No rules of thumb.

Get yourself a Magnahelic or digital manometer,to test the ESP of the system and you'll clearly see if the "better" filter can be added or not.

Test supply and return ESP separately ,to see which side would need duct modifications to add the "better" filter.

Typically we that if the system is acceptable with a cheap filter,we can add a return duct and grille ,to reduce the return static enough to allow the better filter to be used.Here in Florida ,most ducts in the attic ,it's easy to add a ceiling return.

 

Usually if the pressure drop across the filter is more than 25% of the pressure drop across the fan then your filter won't work. You can also just do a CFM test with filter in system to see if you are getting your required CFMs. For a furnace take btu/hr output divided by 1.0505 divided by temperature rise on rating plate to get required CFM of furnace.
(For ex. 100,000 BTU input 80%, 80,000/1.0505 =76,154, 76,154/70degrees=1087, 1087CFM needed for 100,000 BTU input 80% furnace)
For cooling use the standard 400CFM per ton.

 

Pressure drop is only part of the equation, but a big part. I have been measuring air with a flow hood for over 18 years. About HALF of the systems that I have measured have 200 CFM per ton OR LESS!!!! This is even with cheap fiberglass filters. Then someone puts a pleated or washable electrostatic filter in the system & it really goes to hell. Typically on a standard blower and duct system, you can only afford a fiberglass filter with a pressure drop of 0.08 to 0.10 Inches Water Column (IWC). If you have a variable speed blower, you may be able to handle 0.2 IWC.

Remember that most duct systems are under sized to begin with. Even with NO FILTER many systems will not deliver adequate air flow. (That is while the coil and blower are still clean) So any resistance you add is bad.

Get a flow hood and start measuring actual air flow. If you have 400 CFM per ton with a fiberglass filter, you can install a pleated filter and get by. A pleated filter will typically drop 10% to 20% off your air flow with a fiberglass filter and a standard blower.

A 20 x 20 x 1 fiberglass filter with 800 CFM air flow will have a pressure drop of 0.08 IWC. A 20 x 20 x 1 pleated filter with 800 CFM air flow will have a pressure drop of 0.2 IWC, or 2.5 times as much! A washable 20 x 20 x 1 electrostatic filter with 800 CFM air flow will have a pressure drop of 0.12 to 0.35 IWC!! These numbers are all with new, clean filters. The high efficiency filters catch more dirt so they load up faster than fiberglass filters, so don't leave them in for 90 days like the package says you can!

Even a fiberglass filter with enough dirt on it that you can use it to start seedlings and peel the dirt off in a layer has a pressure drop of only 0.15 IWC! That is LESS than a NEW pleated filter!!!!!!


So unless you measure actual air flow, don't sell or install high efficiency air filters except EACs, you will be doing yourself and your customers a favor!!

 

IMHO,testing the static is a more accurate way,with fan data,to decide about adding a filter.

 

Dash,
I will not disagree too much with your statement under these conditions:

1) New install where YOU have the data from the manufacturer on the blower and ESP.

2) new system where the indoor coil is still clean and the blower wheel is still clean,

Note that most of the time, when looking at filters, it is on a system I did not install, where I do not have the blower data with me, or SP data for the coil, where the system may have been installed a while, with a slightly dirty coil or blower (The data is now slighty wrong). Also if the ducts are not perfect at the unit, System Effect will make the blower data wrong by as much as 0.1 inches water column in residential. Using a flow hood still gives good answers even under those conditions.

I always have my flow hood with me.

But if you just installed it and have all the data, pressure drop will be as good as the flow hood.

Kevin

 

I recently read a little about "system effect",I understand ,or think I do,how this effect a blower,but have found nothing on air handlers and furnaces.

If you have any info please post it.

Thanks,dash

 

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?

 

On resi their system is oversized,so the reduction in sensible goes unnoticed.

If you are not the servicing contractor you'll never hear about the problems.



If they are in heat pump markets ,accumilators will save the compressors!

 

I am not against pleated filters, just their mis-application and selling then as a fits-all. I myself sell the Space Guard. I even sell some 1" pleats.

I just like to check the air flow on the existing system BEFORE I install a high efficiency filter. If the air flow is around 400 CFM per ton with a fiberglass filter on a standard blower, it will usually drop to around 350 CFM/ton with a 1" pleat. If I install a Space Guard, depending on the size of the heat pump, the drop off in air flow will probably be betterm maybe to 380 CFM per ton. If they have a Variable Speed blower, the change in air flow will be very small.

If I see a 3 ton system with 600 CFM total air flow, I tell them not to use pleats. So it all depends. In my area, about half the systems I check have 200 CFM per ton OR LESS!!!!!

So I don't recommend pleats on a lot of installations.

Even with pleated filters, most systems will still cool, but if you do a capacity and efficiency check, you will often find that both capacity and efficiency have fallen off!

As Rob Falk, of the National Comfort Institute says "If you are not testing, you are just guessing!"

Note, if you measure actual air flow when you are done selling a pleated filter, this does not apply to you.

 

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.

 

Response;

First;
Yes homeowners and undertrained techs and contracors add filters to systems that can.t handle them.Collectively there's a lot of equipment damage fronm this.
Variables are indeed too many, to come up with guidelines,knowledgable testing is the way to go,before installing.


Second;
You have tested them and I don't doubt your results,but it's not what we find,may be a difference in the products.
4" pleateds inside of cheap throw often are a problem.

1" ,3M filters in place of throw aways ,almost always a problem.

I agree that poor filtration ,will degrade air flow,over time.



Third;
I think the manufacturers have found ,undesized ducts ,to be a problm already.Much more education in the industry will be required.

We saw otherwise good contractors that had no idea how to corect high statis,when Infinities first came out.Many were calling the mfr. to find out what was wrong with the control,when it was undersized ducts.
Some on this and other forums advocate staying away from this product as the consumer can read the static,when it tattletales on undersized ducts or "added" filtration.

Everyone ,filter mfrs,equip. mfrs., contractors,etc., should advise the static must be tested before going with better filtration.

 

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.

 

As you said testing should be part of service calls,etc.,and that's what we do,so no big cost at all,to the homeowner.