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Discussion Starter · #1 ·
Hello helpful people, looking for a rule of thumb for switching to a higher MERV rated filter. If you know the static pressure drop numbers for the original filter.

For example, if the existing filter has a PD of .236 how much higher of a pressure drop would a system tolerate without any problems?

A Honeywell 16" x 25" x 4" has a .236 # and their same sized MERV 13 has a pd of .28.

I'm thinking no one would design a system with that little wiggle room but I'm not sure if there is a standard buffer added in the calculations.

Thanks in advance for any thoughts on the matter.
 

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Not much. Your system likely is designed for a total static pressure of.5 inwc so your filter is using nearly 1/2 the total.
Ductwork is designed with a certain static allotted to the filter, using a more restrictive filter could be a problem. The best idea is to check total static with the filter you want to use and then decide.
 

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Discussion Starter · #3 ·
Not much. Your system likely is designed for a total static pressure of.5 inwc so your filter is using nearly 1/2 the total.
Ductwork is designed with a certain static allotted to the filter, using a more restrictive filter could be a problem. The best idea is to check total static with the filter you want to use and then decide.
Thanks. I was hoping that there was an industry standard of wiggle room just to account for homeowners not changing their filters for years. : It seems crazy not to have some sort of extra capacity but I know around here with mostly vacation homes they cut corners as tight as they can.
 

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Well it just depends on what number they used in the duct calculations for filter pressure drop. I must tell you that with.2wc for a filter and.2 for an evap and.02 for a couple return and supply grilles you only have.08 left for all the rest of the duct run. Add in a tight space and some weird turns and you can see the challenge. Normally I tell customers to use a simple fiberglass filter that will keep dirt out of the evap and secondary heat exchanger. If you need to filter the air in the house then get a whole house air purifier.
As I said it's best to just check static pressure and go from there
 

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Discussion Starter · #5 ·
Completely makes sense. And I'm sure they used the bare minimum design around here, but it also makes sense that they are using the filter to protect the equipment and not lungs. :)

Thanks for your time.
 

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Discussion Starter · #6 ·
Well it just depends on what number they used in the duct calculations for filter pressure drop. I must tell you that with.2wc for a filter and.2 for an evap and.02 for a couple return and supply grilles you only have.08 left for all the rest of the duct run. Add in a tight space and some weird turns and you can see the challenge. Normally I tell customers to use a simple fiberglass filter that will keep dirt out of the evap and secondary heat exchanger. If you need to filter the air in the house then get a whole house air purifier.
As I said it's best to just check static pressure and go from there
Bob, two questions: Is there a simple way to test the static pressure drop? And where does the designed .5 PD come from? Is it a function of the blower/motor capacity? For example anything using x sized blower can only support a .5 PD? And to go higher you'd have to upgrade your equipment?
 

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First, the allowable max total external static pressure will be listed on the furnace nameplate which includes all specs for the furnace. Generally, all residential furnaces will be.5" wc.
The way to check pressure is from the furnace side of filter to furnace side of evaporator for split systems. You would need a manometer capable of something in the range of 1 to 2" wc. Best to find a hvac tech that will check it for you and try the filters you would like to use.
 

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Discussion Starter · #8 ·
First, the allowable max total external static pressure will be listed on the furnace nameplate which includes all specs for the furnace. Generally, all residential furnaces will be.5" wc.
The way to check pressure is from the furnace side of filter to furnace side of evaporator for split systems. You would need a manometer capable of something in the range of 1 to 2" wc. Best to find a hvac tech that will check it for you and try the filters you would like to use.
Thanks Bob,
 
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