[Shop-talk] Furnace Filters

Mon Dec 13 20:44:23 MST 2021

Jim,

I used a Magnehelic, see below, but I found it on eBay for about $40.  I’ve used them forever on industrial equipment.  They are mechanical, just keep on working and are old school like me!  What you are looking at will work for this and other applications too.

Just make sure you don’t put too much suction on the fan, i.e.,  don’t exceed the nameplate value on the fan. This should not be an issue, as you should have a fair bit of headroom to play with.

As far as drilling holes, etc. for the tubes, just get the ends of the tubes somewhere near the center of the duct for this application.

Brian’s filter Whistle is a nice really low tech solution too!  Thanks Brian. I’d never heard of this.

Jack

https://www.amazon.com/Dwyer-2000-00-Magnehelic-Differential-Pressure/dp/B00NI4JUMY/ref=sr_1_3?crid=1XUKMX4911WS6&keywords=magnehelic+differential+pressure+gauge&qid=1639451260&s=industrial&sprefix=magnehelic%2Cindustrial%2C213&sr=1-3

From: Shop-talk <shop-talk-bounces at autox.team.net> On Behalf Of Jim Stone
Sent: Monday, December 13, 2021 5:11 PM
To: Shop Talk <shop-talk at autox.team.net>
Subject: Re: [Shop-talk] Furnace Filters

Thanks again, guys.  I had a feeling that there would be at least one expert on the list, but it looks like I got two, at least in the first couple of hours. And, they both agree!  :)

Jack: a differential meter is an interesting idea, especially given the cost of the bigger filters.  It certainly doesn’t make sense to replace them more frequently than necessary.  I am not a mechanical enginerd, just a plain old ordinary nerd.  Still, I like playing around with new things.  Would something like this work? https://www.amazon.com/Manometer-Differential-Backlight-Ventilation-Measurement/dp/B07K7HT3XJ/ref=asc_df_B07K7HT3XJ/?tag=hyprod-20&linkCode=df0&hvadid=312099766989&hvpos=&hvnetw=g&hvrand=20692614293351478&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9004590&hvtargid=pla-622729475514&psc=1. I’m thinking of drilling a couple of holes in the filter box and semi-permanently mounting it.

Jim

On Dec 13, 2021, at 7:21 PM, Jack Brooks <JIBrooks at live.com<mailto:JIBrooks at live.com>> wrote:

Bingo!  Jim hit the nail on the head.

The 4 inch filter has a lower pressure drop because of overall greater filtration area due to all of the deep pleats. It also has a lower frequency of filter change out for the same reason.  Using two one inch filters together will impact your flow. It may not affect your performance noticeably, but it will change the system flow, drive up fan energy costs, etc.

https://nordicpure.com/ seems to have a lot of sizes in stock and they are not too badly priced either on their website or Amazon.  I’ve used them for a while and am happy with them. They are available in a wide range of MERV ratings too.

I have a differential pressure meter across my filter.  It registers 0-0.25 inches of H2O.  I let that guide my changeout frequency, which often is around a year before the pressure drop indicate to me that it should be changed. Ok, yes, I am a Mechanical Enginerd.

Jack

From: Shop-talk <shop-talk-bounces at autox.team.net<mailto:shop-talk-bounces at autox.team.net>> On Behalf Of John Innis
Sent: Monday, December 13, 2021 2:44 PM
To: Jim Stone <1789alpine at gmail.com<mailto:1789alpine at gmail.com>>
Cc: Shop Talk <shop-talk at autox.team.net<mailto:shop-talk at autox.team.net>>
Subject: Re: [Shop-talk] Furnace Filters

The only real issue is that you will not get as much airflow through a 1" filter of a given effectiveness.  Most of the time that's not a big deal.  In our previous house we did have an issue with the AC freezing up when it was working really hard.  In that case the solution was a lower effectiveness filter because the box could not accommodate a thicker filter.  If you are happy with the performance of the 1" filter and have not had any issues, i'd say stick with it.  I do NOT recommend stacking two one inch filters.

On Mon, Dec 13, 2021 at 4:20 PM Jim Stone <1789alpine at gmail.com<mailto: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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