I need your help on this one,” an HVAC technician said. “I did a tenant fitup on the seventh floor of an office tower this spring. All I did was ductwork and diffusers, and I installed it exactly the way the engineer showed it on his plans. Now everything's going wrong, and they're blaming me. The air handler won't make as much static pressure anymore, and condensate doesn't drain from the cooling coil, so they get a flood in the mechanical room.
“I didn't touch the air handler,” the technician continued, “so this really shouldn't be my hunt, but I've already spent two days up there trying to figure out what's going on, and I'm coming up blank. The only thing I see that's different from the old design is that the engineer increased the airflow from 16,000 cfm to 20,000 cfm. I don't see how that would cause these problems.”
I took as many measurements — static pressure before and after the filters, static pressure at the coil and fan intake, discharge pressures, fan speed, airflow through the unit — onsite at the air handler as I could.
The static pressure leaving the fan was low, and there was enough suction pressure at the fan to prevent condensate from draining. I measured the condensate drain carefully, consulted the manufacturer's fan-performance chart, and rendered my verdict.
“You're off the hook,” I told the technician. “When I measured the airflow, I came up with just about 20,000 cfm, so you're right at design. The low-static-pressure issue is simple: When you increase airflow from a fan, the static pressure goes down. If you cut the airflow back to 16,000 cfm, the static pressure would go right back up. As long as you're getting enough airflow, no one should care that the static pressure is low. If they really want static pressure, they can speed up the fan.”
“On the condensate issue, the condensate trap is shallow to begin with. That wasn't a problem with 16,000 cfm. But when you increased the airflow to 20,000 cfm, that 25-percent increase in airflow made the suction pressure at the fan intake 50-percent greater. That means you need a 50-percent-deeper condensate trap. Make the condensate trap 11/2 in. deeper, and the condensate will drain like it's supposed to.”
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