A new step has been added to effectively interpret manufacturer’s rated total external static pressure. Listed static pressure on the nameplate used to be quite close to the fan capacity found in the fan tables. Not so much anymore. Let’s take a look at the change in fan pressure ratings and why you should be checking the manufacturer’s fan tables.
What we’re seeing in residential air moving equipment is .50-in. w.c. nameplate static pressure ratings on good quality variable speed motors. However, when we refer to the published fan tables for the same equipment, we’re seeing the fan’s ability to deliver 400 CFM per ton up to .80-in. w.c.
Nameplate or Tables?
So which ratings should we use; the nameplate static pressure rating or the published fan tables? From what we’re seeing, they’re telling a different story these days.
It would be easy to glance at the nameplate and see the rated static pressure and then assume as long as the measured static pressure is lower, that we have adequate airflow. Although this is still true, it’s rarely that simple.
With today’s coils and filter pressure drops, the measured total external static pressure will exceed .50-in. w.c. about 85% of the time. So, the need to reduce static pressure is inevitable.
This may not always be the case. If you take a few minutes and pull out the manufacturer’s fan tables on this equipment, the odds are that you’ll find that many of these fans often perform well above the nameplate static pressure. You can be assured of adequate airflow up to .65-in. w.c, .75-in. w.c. and even .85-in. w.c. in some cases.
However, in the field, few of us carry around the several thousand pages of fan tables so we can look up the fan performance on the spot. As far as I know, all this data has never been assembled in one place. If you have it, please give me a call.
But a growing number of savvy techs are carrying paper copies of fan tables they frequently see in their region of the country and many more have taken the time to find and secure passwords for their favorite manufacturer’s websites. These online gold mines contain much of the needed fan tables. A couple of clicks on the iPad or your favorite phone and the fan CFM is right before your eyes.
There’s even a new Droid and iPhone app that “sees” the bar code and can bring up the equipment engineering data in as little as 20 seconds.
Not so Fast
I wish it were this easy. We’ve heard from across the industry that just because the fan may be listed to deliver more airflow, there may be other concerns to check before advising your client that their system is operating as it should.
Since heating airflow will increase as much as 50% from an old natural draft furnace to a condensing furnace, be sure to check furnace rated temperature rise and be sure to set your fan speed correctly in heating mode.
Also, we hear rumors that some manufacturers rate their equipment efficiency based on the extremely low watt consumption of fans when static pressure remains below .50-in. There is a push in California for 500 CFM per ton in cooling mode due to the low latent loads in some weather regions of the state. We have clocked some fan wattage at 500 CFM per ton at more than three times the rated fan wattage under AHRI test conditions. That will turn an 18 SEER into a 12 SEER immediately. Be careful.
Some equipment may have coils that will experience condensate blow off if static pressure exceeds certain parameters. Always check your engineering data for blow off tables.
You’re the Judge
When testing system performance in the field, remember you’re the one that has to render a judgment of the performance of the system. If you don’t solve the problem, consumers will find someone else who can.
One thing is for sure, it’s up to you to check your engineering data that is published for each piece of equipment and uncover manufacturer’s intent and match it in the field.
If all else fails, and you’re not sure what your manufacturer means by a lower nameplate rating than the fan tables show the fan can do, give them a call and discuss the matter. It’s bound to be educational finding someone that really has a handle on fan performance and that you can have a good discussion with. You’re sure to learn something you can carry with you.
Manufacturers are welcome to comment on this subject and help us learn your perspective about the difference in nameplate and fan table static pressure ratings. We are eager to learn and invite the feedback.
Until we hear differently, it seems the best practice is to check the manufacturer’s fan tables for the most accurate airflow and static pressure rating information. It’s time to look past the nameplate maximum static pressure rating. It’s an extra step, but will pay big dividends in the problem solving department.
We all know that many of today’s coil and filter pressure drops when added together exceed .50-in. w.c. If that’s true, it’s impossible to build duct big enough to keep total external static pressure below .50-in. w.c in that case.
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 in how to read a manufacturer’s fan table, 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.