[breathe easy]

I have seen all kinds of figures for acceptable resistance (pressure drop) of filters in residential systems. At the low end I have talked to contractors that believe anything over 0.12 wg will not allow for the new high SEER systems to operate properly. Then I have talked to manufacturers who tell me that their new "air cleaners" that have an initial of 0.31 wg is perfectly OK. I realize there are many factors such as the size of the system, resistance of the coil, resistance of the ductwork, etc. that factor into this answer. The only truly accurate answer comes from measurement and calculation. But what has been the experience in working with systems in people's homes? Are there any guidelines or rules of thumb that people are comfortable with?

 

[breathe easy]

I certainly can't argue with the math. However, let me play the devil's advocate. We have sold many thousands of pleated filters. We have over 100 contractor customers. To my knowledge my customers have never had an "air flow" problem with any of these pleated filters. Have we just been lucky or is some of this discussion like the scientists proving that a bumble bee can't fly?

 

[breathe easy]

Some thoughts:
First, 99% of the filters purchased in the US are done so without the benefit of testing. This can be because of the fact that the contractor does not do the proper testing. But the biggest reason is that homeowners are the ones that make these decisions when they are in the hardware store or the grocery store. That is why I was hoping that someone with alot of experience could help by throwing out some guidelines. "For instance, systems over 15 years old should not use one inch MERV 11 filters." But I guess in reading the responses this is not realistic. There are just too many variables.
Secondly, I wouldn't be so sure about the statement that a throwaway is going to have less resistance than a pleated filter. There are two reasons for this. First, depending on the type of dirt involved the pressure drop on a throwaway will actually increase faster than the pressure drop on a pleated filter. This has to do with the increased surface area of the pleat. If the throwaway stays in longer than one month, this "dirty" filter will oftentimes have more pressure drop than a pleat in the same unit for the same period of time. (But of course everyone in the country changes their filters every month.) Secondly, the deeper media filters have very low pressure drops. In fact in our test chamber the pressure drop on a 4" MeRV 7 pleated filter is almost the same as the pressure drop on a 1 inch polyester throwaway. Also less efficient filters allow more dirt to soil the coil and have an effect on air flow, the cooling efficiency of the system and the pressure drop of the system.
Thirdly, in my opinion the biggest issue with the poor design of HVAC systems in residences is going to occur with the new "air cleaners" from all of the big equipment manufacturers. The "MeRV race" that is going on coupled with the introduction of systems with high initial pressure drops (0.31 IWC) is going to create lots of airflow problems.
Based on this discussion, it would seem that homeowners should insist on pressure testing of their system if they have a PM contract or in the installation of a new system whether for a new house or a retrofit.

 

[breathe easy]

Testing static pressure may be the right approach but I afraid it is not realistic. I just don't see homeowners paying to test static to change their filter. (But, then again, maybe there is a way to package it with a "blower door" test, duct sealing, etc. as an "IAQ service.")

Unfortunately, most homeowners think static is the electricity that jumps off their fingers when they walk across carpeting and touch a door knob.

It certainly should be a part of service calls, PM's and new installs.

Fact is if everyone went by your "Manual D" none of this would be a problem.