Comments on: How Often Should You Change the HVAC Filter?

By: Steve Waclo

Steve Waclo — Sun, 07 Jul 2019 18:10:32 +0000

Something I learned about
Something I learned about filters relates to the use of cold mist humidifiers. We all know how, when using hard domestic water, those humidifiers can create clouds of mineral precipitate that ends up on every horizontal surface, and rapidly clog even low $$$ filters. Distilled water is too expensive, so we just use a CM unit in the bedroom at night.

As an amusing sidelight, I thought condensate from my furnace would be a good alternative…not! It’s nasty stuff.

Doesn’t rain enough around here in a Northern NV so that’s not an option and saving AC condensate is no go as well, since we cool with overnight ventilation.

Sigh…

By: Norman Bunn

Norman Bunn — Fri, 31 May 2019 10:52:14 +0000

In reply to Steve Lindell. Worth a shot if it keeps me
Worth a shot if it keeps me from buying one filter a year. Of course, I still have to go into the crawlspace once a month to check it. :)

By: Nuri Boardman

Nuri Boardman — Thu, 23 May 2019 14:34:13 +0000

Why use a proportional
Why use a proportional pressure drop, instead of an absolute one? .1 to .2 or .25 to .35

By: Mark Johnson

Mark Johnson — Thu, 23 May 2019 01:49:25 +0000

My experience: after I
My experience: after I learned Carrier Infinity controls used change in static pressure to judge when filter needs changing, I tried to do that too. At that time you could buy a Dwyer Model 25 manometer really cheaply (the kind with red oil in a glass tube), and I connected two tubes to measure external static pressure (ESP) for the air handler. My expectation was that I would see ESP gradually rise as dirt got on the filters. No matter how long I delayed changing the filter, could never detect a measurable increase in static pressure. Up to 2-3 years, the ESP remained constant. It was however, an easy way to tell if the system was in 1st or 2nd stage (American Standard Allegiance 18). I was also gung-ho on getting ESP down to a good number, and adding a few new returns was one way of doing that. At this time the filters are sized at 300-400 sqin/ton, or a filter face velocity of 130-180 fpm. I understand that many home AC systems have smaller sized ductwork and filters, and might have different results. But that was how it worked for me.

By: David Butler

David Butler — Wed, 22 May 2019 18:37:41 +0000

Great article. Filter design
Great article. Filter design/maintenance is among the most important factors in system performance, especially for cooling. But as you point out, over doing it can be expensive, especially when using higher MERV filters. To that end, I recommend thermostats that have a filter reminder based on run-time (hours). Many brand/models now have this feature but not all allow the user to adjust the target run-time. It takes some trial-and-error to come up with the appropriate change interval, depending on type of filter and loading rate. A manometer is the best way to figure that out.

By: Steve Lindell

Steve Lindell — Wed, 22 May 2019 11:21:33 +0000

General Filters makes an
General Filters makes an inexpensive G-99 air filter gauge which can be calibrated to measure 0.1″ – 0.4″ of water column pressure:

https://www.generalaireparts.com/amfile/file/download/file_id/2168/product_id/31773/