April 6, 2016

Matthew’s Mysteries – The Case of the Self-Dimming Lights Part II

Editor’s Note: This post is the second half of “Matthew’s Mysteries – The Case of the Self-Dimming Lights” that was originally published on March 24, 2016. To read Part I, click here.

As we continued to examine the lights and what could possibly be causing them to dim when the door closed, we looked at the path and realized what was happening was caused by the plenum space above the restroom. This restroom had plenum return instead of fully ducted return. In a fully ducted return, a duct comes down to a grill in the space and pulls the return air from that space through the duct. In plenum return, there is an opening in the ceiling where the return air will flow to the space above the room so the air handling unit will draw air from the open space.

When we closed the door, essentially the suction from the air handler, which was pulling off the plenum, was trying to pull air out of the restroom through the holes in the lights. As the air handler tried to pull enough air through the small holes in the lights, the speed of the air going past the vertical fluorescent lamps themselves increased greatly.

(Think of it like a household vacuum, when you put on the smaller nozzle attachment, you get the same pull, but the speed increases. When you take it off, you have the same pull but it goes much slower.)

This forced convection had a huge heat transfer coefficient compared to the natural convection that you usually have across lights. We were, however pulling air directly across lights that weren’t designed to handle it. (Return air lights have a path for return air that is separate from the lamps themselves, but are rarely used anymore due to their inefficiency.)

With HVAC systems, if you’re pushing air into a space you need to have a way to get it out to create true circulation. If you push air in and don’t have exhaust or a return path, it would be stagnant in the room. HVAC systems use an incremental cooling process, where air is put in and pulled out until the space reaches the desired temperature through entraining, or the mixing of air in the space

The air speed next to the lamp was so fast and had a much greater heat transfer coefficient, and was cooling the light so much that it was essentially creating a cold artificial temperature around the lights like all of a sudden you put the lights in a refrigerator. Since the fixtures had compact fluorescent tubes with very small amounts of gas in them, it dimmed the lights.

So how did we fix it? Two ways:

  1. We essentially created more exhaust in the toilet room so we could create more of a negative space in there to compensate.
  2. We also created an easier path for the air handler to pull off the other spaces.

The pull from the AHU in that space was stronger than the pull from the exhaust fan so instead of exhaust fan drawing air out of the space, the air handler was trying to suck air out of the space. We had to make a path that was easier for the air handler to get air from the other spaces and make sure that the exhaust fan was strong enough to pull air out of the space. We didn’t want to put a louver in the door due to privacy considerations, so we ended up making a deeper undercut in the door. This allowed the exhaust fan to pull air into the room better, and we increased the amount of air supply and adjusted in the other spaces. We replaced the small egg crate grills that we had originally scheduled with larger egg crate grills to create an easier way for air to get back into space. We created an easier path for the air to get back so it didn’t try to pull air across the lights.

And so solves the mystery of the self-dimming restroom lights!

As told by Matthew Smith, PE to Joyce Fienman