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Building Science

The Basics of HVAC Zoning

What factors to consider, the two kinds of zoning, and issues with equipment

Last year I wrote five articles in a series on duct design and ever since, I keep getting requests to continue it.  Here’s a link to the first article in the series so you can go back and read them first if you’d like.

On GBA, I also published a revised version of the one on total effective length.

I’ll do a quick recap by saying that the focus in those first articles was the physics of air flow that went into determining how big the ducts need to be. We looked at the pressure from the blower that moves the air, the various pressure drops, and how total effective length is a measure of pressure drops, then used that information to calculate friction rate, which determines the duct sizes.

Now let’s crank the series back up with a look at the bigger picture. One of the main points in the earlier articles was that sizing depends on the total effective length of the most restrictive run in the duct system. The equivalent lengths of the fittings matter more than the straight runs of well-installed duct (hardpipe or flex), so you’ve got to choose your fittings well to reduce the pressure drops.

The layout of the system determines what fittings you need and thus the total effective length.  When we’re working on a duct design, we consider a number of factors that affect the layout:  the client’s preferences, location of the air handler, framing issues, zoning, and location of the vents.

I’ll look at some of the other issues in future articles, but today I’m going to focus on zoning.

This is a question you can address when you look at the house plans but you can’t answer it fully until…

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  1. Jeremy Good | | #1

    I'd add another factor: insulation and/or thermal mass. I worked on a community building a few years ago with 3 zones on two heat pumps. It was so well insulated that just a few people in the room with the thermostat would negatively affect the other rooms in the zone. I think that building would have benefited from more zones because the occupancy load was a relatively large proportion of the total for some rooms. Conversely, my 1930s masonry house has enough mass (and no wall insulation) to mask some shortcomings of the retrofitted A/C ductwork and maybe to a lesser extent, the radiator heat.

  2. Kevin Dickson, MSME | | #2

    Let's see, do hotels use ducts and zoning? Generally not. Well insulated houses don't need them either.

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