As I mentioned in my previous post, the Four Schools of Thought for Ceiling Register Placement are 1. Register Over the Window, 2. Register interior to room., 3. Register in Center of Room, and 4. High Sidewall Register. All four schools of thought can work just fine (in terms of comfort), when done correctly. Comfort, however, is not the only factor to consider. Energy efficiency, materials efficiency, ease of installation, and aesthetics are all things to consider as well. This post will look at all of those factors for this particular school of thought: Register in Center of Room. By the way, unless I say otherwise, I’m focusing on cooling mode on a very hot day.
Register in Center of Room
While not very common in California residential design, this ceiling register location has a lot of experience in the commercial world. It is also, by far, THE most common location used in the Las Vegas area and across the arid Southwest where they know a thing or two about cooling. This location has a lot going for it, from very practical (four-way square registers have no direction to worry about, so installers are less likely to install it wrong) to very effective (because the air is coming out in more directions, there is better mixing). Recall from earlier discussions that one of the goals of a supply register is to mix the supply air with the room air as quickly as possible. Four way registers do this better than one-way, two-way and three-way registers.
The center of the room location works the best with a four-way, square register. Using another type of register can potentially lead to problems. I would never recommend a two-way or one way register in this location. It should also be noted that it is usually not possible to put the register in the very center of the room because there is often a light fixture or ceiling fan there. In that case, the register should be moved a foot or so toward the exterior wall. There is nothing wrong with having the register above the blades of a ceiling fan. In fact, if you really want to get the air in a room to mix, just run the ceiling fan while the AC is running! It’s as good as a blender. Hmmm . . . I wonder if anyone has tried putting the supply register directly above a ceiling fan and wire the ceiling fans to the AC fan so that they all run at the same time . . . hmmmmm . . .
The downside to this location is that there is about 4-6 more feet of ducting per register than option #2 (register interior to room), but less ducting than option #1 (register over window). In summary, for cooling dominated climates, option #3 has more upside and less downside than the previous two options.
Duc-Bloc register blocks for duct testing
HVAC 1.0 Book – Introduction to Residential HVAC Systems
Kwik Model 3D HVAC Design Software
Russell King, M.E. 
Aug 01, 2012 @ 09:30:38
Russ,
Very good information here!
Sep 22, 2012 @ 14:15:59
** In fact, if you really want to get the air in a room to mix, just run the ceiling fan while the AC is running! It’s as good as a blender. Hmmm . . . I wonder if anyone has tried putting the supply register directly above a ceiling fan and wire the ceiling fans to the AC fan so that they all run at the same time . . . hmmmmm . . .
I live in the desert and have ceiling fans in each room. But first, I would say most systems in the desert are over-sized because people feel “bigger is better” (another blog for you). It’s difficult to find a home in the desert that really works seamlessly without knowing the system is on, and short-cycling is common. Thus, the variations in the existing system designs have an impact on the theory. Let’s assume we’re talking about the poorly designed existing homes with added ceiling fans. Welcome to my world. Here’s what I have experienced with ceiling fans and A/C:
1) The fan definitely mixes the air better and helps eliminate dead spots. But when short-cycling, the rise and fall in temps seems more obvious because it throws cold air on you. However, because of all of this ill-thinking and poor design, most people find a cool breeze a pleasant relief after the off-cycle, and they welcome it.
2) The fan is typically left on continuously (see #3 below) and allows us to set the T-Stat a couple degrees higher. We’ll run at 82 – 84 degrees because it’s so expensive– and because summer temps are always in the 0’s or teens. When you come inside, 82 feels cold. Air movement from the fan makes the air “feel” cooler and is cheaper than running A/C. The fan makes up for the lost A/C.
3) I try to train the family to turn off the fan when not in the room, as it does not actually cool the room, it just “feels” cooler when it blows on your skin.
So, wiring the six fans in my house to the A/C would have six fans running continuously during the on-cycle. We are only in one room at a time, so that’s five fans running needlessly. Then, when the A/C cycles off your fan would be off at the time you need it the most. I sounds like you would be super-sizing your oversized equipment.
Sep 22, 2012 @ 18:42:13
Hi Roy, Yes, oversized equipment is a whole problem unto itself. One of the problems of oversizing is it makes the supply air much colder, which just makes this issue worse. I would address that problem before playing around with any of my frivolous mind wanderings!