furnace filter efficiency impact

I've read in many places that you should change your furnace filter regularly as a dirty filter will lower the efficiency of your furnace.
Excluding extreme cases where a filter is exceptionally dirty, how much of a loss in efficiency are we talking about for the average "needs to be changed in the next couple of months" filter?
PS: I'm not trying to find a reason to stop changing my furnace filters or anything. I'm simply curious as to how much impact a dirty filter can have. Is it 0.1 - 2% or more like > 10%?
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I don't have a number for you, but you may want to cosider this.
If your filter is blocked, then less air is being fed into the furnace by the cold air return ducts. If you don't have a perfectly sealed furnace chamber, then the furnace might be drawing some of it's air from your basement, which might be pulling air from the first floor dwn to the basement. This might increase the mold growth in your basement (i.e. bringing relatively warm moist air into the cold basement).
Just a thought.
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Jonny R wrote:

Hi, Why don't you experiment yourself? I saw a clogged up filter bringing down room temp. to near freezing point in dead winter. No kidding. When I took the filter out, it looked/felt like a black sheet of slate. Tony
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replying to Jonny R, Iggy wrote: Just so any passersby have a little more answer. In my own testing I found no difference on my electric bill ("efficiency" is only based on the blower motor or air movement, in this case) when I went to annual and now even every 2-years of a change out...I used to smoke and no longer do.

I would've expected 10%, but I saw nothing when I had to wait 2-months after moving-in for a custom filter size. The old filter's corrugations were completely filled and the filter face was flat smooth with dust. I put the new filter in and saw no difference on my bill and heard no relief from the blower motor nor ductwork.

My findings agreed with the basic recommendation. If you can't see through the filter when held up to the Sun or bright light, then and only then it should be changed. Regardless of that, a new filter should go in after carpeting has been changed out or after a dusty home improvement...so yuck doesn't sneak by and into the HVAC system or your air.

I also found that high MERV ratings didn't effect my efficiency at all either. So, I went with 3M's Ultra Allergen filters, even though they're almost twice the MERV rating warned against by the HVAC manufacturer. They've kept my system like new and dustless for a full 10-years now.
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On Monday, September 4, 2017 at 5:14:10 PM UTC-4, Iggy wrote:

That test would seem to depend on what kind of filter you have, if at all. Filters range from the cheap fiberglass ones that are less than an inch thick to 5" thick pleated ones that have a high MERV rating. The latter, you can't see through when they are brand new.
I agree that with any filter you'd have to block it up a substantial amount, raise the pressure drop significantly for it to have any significant effect on efficiency.
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replying to trader_4, Iggy wrote: Yeah, I assumed a 1" filter and not a fat-boy or electrical. I'd suspect a lot of pet hair, drywall dust or sawdust would have a big effect and likely stop air movement even entirely. In my case it seemed to be just a collection of regular light and loose dust and pollen that may have actually helped to filter new particles. Though, I was surprised the MERV rating didn't show up as an efficiency drop.
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The purpose of the filter is to protect your coils or heat exchanger, not to clean the air for humans.
It seems reasonable to me that a dirty filter will catch more dust than a clean filter and be better for your coils, up until the point it impedes air flow.
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On Tuesday, September 5, 2017 at 8:20:04 AM UTC-4, TimR wrote:

yeah i agree,
and if you keep a thermometer in the duct to monitor the air temperature, if the air flow is significantly impeded, the air temperate will rise in the heating season or drop in the cooling season.
If the duct air temp is normal , the air flow is normal.
m
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On Tuesday, September 5, 2017 at 10:19:28 AM UTC-4, snipped-for-privacy@yahoo.com wrote:

The air flow may be normal, but the blower could be using more energy to run. Many newer furnaces have ECM motors, where the motor is programmed to maintain a constant air flow. How exactly they do that, IDK. How much of a factor that is in the overall energy usage, IDK, but if the blower is working harder it will use additional energy.
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On Tuesday, September 5, 2017 at 11:02:36 AM UTC-4, trader_4 wrote:

yep that's a good point, if you have a system like that.
m
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On Tue, 5 Sep 2017 07:17:31 -0700 (PDT), snipped-for-privacy@yahoo.com wrote:

A differential pressure tester or manometer across the filter gives a much better indication.
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On Tuesday, September 5, 2017 at 7:34:37 PM UTC-4, snipped-for-privacy@snyder.on.ca wrote:

Agree. You have one on yours? I've never had one on any furnace I've owned or seen. Of all the people who read here, how many do you think have one? Nuff said.
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On 9/5/2017 9:24 PM, trader_4 wrote:

https://www.alpinehomeair.com/viewproduct.cfm?productIDE3061598&linkfrom=froogle&gclid IaIQobChMI3N3phoqb1gIVj1mGCh0lBAQlEAQYASABEgKDWPD_BwE
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On Sun, 10 Sep 2017 12:53:05 -0400, Congoleum Breckenridge

At least as many as those who have a thermometer on each side of the filter. Filterminders (donaldson) are common equipment on engine air filters
Also Google CleanAlert and Filterscan
or check http://www.cleanalert.com/blog/how-to-install-filterscan-air-filter-monitor
Unlike some of you guys I don't just pull this stuff out of my ass.
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On Sunday, September 10, 2017 at 9:14:25 PM UTC-4, snipped-for-privacy@snyder.on.ca wrote:

thermometers on each side are not needed, there is no temperature change across the filter.
One thermometer is all you need.
If the airflow is restricted by a dirty filter, the temperature will deviate from the normal value.
mark
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On Sunday, September 10, 2017 at 9:27:24 PM UTC-4, snipped-for-privacy@yahoo.com wrote:

I think he means two thermometers, one on the input to the air handler, one on the output. That way you measure the temp drop. The normal output value when it's 78F in the house and 70F will be different. Seems to me that's how you'd do it. But then I just check the filter once a year and see what it looks like. Like I said, I've yet to see a system using air pressure or temp measurements to gauge filters in a residential application.
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On Sunday, September 10, 2017 at 9:44:45 PM UTC-4, trader_4 wrote: Like I said, I've yet to see

(Amazon.com product link shortened)
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On Tuesday, September 19, 2017 at 8:37:46 AM UTC-4, TimR wrote:

Like I said, I have yet to see one of those in use on a system. Have you?
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On Tuesday, September 5, 2017 at 7:34:37 PM UTC-4, snipped-for-privacy@snyder.on.ca wrote:

We have them at work, with alarms when they exceed their limit.
I've never seen one in residential. I just listen to mine, when I start to hear the filter noise rise I change them.
They used to make filters with some kind of a whistle that was supposed to alert you when they were dirty. I haven't seen one in a while.
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On Tuesday, September 5, 2017 at 8:20:04 AM UTC-4, TimR wrote:

Who made that determination? Sure, the cheap 1" fiberglass crap filters that are the minimum were likely put there back in the early days to protect the eqpt. But today there are advanced filters available, from 5" thick high MERV to electrostatic that go way beyond what is needed to protect the eqpt. And they are sold and markteted to clean the air for humans.

Any filter, as it clogs, will start to impede air flow, raising the pressure drop. I think we agree that it won't affect efficiency until it's substantial enough to cause a significant pressure drop.
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