Why Are You Still Using Bypass Ducts in Zoned Systems?


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Last week I wrote about what happens when you try to save energy by closing air conditioning vents in unused rooms. In the end, I recommended not doing it because you won’t save money and you may create some big problems for yourself, like freezing up the coil and killing your compressor. At the end of the article, I mentioned that zoned duct systems do close off vents and that it can be OK with the right kind of equipment and design. But there’s one thing often done in zoned duct systems that’s rarely done well.

The problems with the bypass duct

A few years ago at the ACI conference, I heard John Proctor and Rick Chitwood discuss the issue of bypass ducts. Proctor isn’t a fan of zoning at all, and Chitwood is. On one point, though, they both agreed: Bypass ducts should never be used.

Here are three reasons why they argue for the elimination of the bypass duct in zoned systems:

  • Throwing cold air directly into the return plenum reduces the temperature of the air coming in to be cooled. That makes the evaporator coil get colder, and the colder it gets, the less efficient it becomes.
  • The bypass duct steals air. Even with all three zone dampers open, the bypass duct has a big pressure difference across it, and air is lazy. It’ll cheat and take the path of least resistance whenever possible, in this case the bypass duct.
  • Not only is a colder evaporator coil less efficient, it’s also more likely to freeze up, as the condensation it collects eventually drops below the freezing point. (And if you think a bypass duct is bad for air flow, a frozen coil is way worse. It’s really hard to push air through a solid block of ice.)

Savings from eliminating the bypass

Just recently, Proctor posted an article on zoning and bypass ducts on his website. With the article, he included a video demonstration of a zoned system, showing the changes in airflow and temperatures with and without the bypass duct open. Then he did the calculations to show the efficiency for each configuration. In his little experiment, the three configurations with the bypass duct closed (no air through bypass) were 22%, 27%, and 32% more efficient than with the bypass duct open.

Of course, if you’re sending air to only one zone, you still have the issues of reduced air flow in a PSC blower and increased energy with an ECM blower, as I described last week for the vent-closing scenario. To do zoning right, you have to account for the extra air when one or more zones are closed during operation. Probably the best way to do that is with a multi-stage air conditioner or modulating furnace that can also ramp down the fan speed to send less total air through the system.

My friend David Butler, one of the most accomplished HVAC designers I know, believes that bypass ducts can can be done right…but it’s best to avoid them. “It’s a tool that should only be used when [other] options aren’t feasible or possible.” The bottom line with zoning is that it’s a tricky business no matter which way you go. ACCA has a zoning protocol called Manual Zr, and that’s a good place to start if you’re going to design a zoned system. But be wary of sending any air through a bypass duct, and if you do, make sure it’s only a small amount.

Allison Bailes, III, PhD
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Posted In: Building Performance, Residential Buildings

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