AHU testing and unit pressure profile

[ACD1]

What do you typically look out for when looking at the pressure profile of a unit?

I have a pressure profile for an existing unit that - during testing - generated a much lower pressure than expected at the intake of the unit. The pressure measured was substantially more negative than the return fan was scheduled for. To me, this implies that the return fan is starved for air, running higher on the curve.

 

[GT-EGR]

That is possibly the issue - meaning you have a lot more resistance than you planned for. But that also assumes when you took that profile you were at the correct design airflow. It is also possible that that high negative pressure was while pulling more return air than you wanted, which will correspond with a higher negative inlet pressure - and this is due to your fan being too large or running at too high of an RPM.

Overall without knowing your system you are profiling the basics of what you need are
- static pressures before and after each major component
- the airflow associated with that static pressure
- be sure your unit is in normal operating mode
- be sure to read the actual airflow in each location, there is differences between the total airflow back to the unit vs what is recirculated or relieved vs what is supplied


Generally speaking a designer is responsible for knowing all the static pressure requirements outside the equipment, and while the designer should also be aware of the unit profile - each manufacturer is somewhat unique so the pressure drop associated with a cooling coil is something you need their submittals for.

 

[willard3]

Your pressure plot should be in total pressure which equal static pressure + velocity pressure.

 

[ACD1]

So - The unit is running at approximately 45% outside air. I asked this question because, in the past - I ask for testing on an existing unit, and I rarely see anything that is crazy high or low from a pressure perspective. For this test, I had all the boxes placed at maximum to see what the unit would do. It's an office space, so - it shouldn't be running with that much OA. Now, you could argue that it will never run at max, but I also don't think that the boxes were scheduled with enough maximum cooling cfm. GT-EGR - what do you mean when you say the fan is too large? You don't mean too much diameter, do you? The unit was scheduled, for an office unit, far too little return fan. The units are scheduled with return fans that are 75% of the cfm of the supply fans. For an office unit, I would expect much closer to the supply cfm, if not the full supply cfm so as to account for return duct leakage.

 

[GT-EGR]

What I meant was that the return fan size and speed setting is maybe pulling more airflow at a higher static than you designed for.

If you designed the return side ductwork to return 1,000 CFM at 0.5” static, but if your fan is capable of much more than that, it for example could pull 2,000 CRM through the return duct and you’d see a reading of 2.0” negative at the return inlet.

The actual return CFM you want depends on your setup - if your unit has relief air sections built in, or if that is done by a separate fan