March 23, 2016

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

Editor’s Note: This post is the first in a series entitled “Matthew’s Mysteries.” Each of “Matthew’s Mysteries” tells the story of a conundrum encountered by Matthew Smith, PE, Cleary Zimmermann Design Director and Associate Principal, a registered Professional Engineer with both mechanical and electrical designations.

Each “Mystery” will be presented in two parts: the mystery, and how it was solved. We invite you to play along to solve the mystery and contact us with any ideas, questions, or your own mysteries that need solving!

Several years ago we had a church project where we put typical non-insulation contact down lights, which have holes in the top of them to allow the heat from the lights to vent. We put those lights in a typical restroom with an exhaust fan. The areas above and adjacent to the restroom were meeting room space, storage, and space for minor church functions. All of these areas were plenum return, meaning that the return air that comes through the space would go up and into the return systems of the air conditioner without being funneled through a duct.

The owners constructed the project and noticed right away that when you first opened door to go in the restroom the lights were on bright, but when you closed the door behind you the lights would dim. When you reopened the door, the lights would get bright again. The owners then called us asking, “Why did you put a dimmer in the restroom? When we close the door the lights get dim. They don’t turn off, but they just dim really low.” I responded, “Why would I dim lights in a restroom?” to which they replied, “We thought the same thing.”

I asked Jerry Katz, former Director of Construction Administration, why would our lights be dimming when they go in the restroom. He replied that he had never heard of that phenomenon before and he had been doing construction longer than I’d been alive.

The next day, we went out to the church. We went to the restroom, opened the door, and found the lights were nice and bright. When we closed the door from the inside the lights dimmed substantially, just as described, so much so that we could barely see each other.

So we looked at lights, and found nothing wrong with the ballast. “Do we have the wrong lamp?” we wondered. Nope, nothing wrong with the lamp, it worked just fine. The contractor had replaced two of them just in case, with no change. It wasn’t particularly cold in the space, which can cause light output losses in the tubes of fluorescent lights.

Spectrum Lighting had provided the fixtures so we talked to Spectrum, but they had never heard of this phenomenon either. Spectrum then called the manufacturer who said, “It sounds like you’ve got them in a cold environment.” So we had to go back and look at what was really happening…

Do you have any idea why the lights could be dimming? Check back next week to see how Matthew solved the Mystery of the Self-Dimming Lights!

As told by Matthew Smith, PE to Joyce Fienman