The Case of the Misplaced Air-Handling Unit

Dec 1, 2008 12:00 PM, By Leonard Greene, P.E., Orbital Engineering and Consulting

Maintenance technician makes shocking discovery while performing routine service call on middle school's A/C unit.

When the work order came across his desk, it seemed like any other routine maintenance call to the school district's service technician — certainly nothing out of the ordinary. The complaint was not uncommon for the old middle school that housed the problem: a noisy above-ceiling air handler.

After working for the district for several years, the technician had visited the building in question many times prior on various service calls. Arriving onsite just after the students had been dismissed for the day, he thought to himself it was probably just a bad fan belt. Little did he know that the air-conditioning (A/C) unit he was about to troubleshoot was not the same one he'd worked on in the past — a minor detail that would prove to cause him major trouble as the job unfolded.

Setting the stage

Approximately 50 years old, the middle school had been retrofitted for A/C by removing corridor plaster ceilings. Original air handlers had been installed close to bar joists in corridors and ducted to the individual classrooms. Then, lay-in ceilings were added to conceal the units and accommodate troffer-style luminaires.

At the time of the accident, unbeknownst to the service technician, these original units had come to the end of their expected service life and had just been replaced prior to the beginning of school that year. The school hired a large mechanical engineering firm, located in the state capital about 100 miles away, to perform the design work for this air-handler replacement project.

The engineering firm prepared contract documents for the equipment replacement for a relatively small fee. Design work was based on a site visit and out-of-date plans furnished to the firm by the school district. One additional site visit was made during construction after the ceilings had already been closed by the installing mechanical contractor.

The accident

Not privy to any of the recent updates, the technician arrived at the school ready to tackle the problem — with a 6-foot ladder in hand and replacement belts on his service truck. Once in the building, it was not hard to locate the culprit, as the unit on the second floor provided a robust acoustical signature. However, the technician did find it odd that the location of this sound seemed a bit off from where he'd recalled the unit sitting on earlier visits. Quickly dismissing this slight confusion, his next step was to locate the service disconnect, and kill the power before accessing the unit. What he did not realize was the noisy new unit was actually located 10 feet to 15 feet from where the original unit had been positioned.

Photo 1. Uneven spacing of troffer-style luminaires indicates missing unit at accident location.

Disconnects were typically located on one of the corridor walls above the ceilings within 10 feet of the unit. Because the units were 6 feet wide and only had a few inches of clearance above the ceiling tiles, removing a center tile and looking each way was not an option. Ductwork presented similar interference problems. After climbing up the fiberglass ladder, he began to move one of the tiles above the water fountain (Photo 1). As he slid it forward, his hand unexpectedly hit an exposed 277VAC conductor. The resulting shock caused him to fall off the ladder and sustain a severe head injury.

Although no one witnessed the accident, some staff members were still in the building at the time of the incident. Luckily, one of them came on the scene soon thereafter and called 911. Dazed but conscious, the man was transported by ambulance to a local hospital where doctors treated his burns and contusions. They determined and documented that the cause of his injury was due to electrical shock. Following his release from the hospital, the victim required extensive physical therapy and follow-up neurological evaluation. In fact, he was still receiving treatment two years later when the lawsuit that ensued finally settled.

After the accident, the victim filed suits against the design firm and mechanical contractor. I was retained by the victim's attorney to determine the cause of the accident and responsibility of all parties involved.

Investigation and analysis

Photo 2. Unacceptable conditions above the drop ceiling were still present at the time of the forensic investigation.

Upon investigation, I found the scene above the ceiling tiles quite disturbing. In addition to the abandoned items, uncovered boxes, exposed wires, and lousy cabling, the fire-rated corridor walls had multiple unstopped penetrations (Photos 2 and 3). It was obvious to me these conditions had not been observed by the consulting engineer during his field visits. Otherwise, he would have immediately required corrections based on his contractual responsibility and ethical duty as an engineer.

Photo 3. How about this workmanship? Note lack of fire stopping or fire damper at duct penetration.

I determined the cause of the accident rather quickly and easily — as well as confirmed the occurrence of electrical shock (see Accident Reenactment). The district investigation discovered a fixture whip that was abandoned and energized above the ceiling. Subsequent to my arrival, it had been found and made safe by an on-staff school district electrician (Photo 4). This whip had an overall length of about 6 feet with No. 12 AWG THHN conductors. Because the conductors protruding from the fixture plate were not covered with dust, I concluded this was the previous location of a missing luminaire. A check of the original lighting plans confirmed my suspicion — that a fixture had been planned for this location. Additional investigation revealed this whip was still hot and part of the corridor lighting circuit controlled from a panel in a locked electrical closet.

Photo 4. The exposed 277VAC conductor (shown after new twist-on connectors were added to the exposed live wires the HVAC technician made contact with during his investigation).

Based on my discoveries thus far, I believed the evidence overwhelmingly supported the conclusion that a luminaire had been removed for the purpose of installing the new air-handler unit. Challenging my assertion that removal was recent and coincidental to the installation of the new air handler, the defense promptly questioned when it had been removed and by whom. In countering, I showed the air handler would not have fit in the ceiling space had the luminaire remained in place (Photo 5).

Photo 5. Could you fit a lighting troffer in this space? No way!

My opinion that the lack of dust on the fixture connector plate compared to very dusty plates in other areas also came subject to severe challenge by the defense, as they promptly questioned my level of “dust” expertise. In response, I simply stated that the difference was obvious. You don't have to be a weatherman to know which way the wind blows! Furthermore, other images I produced during my four-day deposition clearly showed fingerprints on the exposed side of the plate that were very fresh in nature, but with enough dust coverage to rule out recent handling.