Interpreting Duct Static Pressure

July 30, 2007
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The air speed going into the single return grille was over twice the 400 feet per minute recommended by ACCA. The undersized return created noise and high static pressure.

[Editor’s note: When we last visited with lead installer John and his helper Charlie (in the June 11 issue), they had learned how to measure external static pressure on a new system they had just installed. They were shocked to measure a high 0.98 inches of water column - and surprised when their trainer David asserted the airflow through the unit was probably OK.]

“The half-inch rule of thumb for maximum static pressure doesn’t always apply. What kind of indoor blower motor is in this unit?” asked trainer David, pointing at the “V” in the model number.

“Oh, yeah, variable speed,” answered installer John. “They can handle a lot more static, right? And I set the DIP switches for 400 cfm per ton based on the compressor size. So are we actually OK then?”

“Well, yes, and no,” answered the trainer. “From an airflow standpoint, you’re probably barely OK. Most electronically commutated motors (ECMs) will deliver the airflow up to 0.9 inches, and usually a bit more. But there’s a lot more to consider. This is definitely not a happy system. There are a lot of reasons to get this static pressure back down.”

FIXING A NOISY SYSTEM

“First off,” said David, “have you noticed we’re having to speak kind of loudly?”

“Sorry, what did you say?” joked John. “But, seriously, I did notice this new system seems noisier than the one we pulled out. Why is that? Is it caused by the high static pressure?”

“Partly,” replied David.

He explained there were two key reasons for the noise.

“The high static pressure is causing this poor variable-speed motor to ramp up to maximum revolutions per minute (rpm) to get the programmed airflow. More rpm means more noise.”

He then pulled out his anemometer. He placed it over the single central grille under the platform and measured an average of 849 feet per minute (fpm) air velocity.

“I think the other big noise maker is this high air velocity at the grille,” said David. “ACCA Manual D tells us this should be under 400 fpm. After sealing the ducts, and going to a variable-speed motor, there’s now a lot more air going through this single return grille. Too high of a velocity like this, and you get air noise.”

“So should we drop it back to a lower airflow setting?” asked Charlie.

“No, we should give it a tracheotomy,” replied David.

“A tracheotomy?” questioned John. “Isn’t that when you help someone who’s choking by cutting a hole into his wind-pipe … oh … I see what you mean! We need to add a hole. We could cut in a second return grille over on this other wall into the return box.”

“Exactly,” replied trainer David. “I’d literally like to see double the return air grille size they have now. A good rule of thumb is to aim for 1 ¼ to 1 ½ feet of return grille per ton of air conditioning. By the way, one clue that the return grille is too small is that 0.70 inches out of the total 0.98 inches is on the return side. Let’s see how much of that is caused by the grille versus the media filter.”

David reinserted the two static probes, one on each side of the brand new filter, and measured a drop of 0.15 inches. He pointed out that although it was currently 0.15 inches, the pressure would rise as the filter loaded up. This was another reason to get the total static down. He then returned to the subject of the grille.

“So if the total static on the return side is 0.70, and we know that 0.15 of that is caused by the filter, how much is caused by the rest of the return system?” asked David.

Charlie answered correctly that it would be 0.55 inches, which they then confirmed by placing one probe at the inlet to the filter, the other in the room in front of the single grille, and getting a reading of 0.55 inches.

“In addition to adding a second return, I’d replace this old stamped return grille that’s been bashed by the vacuum cleaner over the years. Use a new low resistance grille with more net free area. This will cut the velocity and noise at the grilles, cut the static pressure, lower the rpm and also save energy,” said David.

David recommended adding a second return, and replacing the original stamped grille with a less restrictive, more aerodynamic design.

LOWER STATIC PRESSURE SAVES ENERGY TOO

“I can see how added return will help with the noise and static pressure,” said John. “But how exactly will it save energy?”

“Two ways: the circulating blower motor will use quite a bit less energy at a lower static,” said trainer David. “A good rule of thumb for an ECM is it uses about 40 percent more electricity to move the same amount of air at 0.90 inches than at 0.50 inches. And the customer actually pays for that wasted electricity twice: once to make the air move, and then again to run the a/c to remove the heat Btus the electricity turned into.”

“Makes sense,” said John. “What’s the second way adding return will save energy?”

“It will reduce the pressure across those return duct leaks that you didn’t manage to seal,” answered the trainer. “Air is proportional. In essence, if you make the good holes bigger, the bad holes don’t have as much significance.”

“OK. Although we’re going to seal everything we can, I can see that some small leaks will still be there,” said installer John. “So getting the static pressure down is actually a way to reduce duct leakage. Interesting angle. I’d never considered that.”

“But that brings up a point,” said Charlie. “Was this high static pressure actually caused by the duct sealing we did?”

“Static pressure always goes up somewhat as an inherent side effect from sealing ducts,” answered David. “Some air used to enter the system enters through the bad holes. Now it’s all trying to move through the good holes - by that I mean the grilles and registers. With regular permanent split capacitor [PSC] motors, airflow through the coil usually drops at least a little after duct sealing. But in some cases, it can drop a lot, especially if the ducts were way undersized to begin with.”

“So did the airflow go down because we sealed the ducts?” asked Charlie.

“Actually, with this new variable-speed motor, this system’s probably moving more air now than it did before,” said David. “The ECM is probably the bigger cause of the high static we’re seeing. When it sees undersized returns or other restrictions, it ramps up to compensate. In fact, it’s a good thing you did seal the ducts, otherwise the higher pressure caused by the ECM would have caused the ducts to leak even more.”

DUCT RENOVATION, NOT JUST DUCT SEALING

“So bottom line, what do we do now?” asked John, before answering his own question. “We need to bring the high static pressure and undersized return grille to the customer’s attention, and offer to solve it. Would have been better to recommend this before you sold the job, but still, this is their problem, not yours. The return has always been undersized. It’s just showing up more now that the ducts are sealed and the airflow is right.”

David then gave them some tips on how to approach future jobs.

“Most duct systems need more done to them than just sealing, especially if the airflow was already low before the repairs. We prefer to use the term duct renovation, not duct sealing, when describing what we offer. In this case, it would have been good to recommend added returns as part of the job.

“Sometimes the supplies need to be straightened out, or fittings replaced. With attic systems, it often makes more sense to start over than to try to patch up a dirty, old, leaky, undersized, poorly insulated system. Testing it for leakage, pressure, temperatures, and airflow will guide your recommendations.”

After a pause, David added, “By the way, as part of the commissioning after a duct renovation, you’d better check the refrigerant charge. Even if it had been adjusted before, once you get the leakage solved and the airflow right, it will likely need to be changed. And, finally, always measure building and combustion zone pressures to ensure there isn’t a negative pressure problem.”

So John and Charlie learned again that there’s more to fixing ducts than just painting on the mastic. A true duct renovation requires looking at the whole picture.

Publication date: 07/30/2007

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