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 Interior to Room.  By the way, unless I say otherwise, I’m focusing on cooling mode on a very hot day.

This is also a very common location.  Though it is not my preferred location, this is where I located most of the registers in my designs.  This location does not have such die hard supporters as the register-over-the-window location.  It does however have a lot more benefits when done correctly.  Again, it is very easy to screw these up. To do this location correctly, it should be a three-way register or the less common one-way register.  It should blow toward the wall opposite of the door and/or toward the primary load (window).  I highly recommend a curved blade register that allows the air to hug the ceiling more, like this one:

Rather than one like this one:

I also highly recommend that you find some register manufacturer catalogues and learn how to read them.  There are some very important performance factors that you should understand.  Throw distance, sound rating, pressure drop, etc.  It’s more complicated than you think.  ACCA has a Manual T (Terminal Selection) that discusses all of these, but it is a bit out of date.

Like I said, it’s easy to screw these up.  I’ve seen two-way registers put in this location.  Not good.  Half of the air goes right out the door.  The other half never helps the room volume near the window.  If you notice in the top view diagram, above, some of the air goes right out the door.  I call this “short circuiting”.  It would be better if that air stayed in the room longer.  The one-way register does not have this problem.  It can be minimized in the three-way register by using the adjustable control dampers behind the face blades to direct the air toward the exterior wall.  I have set up a pretty simple test rig using a duct tester fan and a fog machine to show that you can get most of the air to go that direction without reducing airflow significantly.

I almost forgot!  I couple posts ago I posed a little quiz question:   What’s better for heating a room, floor registers or ceiling registers, and why?  Most people will say that floor registers are better because hot air rises.  Sorry, that is incorrect.  Yes, hot air does rise, but you have to remember the sole purpose of a supply register: to efficiently and effectively MIX the conditioned air with the room air.  One very good rule of thumb (as much as I despise most rules of thumb) is to blow the air in the opposite direction that it will naturally want to go.  If hot air comes out of a floor register it will go up . . . and stay up.  This does not promote good mixing.  In fact, it promotes stratification.  If you blow hot air downward, it will reach close to the floor (with a properly selected register) and then begin to rise, but by that time it has mixed with the room air making it less likely to stratify.

This same rule of thumb can work for a register in a room.  Blow the air in the opposite direction that it will naturally want to go.  In a typical room the natural direction is out, toward the door, back to the return.  Assuming that the return is out in the hall, better mixing is achieved by putting the register near the door and blowing it away from the door.  This is why the interior register tends to work better than the register over the window.  Another benefit is less ducting, which equates to less resistance to airflow and conduction.

Next post: School of Thought #3 – Center of Room.  The most common location for new homes built in Las Vegas!

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