[Shop-talk] Furnace Filters

Mon Dec 20 12:50:44 MST 2021

The 4” filter will give you far more filter area.  Generally the media for a given MERV rating will be about the same (differs between brands of course).  A thicker filter has deeper pleats, probably almost as many in a 1” or a 4” cartridge, but the 4” will have roughly 4 times the area.  Thus a thicker filter of the same MERV rating will generally last as much longer as it is thicker.

Remember too that the more filter area the less the pressure drop across the filter.  Besides blower life, the lower the pressure drop the more effective the filtration will be.  I used 5” filters in the old furnace because they were the thickest that would fit.  I think they were about $35 apiece for 20x25.  New furnace has Carrier’s mucho-expensive air filtration system which is a box that holds a 5” filter and some electrostatic wires on the clean side.  While the 10-year warrantee is running I’ll bend over to buy theirs, but I doubt they’re anything special other than that they connect with a sensor in the unit that tells it that it’s eating the expensive stuff.

Karl

From: Shop-talk [mailto:shop-talk-bounces at autox.team.net] On Behalf Of Jim Stone
Sent: Monday, December 13, 2021 6:03 PM
To: Shop Talk
Subject: Re: [Shop-talk] Furnace Filters

Thanks, guys.  That is all good feedback. David: I hadn’t considered using a washable filter, but it is an interesting idea.  It looks like a 4” one will cost me around $100, so the breakeven point is after a year.  Not too bad.

But, here is an interesting point that showed up tangentially when I Googled “washable furnace filters”: 

Fortunately, most 20x25x4 air filters last between six to eight months. By that calculation, the average homeowners will not need more than two 20x25x4 air filters in any given year. In contrast, a one-inch air filter will need to be changed every two to three months.  https://servicechampions.com/benefits-of-a-20x25x4-air-filter-installed-in-your-furnace/

If that is true, the cost difference between the 4” and 1” disappears.  Thoughts?

Jim 

On Dec 13, 2021, at 5:32 PM, Pat Horne <patintexas at icloud.com> wrote:

My shop air handler is set up to use a 2” filter but the installer said to use 2 one inch filters, both good ones. I watch the first one & remove it when it is too dirty, move the second one up & install a new second one. 

There is some discoloration on the second filter, so it is doing something. 

If you want to go with two different ones, I’d put the poorer one first, followed by the better one. 

My HVAC guy says to not use the corrugated filters, nor the fiberglass ones that you can see throughout. He sells me Purolator 312 filters by the case. 

Peace,

Pat

Pat Horne 

We support Habitat for Humanity

On Dec 13, 2021, at 4:21 PM, Jim Stone <1789alpine at gmail.com> wrote:

The company that installed my HVAC system a few years ago used a filter box on the return line that takes 20” x 25" x 4” filters, which is an unusual, and thus expensive, size - filters generally run about $25 each - that is not readily available other than online.  During construction, my general contractor just had a cheap 20” x 25" x 1” filter in there that seemed to work just fine.  I checked the installation instructions for the furnace and there are no specifications for the filters, other than that they need to be changed regularly, which is getting pretty expensive.  Is there any reason I couldn’t just use 1” filters, which are less than half the price?  Maybe even put two of them in the box, a good one first to catch most of the dust and a cheap one like the contractor used as a secondary one.  It is a snug fit and I don’t think there is any danger of them tipping, but two would make it impossible.

Thanks,

Jim

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