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School of Thought #3: Register in Center of Room

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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.

 

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School of Thought #2: Register Interior to Room

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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!

School of Thought #1: Register Over the Window

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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 Above the Window.  By the way, unless I say otherwise, I’m focusing on cooling mode on a very hot day.

Putting a register above the window seems to be one of the most common locations in homes for many, many years.  It also seems to have the most ardent and dedicated (aka, stuck in their ways) practitioners.  Having put about 2000 residential HVAC designs to paper, I’ve received a lot of, shall we say “comments” about my plans.  No matter where I put a register, there was always an HVAC contractor who did not like that location.  The one location that most contractors would insist on was over the window.  The reasoning went from logical (this directly addresses the major load in the room), to rule of thumb (I was always taught that you had to “wash the windows”), to experience based (I’ve been doing it this way for 30 years and it has always worked fine), to nutty (it pushes the heat/cold back out the window).

When done correctly it can be very effective and maintain good comfort, but it does have some serious drawbacks.  The correct way to do this option is to use a two-way register oriented parallel to the window.  alternatively and bar-type register can be used with the air directed in a manner similar to a two-way register.  Using the wrong register can seriously screw this option up.  I’ve seen three way registers located here, but blowing back into the room or worse, blowing directly on the window.  Both of these can result in serious comfort and energy issues.

The down sides to this school of thought include:

  • compared to other locations, it requires the most ducting, which increases materials costs, conductive losses, and pressure drop.
  • If the roof pitch drops down over the window, the register boot can be very close to the roof decking.
  • Because the air only comes out in two directions it doesn’t mix as well and can cause cold spots if directly in the path of the airflow.
  • If located too close to the window, it can blow air directly on the window.  This increases the delta-T across the window, increasing conduction through the window.

Next Post:  School of Thought Number 2 – Interior to Room

The Four Schools of Thought on Ceiling Register Locations

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As promised, here are the “Four Schools of Thought on Ceiling Register Locations”.  See diagram below.  OK, so the fourth option is technically not a ceiling register, but it is often used as an option.

First some assumptions: 1. slab on grade house, floor registers are not an option; 2. It is a standard sized bedroom with a door on one side and a window on the opposite wall; 3. the return grille is outside the room; 4. the diagrams show cooling airflow (heating would be similar.

The blue arrows are the primary flow direction, which is the direction of the airflow caused by the velocity and momentum of the air as it leaves the register.  The purple arrows show the secondary flow direction, which is the direction of the airflow after it has lost its initial velocity.  Another way to think of secondary flow direction is the direction the air already in the room moves right when the system fan turns on.  Generally speaking, this is the direction the air is going to go to reach the return grill.

I’m sure all of you are dying to know which one of these is the best!  Well, I hate to burst any bubbles but, when done correctly, they all are very, very close.  They all have pro’s and con’s, which will be discussed over the next few posts, but in terms of comfort, which is based on even temperature in the room, they are all very close.

Notice that I was very careful to say “when done correctly”.  It is very easy to screw up any one of these by using the wrong type of register.  The first option should always be a two-way register, never a three-way or one-way.  The second option should always be a three-way, but one-way registers can work too.  It should never be a two-way.  The third option should always be a four-way, possibly a three-way or circular (radial) register.  The fourth option should always be a bar-type register directed perpendicular to the wall.