The Achilles’ Heel of Zoned Duct Systems
The way that many zoned duct systems handle excess air can create a few problems
Last week I wrote about what happens when you try to save energy by closing air conditioning registers 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 registers, and that doing so 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.
What is zoning?
Before we find out what that thing is, though, let's be precise in our language and clear up exactly what we're talking about. The word “zoning” is used in more than one way in the context of heating and air conditioning systems in a house. First, larger houses are almost always zoned. That is, they have more than one thermostat so you can control the conditions separately in different parts of the house. In a two-story house, for example, there will probably be at least two thermostats — one upstairs and one downstairs.
The other way that the term “zoning” is used is to describe a single duct system attached to a single HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. system that serves multiple zones. In most homes with forced-air HVAC systems, each thermostat is connected to its own heating and cooling system. The home is zoned, but the HVAC system is not. In a “zoned system,” a single heating and air conditioning system is controlled by multiple thermostats in multiple zones.
In the photo above, the three green lights are part of three zone dampers that control the flow of air to three separate zones. Depending on the needs of the house, any combination of one, two, or three zone dampers may be open and sending conditioned air to their respective zones.
If only one or two of the zones are calling for air, most air handlers will create extra static pressure because one or two of the pathways are closed off. Enter the bypass duct (shown in Image #2, below). When the system is running but not all of the zone dampers are open, the bypass duct — in theory — is supposed to relieve the extra pressure and maintain good air flow throughout the duct system.
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:
- 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 this week, 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 performed the calculations to show the efficiency for each configuration. In his little experiment, the three configurations with the bypass duct closed (with no air through bypass) were 22%, 27%, and 32% more efficient than the configuration 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 register-closing scenario. To do zoning right, you’ve got 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 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 be done right... but it’s still 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.
Allison Bailes of Decatur, Georgia, is a speaker, writer, energy consultant, RESNET-certified trainer, and the author of the Energy Vanguard Blog. Check out his in-depth course, Mastering Building Science at Heatspring Learning Institute, and follow him on Twitter at @EnergyVanguard.
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