[Shop-talk] Furnace Filters

[Tom Coradeschi]

Now you’re getting to the point. Filtration efficiency is going to be measured in MERV (and/or MPR, if you’re 3M) and should be independent of filter thickness. 

What the 4” filter buys you is (for the same filtration efficiency) longer filter life, and, maybe, lower pressure drop across the filter. The $64k question is: how much lower, and does it really make a difference? If you do end up buying a differential pressure gage, would you be interested in running 1” vs 4” and reporting out what you see? You’d have to conjure up a way to seal up the duct so you get a good upstream reading with the 1”, I think. 

Another ME here, albeit one who is not crazy about doing sheet metal work … at least not crazy enough that I see the point in going to a 4” filter, sans data showing it to be worth the effort. And I have a box full of 1” MERV 13 filters to use up - on a 3 month change cycle I am good until early 2023, at this point:-)

Whatever you do, don’t stack 1” filters. They will act like two resistors in series - your blower motor will almost certainly not respond well to that. 

Regards,

Tom Coradeschi
Tjcora at icloud.com

> On Dec 13, 2021, at 7:03 PM, Jim Stone <1789alpine at gmail.com> wrote:
> 
> 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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