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
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. 
Apr 10, 2018 @ 06:28:00
So just to clarify I have my master bedroom with two vents one over each window on my
Exterior wall, I should have a 2 way on both so that the air can sweep the room? Right now there are 3 ways above them pointing towards the Windows.
Apr 11, 2018 @ 07:15:58
Precisely. What you have is an old school of thought called “washing the windows”. The theory was that the windows are the primary source of heating or cooling load for the room and if you blow air directly on the windows, you address the load more effectively. The problem is that by blowing very warm or very cold air directly on the window you increase the load by increasing temperature difference across the window (delta-T). Heat transfer through a window is U x A x delta-T. U-factor is a fixed performance value of the window, A = area (doesn’t change), delta-T is temperature on one side minus the temperature on the other side of the window.
In a well-mixed room the indoor temperature might be 70 degrees inside and 40 degrees outside, delta-T = 30 deg. If you blow air directly on the window in heating mode, the inside temp at the window might be 90 deg or higher and the outside 40, delta-T = 50 deg. You just increased heat transfer through the window by 67%! You are basically forcing heat through the window. Heat that would be better used by mixing with the air in the room. A similar thing happens in cooling mode.
It would actually be an improvement to flip them 180% and have them blowing into the room, but it would not be ideal. 2-way registers blowing parallel to the windows would be the best.
Thanks for your question! ~ Russ
Apr 14, 2018 @ 18:50:08
Thanks for the quick response! Just one more question with the vents in my front Room should those also be two ways? They are over my backyard wall view windows.
Apr 14, 2018 @ 18:54:02
Also my front Room is open to the kitchen on the side where the ac vents are over the windows so for instance I did put two ways on them and one side the window closest to my kitchen one side of the two way would be blowing in the kitchen.
Apr 17, 2018 @ 13:10:45
Hi sry I know I wrote slit just to make
It clearer just trying to see what kind and what way should 2 vents in a front face being that they are over 2 windows and the front Room is open to half the kitchen. Thanks again.
Apr 17, 2018 @ 13:18:15
It’s kind of hard to visualize, but as long as they are not blowing directly on a window they should be fine. You can always change them around later if it doesn’t seem to be working.