thread: TWO ZONE- NO BYPASS DUCT

Inspected a new North Carolina house the other day- two story, 2 zone heat pump- no bypass duct and damper installed. With one zone closed, very high noise/flow level at the ON zone ceiling registers. I mentioned it in the Inspection report saying high noise/flow noted and that Honeywell (zone control manufacturer) recommended use of a bypass with damper, I also included a picture of the Honeywell instructions with diagram mentioning a bypass duct was recommended.

Builders super says they don't do bypass's, although I have told my client they did in one of their houses in a different development that I inspected in January.

Questions-

1- is a bypass and damper automatically required in a two zone?

2- where would i find any duct maximum air flow/pressure limits?

Thanks, I am confident in how I have handled this, but I would be interested in learning more as to what is required.

Originally Posted by JOHN PAVAN

Inspected a new North Carolina house the other day- two story, 2 zone heat pump- no bypass duct and damper installed. With one zone closed, very high noise/flow level at the ON zone ceiling registers. I mentioned it in the Inspection report saying high noise/flow noted and that Honeywell (zone control manufacturer) recommended use of a bypass with damper, I also included a picture of the Honeywell instructions with diagram mentioning a bypass duct was recommended.

Builders super says they don't do bypass's, although I have told my client they did in one of their houses in a different development that I inspected in January.

Questions-

1- is a bypass and damper automatically required in a two zone?

2- where would i find any duct maximum air flow/pressure limits?

Thanks, I am confident in how I have handled this, but I would be interested in learning more as to what is required.

Its funny how when you mention requirements to a builder they often respond that they do not do it that way. Its as if the way they choose to do thinks is somehow magically correct.

Here are some ACCA recommended velocities;

Main Plenum-Supply 700 for rigid ducts/600 for flex. 900/600 maximum
Main Plenum-Return 600/600; 700/600 max
Branch Supply 600/600; 700/600 max
Branch Supply-Return 400/600; 600/600 max
Supply grilles-500 recommended/750 max
Return grilles 400/600 (300/300 for filter grilles)

You typically use 400 cfm per ton for AC and I think 440 for HP.

Static pressure determines the CFM output of the blower. That is typically based on the manufacturers info for each system.

I sometimes find zoned systems without bypass dampers. I think the subcontractors just do not know any better or sometimes need to cut corners any way they can to make a profit based upon what builders are willing to pay.

I have only come across this once, and 3 of the 4 zone dampers weren't working.

What is the bypass damper doing? I picture it as allowing air to flow back into the FAU when one of the zone dampers is closed. Sort of like on a hydronic heat system? The system I saw was really old, and wasn't working very well to begin with. I guess I could look it up on line, but this is more fun.

Questions-

1- is a bypass and damper automatically required in a two zone?

No, but there should automatically be some method to prevent excessive noise/flow at all operational combinations. I run into these quite often with one unit providing both upstairs and downstairs with zone dampers. It can be quite effective to simply size the ductwork large enough to handle excess flow if both zones are close to 50% of the load. Another method is to provide a "dump" duct to a common area to relieve excess pressure but this means the common area is almost always too hot or too cold. The preferable method to me is the bypass duct with barometric damper. No matter what, excessive noise is not a problem when proper design is used.

Originally Posted by JOHN PAVAN

Inspected a new North Carolina house the other day- two story, 2 zone heat pump- no bypass duct and damper installed. With one zone closed, very high noise/flow level at the ON zone ceiling registers. I mentioned it in the Inspection report saying high noise/flow noted and that Honeywell (zone control manufacturer) recommended use of a bypass with damper, I also included a picture of the Honeywell instructions with diagram mentioning a bypass duct was recommended.

Builders super says they don't do bypass's, although I have told my client they did in one of their houses in a different development that I inspected in January.

Questions-

1- is a bypass and damper automatically required in a two zone?

2- where would i find any duct maximum air flow/pressure limits?

Thanks, I am confident in how I have handled this, but I would be interested in learning more as to what is required.

1. A bypass damper is not always required. In fact, is is usually never installed in newer units. Most modern units are variable volume or multi-speed fan units that adjust air flow based on static pressure. Also, some backward inclined fan designs will "ride" the fan curve to accommodate any increase in static pressures.

2. If you are referring to the recommendations for max cfm in a particular duct size for low pressure systems, then there are guidelines. These guidelines are typically in reference to velocity for noise control and recommended pressure drop per 100ft of length for design considerations. In general, for sound control velocity within the duct should be less than 800-1000fpm +/- and velocity at the register should be approx 350-550fpm depending on register type, room dimensions and throw characteristics. You should design the ductwork using .1 wg/100ft of equivalent duct length for friction loss as a guide.

Bottom line is if you are not a design engineer, don't try to be. If you hear excessive noise or feel low or high flow at a register I would recommend you suggest review by a licensed contractor.