Why Is This Thing Dumping Air?

Titus Chief Engineer, Randy Zimmerman observes that after many years of solving comfort issues, he’s found that the most important step is knowing exactly what you’re dealing with and don’t make any quick assumptions.

That was the question being asked by a legal assistant who happens to be married to one of my co-workers at TITUS. The thing that she is referring to is a TITUS model PTQP. This is a parallel fan-powered terminal unit with pneumatic controls. See the accompanying photos that she snapped. The unit in question is oddly enough the only unit mounted in a fully exposed manner, with all the other similar units mounted above finished ceilings.

Her comfort complaint involved cool air dumping from the induction port into her workstation located directly below. A MERV-rated filter was subsequently fitted to the unit. It seemed to reduce the amount air leaving the induction port, but it didn’t really solve the problem.

So, what would cause this situation to occur?
As most readers are probably aware, a parallel fan unit supplies cold primary air to the temperature control zone during the cooling mode. The unit fan does not operate during the cooling mode. As the cooling demand is reduced, the primary air damper begins to close, eventually reaching the minimum ventilation airflow limit. On a further drop in room temperature, the unit fan is energized to pull room air into the induction port. Any further drop in temperature would cause any optional heat to begin sequencing on. This unit happens to have an electric heating coil on the discharge.

Most parallel fan units have an issue with air leakage – some worse than others. Unlike series fan units where casing pressures vary from neutral to slightly negative, parallel units typically have at least half of the casing under positive pressure during the cooling mode. This is problematical because fan terminal casings often have removable access doors, knockouts, wiring passes, and other sources of leakage. And unlike series units, parallel units typically include backdraft dampers. The purpose of the backdraft damper is to prevent primary air from traveling through the blower and out of the induction port when the unit fan isn’t running.

Because the backdraft damper typically relies on gravity to get a reasonably tight closure, units mounted upside down usually result in backdraft dampers that cannot close. Obviously, this would result in massive air loss unless modifications were made during the installation process. That’s not the problem here. The orientation markings indicate that this unit was mounted in the orientation for which it was intended.

Here’s the answer to the question.
I would presume that the air dumping from the induction port is due to a backdraft damper that is not getting a reasonably tight seal. They never really get a tight closure, but if a strip of gasket is missing or the backdraft damper is getting caught on surrounding insulation, that would explain the air leakage. And the addition of the MERV-rated filter simply added some pressure drop which would reduce the amount of air traveling that direction. The recommended fix would be to remove the bottom access door located closest to the unit discharge, so that the backdraft damper can be inspected, and any necessary repairs made to allow it to close properly.

When this question was originally posed to me, the first thing I wanted to know what the unit type. I couldn’t tell from the photos whether it was a series or parallel unit, and my answer would be very different depending on the unit type. If this had been a TITUS model PTQS series fan unit, I would suspect that the air was exiting the induction port due to a balancing issue.

Series fan units have unit fans that run throughout the occupied mode, typically providing a constant air volume to the space being served. The primary damper modulates between minimum and maximum airflow limits in response to a thermostat to regulate the amount of cooling being provides. As the damper closes, the unit fan draws make-up air through the induction port. The unit fan is typically set for an airflow matching the primary maximum flow, so that at design cooling the induction port should be under a neutral pressure condition with little to no air being drawn in.

When service technicians or building personal observe air blowing out of an induction port, they often wrongly assume that the unit fan is running backwards. In rare situations it’s possible for a forward curved fan to run backwards, but even if it does it will still move air to the room – at a about 50% of it’s balanced airflow. So, the fan won’t blow air out the induction port, but primary air that isn’t being delivered to the room must go somewhere. In this situation, the primary air follows the path of least resistance and dumps out the induction port.

Since it’s rare to see a blower running backwards, a more likely scenario involves an improperly balanced unit. Let’s say that that a controller was replaced, and the installer didn’t know how to set the airflow limits. They might just get it working so that the damper modulates from full open to full closed and walk away not knowing that flow limits are required. So, on a call for full cooling, the primary damper goes full open delivering far more air than the unit can move, and the excess air dumps out the induction port.

After many years of solving comfort issues, I’ve found that the most important step is knowing exactly what you’re dealing with and don’t make any quick assumptions. Experience can save a lot of time getting to a solution but no matter how experienced you are, you may run into a situation that you’ve never encountered before. And I guess that keeps things interesting.

Read the full blog post here.