Navigating the Twilight Zone: The Hidden Flaw in a Zoned Duct System
Ah, so what exactly is this hidden flaw? "My HVAC guy put a zoned system in my house and told me it was the cat's pajamas," you may be thinking about now. Or maybe your HVAC installer described it as the bee's knees, the eel's ankle, or the elephant's instep. Doesn't matter. However they described it, there's one piece they absolutely should NOT have installed.
First of all, 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 always zoned. That is, they have more than one thermostat so you can control the conditions separately in different parts of the house. If you have a two-story house, for example, you probably have 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 system that serves multiple zones. In most homes, 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.
"Come on, man, just tell me what the flaw is!"
Hold on. Hold on. We're getting there.
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 1, 2, or 3 zone dampers may be open and sending conditioned air to their respective zones.
If only 1 or 2 of the zones are calling for air, most air handlers will create extra static pressure because 1 or 2 of the pathways are closed off. Enter the bypass duct, shown at right. When the system is running but not all 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.
At the Affordable Comfort conference this year, I went to a talk on zoned duct systems where John Proctor and Rick Chitwood discussed the pros and cons of these systems. Proctor's take is basically that zoned systems are horrible and shouldn't be used. Chitwood likes them and says when done right, they provide exceptional performance.
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.)
The bottom line is that zoned duct systems are tricky. I do believe that Chitwood is right, but so is Proctor. I think Proctor's main objection isn't that zoned systems can't work; it's that they're done wrong so often. In the end, if you do get one, make absolutely sure the installer doesn't put in a bypass duct.