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

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