A relief fan that never stopped.

The symptom

The elevators started faulting and trapping people. The elevator contractor inspected both cars and every landing door and concluded that excessive pressure across the hoistway was holding the doors from closing. The elevators were taken out of service until they could be confirmed safe.

The first suspect was the air handler's supply fan drive, which had tripped after a power event — leaving the exhaust fans running with no supply air. But entrapments had happened at other times too, so the drive trip couldn't be the whole story. The owner brought Bluefin in to find out what was.

What the BAS showed

On the building automation system's air handler graphic, nothing looked alarming: supply and relief fans running, the outside air damper at its 5% minimum, building pressure close to setpoint. What the screen didn't show:

• The relief fan couldn't modulate. Its minimum speed was clamped at full speed, so it ran at 100% all the time. The building pressure loop had never actually been in control.
• The outside air damper wasn't at 5%. In the field it was fully closed. The only outside air the building got was leakage through the closed blades.
• The toilet core exhaust fan wasn't on the BAS at all. It ran around the clock, moving close to 8,000 cfm with no status, no alarm and no place in the control logic.
• The building pressure sensor was in the wrong place. It was referenced in the ceiling return plenum rather than the occupied space, so it couldn't see what the elevators were feeling.

As found vs. after commissioning

Simplified diagram, redrawn from the field BAS graphic. Building and owner not identified.

What was really happening

Instead, two fans were exhausting at once — the relief fan pulling plenum air straight outdoors, and the toilet exhaust running unmonitored — while the outside air damper sat closed. With far more air leaving than coming in, the building made up the difference through every crack in the envelope and through the elevator hoistway. That pressure difference is what was holding the elevator doors.

The fix

1. Relief fan locked out. It was running at 100% with no control, so the first step was to stop it.
2. Outside air matched to exhaust. The air handler's minimum outside air was set to match the measured toilet exhaust, roughly 8,000 cfm, so air in equals air out.
3. Pressure sensor moved to the occupied space. The building pressure reference now reads the space people actually occupy, with the loop targeting +0.01 in. w.c. to outdoors.
4. Economizer sequence rewritten. A revised sequence was issued so the relief fan runs only in economizer mode, modulating on building pressure — and the air handler, relief fan and exhaust work together instead of against each other.
5. Pressure monitoring installed. A differential pressure sensor in the hoistway core and building-pressure alarms in the sequence now alert the owner if the building drifts out of balance again.

The result

The building now reads neutral to outdoors. Outside air brought in matches what the exhaust removes, and the relief fan is back to its intended job. The underlying lesson: the controls were only managing the fans they knew about — and one of those was doing the wrong job.

What to check in your building

• Is every exhaust fan on the BAS — or at least accounted for in the air balance?
• Does your relief fan actually modulate, or is its minimum speed set so high it can't?
• Where is your building pressure sensor referenced — occupied space or return plenum?
• Does the outside air damper really do what the graphic says? Has anyone looked?
• Does minimum outside air cover the total exhaust leaving the building?

Related: Owner's representative & engineering advisory · Retro-commissioning