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Static Pressure – Why Measure It?
This article was written by Neil Comparetto. Neil is one of the smartest and most thoughtful techs I know online. Thanks, Neil.
I enjoy drilling holes in things. I rarely leave a house without drilling a hole in something. I also believe it’s an essential step to commissioning and diagnosing a forced-air piece of equipment. Let me explain why.
I think we all can agree that proper airflow is necessary across the indoor coil. You should set the airflow before adjusting the refrigerant charge, right? Yes. Well, how do you know what fan speed to set the blower at?
Whether it's a PSC, X-13, or an ECM motor, you have fan speed options. The easiest way to set the blower speed is to measure TESP (total external static pressure), cross-reference the TESP to the manufacturer's blower chart in the installation manual, and adjust the blower speed. Sure, there are other ways of estimating or measuring airflow, but for commissioning a system in cooling, using static pressure and a blower chart is easy and accurate enough.
350-400 CFM (594.65 m3/h-679.6 m3/h) per ton works in my neck of the woods. If you are in a very dry climate or at a high altitude, the CFM per ton requirements may be higher, often 450-500 CFM (764.55 m3/h-849.51 m3/h) per ton.
Even when commissioning a furnace in heating, I like to set up my airflow first, or I at least like to know how many CFM the blower is moving. Typically, it's between 130 and 150 CFM (220.87 m3/h-254.85 m3/h) per 10,000 input BTUH.
How many times have you serviced a system installed by others, there is no evidence of airflow being measured, and the blower speed is set too high? My guess: every day.
There's a better than good chance the airflow is wrong and has been since day one.
The airflow is set. Now what? Take it to the next level; benchmark your pressure drops.
The pressure drops across the return duct, air filter, indoor coil, and supply duct can be valuable pieces of information when servicing the equipment in the future.
Imagine knowing exactly what the pressure drop across the coil was when commissioned 7 years ago. In a typical arrangement of the evaporator coil on top of a furnace, a visual inspection of the coil for cleanliness can be difficult.
Knowing the original pressure drop can save time diagnosing and justify taking further action. TESP by itself will not give you this information, only that there is an issue somewhere in the supply air side of the system. (It is recommended to record the dry and wet pressure drops across the coil, they will be different.)
Air handlers can typically handle a higher pressure drop across the filter (because the coil is included in the TESP rating) but at the cost of filter efficiency. Generally, these filters are good for 6-12 months. Knowing the before and after pressure drop of the filter in this system will help you determine how frequently it needs to be changed.
Knowing the supply and return duct pressure drop can be useful as well. It's not unheard of for vents to be closed, return grilles blocked, internal liners to collapse, or flex duct to get smashed or even disconnected. A static pressure reading of the ducts referenced to the pressure drops when commissioned can quickly tell you if there are any discrepancies. Better yet, it tells you what actions to take.
Does benchmarking lengthen the time it takes to commission the system? Yes. Does it give you the information necessary to quickly and accurately diagnose airside issues during future servicing? Yes, it does. In reality, once you do it a few times and develop a system, it doesn't take much longer at all.
If you have not listened to Bryan Orr and Jim Bergmann’s podcast on checking the refrigerant charge without gauges, please do. They make a case for an even more comprehensive benchmarking procedure. Listening to it was one of those aha moments for me.
Take TESP to verify airflow against benchmarks and/or blower charts. In my experience, airflow is incorrect most of the time. If this system is new to you and will be part of a service agreement, I recommend that you check all four pressure drops (return, filter, coil, supply) for reasons mentioned earlier.
I find a lot of air filters that are too restrictive (small) on service calls. Air filters can be low-hanging fruit if the equipment is not getting to proper CFM. It's not uncommon to get .30” pressure drops on new filters.
If you find issues with the existing duct system and it's exposed, static pressure readings can help pinpoint where the restriction is.
Many times, the restriction is obvious. A nasty reverse elbow then it turns twice, transitioning from 20” to 10” into some kind of cap-and-tap contraption. Sometimes the restriction is internal or not obvious. Collapsed duct liner or a closed damper can be found with static pressure strategically measured across portions of the duct system.
If there are issues with the duct system, let the homeowner know. This conversation might expose some comfort problems that they are experiencing. At the least, it will make them (and you) aware that there are issues that may need to be addressed when it's time to replace the system.
I'm not advocating checking static pressure every time you run a service call. I know that's not always practical. I am advocating for installing the pressure ports and benchmarking on commissioning, as well as measuring airflow against the benchmarks during service when the call type calls for it. Future service techs will thank you, and you will come to a faster and more accurate diagnosis.