Static Pressure and filter changeouts

[Notasupertech]

Is there a way to read a static pressure guage and know for a fact that it is time to change out your filters? We have Magnahelics in place to read the pressure differential but do not go by these for filter replacement... I know there has to be a rule of thumb on this one...

[dlove]

to answer your first question: YES. than I would say there is no rule of thumb here, unless you say change when visibly dirty. I assume you have a large filter bank? with pre-filters and than box or bag filters. One must know the design static for the system, the design pressure drop across the coils. are these fans constant with vortex dampers or VFD, driven? with so many questions, there are so many answers. There are many that change filters because they look dirty and really don't need to be changed. need a wee bit more info

[hvacker]

The fan speed has to be constant. VFD's Etc won't work. Some place cardboard over some of the filters, usually half of them, and read the pressure drop. Put a note by the mag or a mark.
Different filters have different drops so rules of thumb aren't done.
A lot also depends on the system. Some designs are so marginal that even partial plugging affects performance.

[hands]

The rule of thumb is 1" w.c. above the differential pressure with new filters in place. Use a magnehelic gauge with the pressure tips on each side of the filters only. If you have vortex dampers or a VFD, you have to run the fan at full speed when you check the pressures. Say you start out with new filters and the VFD is at 60 Hz. and you have a 0.2" differential pressure reading. When the reading gets to 1.2" you check or change the filters. You need to check the filters because some of them could have pulled through and the differential pressure would be off.

[dlove]

The rule of thumb is 1" w.c. above the differential pressure with new filters in place. Use a magnehelic gauge with the pressure tips on each side of the filters only. If you have vortex dampers or a VFD, you have to run the fan at full speed when you check the pressures. Say you start out with new filters and the VFD is at 60 Hz. and you have a 0.2" differential pressure reading. When the reading gets to 1.2" you check or change the filters. You need to check the filters because some of them could have pulled through and the differential pressure would be off.

I would take it a step further. On a VFD system if your maintaining static down line where your probe is and your HZ on your VFD is not maxed out then i would not changed my filters. My bench mark is knowing my PD across the coil and clean filters. lets say my VFD may only run at 30hz to maintain say 3" of static (when i have clean coils and filters). When my filters start getting dirty my HZ will increase to maintain my static set point. i start checking my filters making sure they are ok. When i get close to 50hz I will at this point change my pre's and then check my HZ and continue to do that. The pre-filters are the front line and the cheapest. The box, bags or hepa are the real cost. You can sometimes go 6 months with out changing your main filters sometimes longer. On a bank of filters, some filters will be dirty than others, and air, like water will travel the least resistance. So sometimes using a magnehelic in some cases don't really give you a good sample.


However, you must monitor bypass making sure your not dirtying the coil or your filters are not damaged. I will spend a little more investing in good filters then having to clean coils. But for all this to work you need to know the design of the system. use a good manometer, perform routine inspections and check, check your static sensor for calibration..

[Notasupertech]

Thanks to everyone on replying, I appreciate the help.