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HVAC Zone Controls & Zoning Dampers

Eliminate BYPASS on a Zoning System?

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There has been a lot of buzz around eliminating bypass more so lately, but it has been talked about for 20+ years.  Some states have even mandated that all new Zoning systems be installed without bypass in certain types of buildings.  Others have argued against bypass for many years but only recently have HVAC zone control manufacturers offered products specifically designed to eliminate bypass.  Although these products are new, the principals they employ are NOT.  The different products on the market are just elaborate ways of "leaking" air into zones or areas that are not calling for it.

At ZoningSupply.com we rely on more than 2 decades of HVAC Zoning experience to come up with our recommendations and with regard to eliminating bypass, we do have some suggestions of applications with SmartZone that have worked well.

First of all, how many zones do you have?  The more zones you have the more difficulty you will have operating without a bypass.  It becomes more challenging because the amount of surplus air and air pressure in your duct work increases when (worst case scenario) your smallest zone is the only zone calling and all other zone dampers are closed.  A zone system with more than 4 zones needs bypass almost certainly.

Secondly, how large are your zones ducts?  Are they capable of handling the excess CFM of air that will be generated when other zones close?  Most duct can handle much more airflow than it is designed for but only up to a point.  Make sure that the smallest zone is capable of handling most or all of the airflow when the other zones are closed.  If the smallest zone is NOT capable of handling ALL of the airflow then there are at least a couple of options of techniques that can be used to possibly still eliminate bypass.

Airflow management techniques that can be used in conjunction with each other to reduce or eliminate the need for bypass on a zoning system:

  1. Over-sized ducts - increase the size of ducts and dampers beyond normal spec.
  2. Leak into zones - adjust the dampers stop screws so the dampers "leak" a fixed amount of air into some or all zones (no more than 25% recommended into each zone).
  3. Dump zone -  a room or rooms where the duct does not have a damper and will receive air flow any time the hvac system is running.
  4. Managed dump zone - use a "bypass" damper connected to "dump" duct into any or all zones.  Adjust the bypass damper to stabilize the static pressure when the smallest zone only is calling.  This is much preferable to the other "Dump Zone" because you are not dumping air when it isn't necessary which can affect air speed.
Use set screw on ecojay motor to "leak" air into NON-calling zones.

Use set screw on ecojay motor to "leak" air into NON-calling zones.

Multi-stage HVAC equipment - If your current hvac system has multi-stage (2 or more speeds) SmartZone can select the appropriate speed based on the number of zones calling (if set to 2nd-Stage Lock).  This capability can significantly reduce the amount of surplus air volume and pressure that would normally be bypassed because when only 1 zone is calling, the equipment will be in low speed.

Even with all of these techniques there are some systems and applications that just must have a bypass & for that we recommend the static pressure controlled version.  You can find more about why this is best in another blog post on ZoningSupply.com.

When it comes down to it, with Bypass or any other type of pressure relief on a zoning system, there is a balancing act between air velocity/noise.  You want to relieve or bypass as little air as possible to maintain efficiency of the HVAC system, however, with zoning it is inevitable that the air velocity will be come too high when only one zone is calling and that is why some combination of the techniques discussed should be used with any zone control system.