The 2nd biggest problem in our industry is Airflow. The first being laziness.

We REALLY need to do a better job in understanding airflow, how to measure, how to set it, and how to correct airflow problems. The furnace and AC systems will not function properly if the airflow is not correct. These improperly installed/adjusted systems are a major waste of energy, money, and time. Not to mention, if the system you sold your customer is not setup correctly, you have in fact NOT provided the professional service your customer paid for. 

Lets start with TESP. TESP is Total External Static pressure. In short, its the pressure of the air system that the blower wheel "sees", which includes, the Evaporator coil, Filter, both return and supply ducts and registers. Each component has a specific pressure rating based on face velocity (how fast the air moves across it). If the incorrect components are combined together, we can have high TESP which results in low CFM and a poorly performing system.

Regardless of brand, the Max Target TESP for residential systems is .5"wc. Most efficiency ratings are based on this number, that's why its stamped on the rating plate of the furnace/airhandler underneath model/serial information. It has been on the rating plate of the equipment since AT LEAST the 60's if not longer. 

As I stated before, each component has a pressure drop rating from the manufacture. Most evaps will have a rating of anywhere between .25 and .33" wc. Most air filters will be less than .2"wc More closer to (.15"wc). If the duct design is done at .08"wc and the registers have .1"wc, We have a potential total of .63"wc. A minor change or adjustment to any of these pieces can result in better or worse airflow.

ESP is critical to CFM. Just because the curtain by the register is moving, DOES NOT mean our airflow is "good". Its just a visual perception.

High ESP (External Static Pressure) will result in the following: High noise, high velocity, low cfm. Reasons for this could be: Duct system too small, evaporator to small or dirty, registers/grilles blocked or closed, improper blower set-up.  Low ESP (External Static Pressure) will have the opposite affect: Low noise, low velocity, High cfm. Reasons for low external static pressure could be: Improper blower set-up, incorrect probe placement, duct system too large or massive leakage, dirty blower wheel, or dirty secondary coil. 

If you look at the blower performance chart in the manual of the air handler or furnace, you will see that when the ESP goes up, CFM drops. In some cases when we speed up the blower, we can actually make our CFM way worse than it was when we arrived. So we must first take a TESP reading before we make ANY adjustments. The components of the air handler must be inspected as well before we perform this test, blower wheel needs to be clean, secondary heat exchanger (if applicable) must be clean as well, if they are dirty, we will not get correct readings. 

We can understand a lot from these readings. We can determine if the evap is too small, dirty ect. Same goes for the air filter and duct system. Or maybe, just maybe, we have a blower that is just way too big for the airflow system we are working with. Understanding the components we are working with is key as well. 

Attached are some slides to reference. Feel free to contact us to discuss further and keep an eye out for upcoming classes on Airflow. 

Carlos