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The era of the typical residential fan being rated at .50-in. is coming to a close. With the increasing pressure drops over today’s cooling coils and filters, it’s not uncommon to find filter and coil pressures exceeding rated fan pressures…that’s excluding the pressure drop of the duct system! The result is fans that are delivering dangerously low airflow, which results in poor system performance and shortened equipment life.
If you think the little green emblem on your equipment is an assurance that the fan has the capacity it should, it’s time to take a closer look. If you’re sure equipment that is AHRI rated, and matched equipment had “someone” check the fan capacity to be sure the fan capacity matches the coil pressure drop, check again.
Nobody’s got your back on this one. It’s time to take a closer look at some engineering data and to be sure your system will actually operate as you think it will, once it’s installed in the field.
Who’s Fault Is It?
The EPA and DOE don’t seem to understand the problem. Few in committee meetings seem to care. The prime objectives are Higher Efficiency Equipment Ratings and don’t upset the apple cart. Everyone at the top is too busy keeping their plates spinning and their programs funded to slow down and see what’s actually happening in the field. The focus is on the refrigerant cycle being the end-all to verification, not actual system operating efficiency. So when it gets down to it, systems installed in the field are not saving energy or keeping consumers comfortable, and its up to us to change that.
We design and install the systems, so we’re going to have to provide our own solutions for the problem. The buck stops here. We’re on our own.
What Actually has Happened?
When a job begins, a typical contractor simply chooses the equipment based on the highest equipment efficiency matchups assuming the manufacturers or distributors checked to make sure this equipment would actually function together when installed in the field.
At the same time, the manufacturers believe that we follow industry design practices and will be sure to design and install humongous duct systems to compensate for their undersized fans. ACCA manual D even states that “the duct system is to be sized to match the blower that comes with the equipment.”
Oops! A little assumption here and a little assumption there and the system is a mess. There’s no way the typical system can possibly function as claimed in the manufacturer’s specifications or as the contractor promised their customers in the field.
Real Numbers
Here’s a scenario that will bring this situation into focus.
There’s a 95 AFUE furnace coupled with a 19 SEER coil and condensing unit. The fan is rated for .50-in. total external static pressure; it says so on the equipment nameplate.
In essence, the manufacturer is saying: “Here’s some amazingly efficient equipment. However, right here on the nameplate, as we individually tested these components, we left a little note that warns, if the pressure drop of the coil, the filter, the plenums, the ducts, the fittings, the dampers, grilles and registers exceeds .50-in. this system isn’t going to work.”
However, if we look at the coil rated with this system, the wet pressure drop at the rated system airflow is .41-in. Add to that the manufacturer’s top quality high efficiency super duper air filter system rated a pressure drop of .32-in. and the total external static pressure of this equipment when installed in the field will be .73-in. This is almost 50% in excess of the rated fan pressure.
Then add it to a typical residential duct system with a national average pressure drop of .32-in. and this system will be operating in the field at 1.03-in. of total external static pressure. Exceeding the manufacturer’s specifications by 200%!
This system, although qualifying for the maximum in state and federal energy and tax incentives, will operate far below the manufacture’s listed efficiency and well below the assurance of energy efficiency implied by the government assurance expressed by the incentive program to their citizens.
Who suffers? The consumer. And they’ll unknowingly suffer for the next 20-years unless some savvy service tech measures system performance and diagnoses the problem. Few verification programs will identify the problem. Get the picture?
See Why .50-in. Fans Gotta Go?
As years passed and the push continued for higher efficiency equipment, and better filtration, the scales tipped and nobody caught it. Everyone forgot to set a standard requiring that the fan capacity has to increase to compensate for the more restrictive coil and filter pressure drops.
Will manufacturer’s self police this issue? Don’t count on it. Will our regulators finally take a realistic look at this issue? Don’t bet on it, my friends. Our best choice is simply to educate each of our customers to the reality of the situation, so they will eliminate the low bidders that will try to slip in poorly performing equipment and attempt to compete with us.
How’s that for self defense?
Quick Test
One way to get a quick peek at the static pressures in an HVAC system to help you select the right fan, is to use the following formula:
Add together the rated pressure of the cooling coil when wet, plus the rated filter pressure drop (check the manufacturer’s engineering data) plus the estimated pressure drop of the supply and return duct system. The total of these pressure drops will tell you the absolute minimum pressure rating of the fan needed to move adequate airflow through the system.
Here’s an example:
Wet coil rated pressure .31-in.
Filter rated pressure .15-in.
Supply Duct Pressure .16-in. (National Average)
Return Duct Pressure .16-in. (National Average)
Total .78-in.
Select a fan for the system that can deliver the required system airflow at a minimum of .78. So, probably an .80-in. variable speed fan. Then measure the operating system static pressure when the system is built to verify actual system performance.
One Last Note
There will probably always be use for some equipment made for condos without duct systems or furnaces used for heating only with fans rated at .50-in. But generally speaking, when selecting a fan for a ducted system with a cooling coil and a higher efficiency filter, carefully consider a good quality variable speed fan. When you do the math you’ll find most systems will do best with a fan rated near .80-in.
Rob “Doc” Falke serves the industry as president of National Comfort Institute, a training company with technical and business level membership organizations. If you're an HVAC contractor or technician interested an procedure to help redesign a retrofit HVAC system, contact Doc at [email protected] or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, articles and downloads.