In my article, Under Pressure, I talked about available blower static pressure and PSC motor maximums. But, what if you have correctly designed the ductwork and set up your airflow perfectly - only to get a call years later about an uncomfortable couple of rooms? A contractor in RI had just this problem, but he was armed with the knowledge of static pressure and the ability to measure it!
Static pressure is a simple measurement that can be taken with a static pressure probe and a dual-input manometer. I have seen guys out there still using magnahelics, u-tube and incline manometers. These mechanical means will give you the same answer - but I wouldn't use an abacus to do what I can with a calculator! Anyhow, the manometer you are using to measure gas pressure can be used for static pressure too, as long as the resolution is .01" w.c. (25 pascals) or better. With static pressure, we are able to see how much (or little) pressure is in the ductwork. We can also identify restrictions like clogged evaporator coils or hidden air filters. I once used a static pressure probe to locate a filter that had been insulated over and remained untouched for years! When measuring, I had a very low negative static pressure before the filter, and extremely high negative static after. This identified an abnormal restriction. Most pleated filters have a .12"-.15" pressure drop when clean.
I got a call from the contractor in RI with a couple of static pressure measurements that didn't sound quite right. He explained that during the recent cold spell, most of the rooms being fed by this unit were uncomfortable. He checked the static pressure in the supply duct and only had a .01" w.c. - almost nothing. Measuring before the add-on A/C coil, doing so by removing the high limit switch of the furnace, the tech was able to record a static pressure of .77"w.c.! Most coils have a pressure drop of .20"-.30"w.c. when clean. Since the pressure drop of the coil is .76" (.77 - .01), it was easy to determine that the clogged evaporator was the culprit. Since removing this 10 year old coil that was obviously never cleaned was so labor intensive, the tech was able to sell a new coil.
If the technician was never armed with the necessary tools and knowledge, it would have been easy to start replacing motors and searching for disconnected ductwork. After all, mechanical systems are much easier to fix when you know what the answers are. Static Pressure, like temperatures or amperage, is an invisible thing and we must have the right tools to even quantify them! Start measuring static pressure when commissioning the unit, and during maintenance. It really is amazing what you will find!
Dirty Coil Angle #1
(.76"w.c. pressure drop)
Dirty Coil Angle #2