A recent conversation with a high quality HVAC contractor reminded me that every piece of equipment (and people for that matter) has their limits. A 1/2 ton truck can only pull so much weight, a man can watch only so many movie award shows, and a Permanent Split-Capacitor (PSC) motor can overcome only so much static pressure. The conversation started centered around figuring the design friction loss for a newly installed unit. For those seasoned system designers, you should be aware that this very important number is traditionally figured prior to duct installation. Anyhow, in order to calculate the friction loss, one must know the available blower static pressure and the total equivalent length of the longest duct run.
Here in lies the problem: How can a blower overcome more static pressure than it is "designed" for?
Most technicians have looked at furnace manufacturer data plates to write down model numbers and even find the required temperature rise. System designers usually peer over manufacturer ratings and instructions, long before the homeowner makes that last payment - never mind by the time a technician completes a service call. The issue is that many system designers and technicians overlook an important detail on every forced-air furnace's label: Maximum Total Static Pressure. This is the highest amount of restriction the motor can overcome before seeing performance issues, like frozen a/c coils for instance. When designing the system, and figuring the available blower static, it is most important to be able to deliver your design airflow (CFM) at half of an inch of water column (.50" w.c.) on medium speed. All PSC blower motors have manufacturer blower charts that generally end at .50" w.c. and have clearly labeled data plates stating this maximum restriction.
Sounds like a lot of pressure to go around, right? Well, the maximum is not all that is available. What if we added a new high efficient 4" or 5" pleated filter? This is a restriction in our airflow, and the higher the airflow the more of a restriction! In this particular example, we lost .20" w.c. for the air filter, when it is clean and new! Also, since there is ductwork on that furnace, why not add an evaporator for air-conditioning? That's right, another restriction - we lost .25" w.c. for this device. Oh, and don't forget the return grille, balancing damper, and supply register on that longest duct run. This added an additional loss of .09" w.c.
Some quick math, even for the greenest of technicians, will show that we now have negative available blower static pressure (.50" - .20" - .25" - .09" = -.04"). This means there is no static to overcome any ductwork!
Unfortunately (typically), by this point all of the ducts are installed and the unit is operating - providing the much needed heat over the past week of single digit weather. So, it is determined that the ductwork is undersized and modifications are needed. This is usually quite the conversation to have with the homeowner, and do you think they will be paying to fix that ductwork? Good luck!
Think about these things before setting your equipment up for failure! Remember: a PSC motor can only overcome .50" w.c. and most men have a (1) movie award show limit per year...