Higher Tech Duct Diagnostics Stressed
One such company, National Comfort Institute (NCI), headquartered in Sheffield Lake, Ohio, gave a presentation at a seminar held by the South Carolina Association of Heating and Air Conditioning Contractors (SCAHACC). NCI President Rob Falke spoke on "Effective Duct System Diagnostics." The company markets its duct diagnostic training programs to HVAC contractors.
"If you are a typical HVAC contractor, the last system your company installed is only delivering 58 percent of the Btu's that you told your customer it would," he stated to a roomful of contractors whose interest in training was made apparent by their attendance.
Indoor air is generally invisible; however, that doesn't mean that it can't be measured from ductwork. Measuring it with appropriate tools, he stated, is the only way to know if the duct portion of the entire HVAC system is functioning properly. It can be a real blow to the ego at first, he said.
When the contracting company he used to work for started measuring airflow, Falke said he found a large discrepancy in the measured airflow and the amount he assumed he would find. "As they say in California, ‘Bummer, dude.'"
After being humbled and getting past the rebellion of his own ego, he said he started looking at airflow measurement as a challenge, a sport. "It's pure sport. You don't know what a system's doing until you measure it.
"All the engineering and design in the world isn't worth zero without proper balancing."
Difficult Questions
Measuring one's own mistakes means that a contractor must ask himself questions - "mean, nasty questions," Falke said. "They hurt real bad." For instance:Then there are the assumptions many HVAC contractors make:
Measuring airflow also can take the contractor out of the realm of selling a commodity - a box.
"When a contractor sells boxes, he sells a commodity," Falke said. "Manufacturers have done a great job teaching us to fix the box. It's time for us to get our heads out of the box and look at the duct.
"The duct system - that's your baby. The equipment is just a component."
When it's done right, customers get the Btu's they paid for, he continued; they get systems that operate properly, adequate airflow, low leakage, comfort, quiet operation, increased efficiency, and equal temperatures.
The contractor, on the other hand, gets satisfied customers, increased profits, increased referrals from satisfied customers, employee retention (employees worth keeping appreciate knowing that they are doing things correctly), and "little or no competition."
Diagnostic Tools
Falke cited the following as required diagnostic tools and instruments for measuring airflow:Air balancing hood - measures airflow from grilles and diffusers.
(Note: In areas such as toe-kicks, where the hood can't be sealed against the ceiling or wall, Falke recommended blocking either side with something like large, fuzzy dice. This, he said, is the extent of how far he ventures into low-tech solutions.)
Digital manometer - measures positive and negative air pressures in ducts, from room to room, and for taking traverses. Dwyer Magnehelic® gauges perform similar functions, he pointed out.
Hot wire anemometer - measures air velocity in grilles, filters, coils, and ducts.
Rotating vane anemometer - measures air velocity by use of a rotating vane or fan.
Static pressure tip and Pitot tube - measures static, velocity, and total pressure; metal tubes attach to a manometer with rubber hosing.
Thermometers - measure and evaluate temperature.
Smoke devices - test duct tightness and/or observe air patterns.
Amp/ohm/volt meter - measure motor amperage and voltage and test controls.
Hand tools - good drill with 3/8-inch bit, 3/8-inch test hole plugs, additional hood sizes, good flashlight, calculator, 6-foot folding ruler, two-way radios, clipboard.
How To Do It
"Cover the hole, mash the button" is Falke's simplified, tongue-in-cheek explanation of how to use some of the equipment. Of course it's more complicated, he pointed out, but not by much.Before a contractor uses an air balancing hood, he must read the manufacturer's instructions, Falke said. "Hoods have varying capacities, limitations, and reading characteristics." If a grille is in a place where a hood cannot be used - in tight bathroom spaces, for example - use an air traverse instead to get an accurate reading.
Cover the grille's open area completely with the balancing hood. Make sure you have a tight air seal. Then, read and record the cfm reading on the meter.
Hoods can be adapted to many types of field conditions, Falke explained. If the grille is too large for the hood, for instance, but the air velocity is less than 500 fpm, "it is also acceptable to split the grille in half, take two readings, and add them together for total airflow."
If the hood readings are questionable, double-check them by performing a traverse in the ductwork leading to the grille.
He identified static pressure measurements as the key to finding many system problems. "If you measure static pressure on your service calls, you will find more messed-up systems than you could ever fix."
Publication date: 11/17/2003
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