How to inspect furnace filters?

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In alt.home.repair, on Mon, 5 Oct 2015 08:04:12 -0500, Bill Gill

It wasnt' off topic.

It's like you didn't even read my last 5 lines, that you snipped so no one else would re-read them.

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On 10/6/2015 12:18 AM, micky wrote:

furnace filter. The topic to which you replied was specifically about how to inspect furnace filters. Look at the subject line above. When you asked I told you and then wanted to know what it had to do with inspecting furnace filters The answer is nothing. If I use dried manure to warm the house it still doesn't have anything to do with how to inspect the filters.
Bill
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In alt.home.repair, on Tue, 6 Oct 2015 08:09:57 -0500, Bill Gill

No, it's not. If the air in the house is dirtier, the filter will be too. The filter filters the air in the house.

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On 10/6/2015 12:00 PM, micky wrote:

how to inspect them to know when they are dirty enough to be changed. This has nothing to do with how much dirt there is in the air. If there is a lot they will have to be replaced oftener than if there isn't. But the inspection procedure will be the same.
Is there some reason that you insist that you know better than I do what my question was? It seems more like the activity of a troll than a desire to help others. I give people a chance to show that they are really interested in helping. But I don't do it forever when they refuse to actually address the question.
Bill
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On Tuesday, October 6, 2015 at 6:33:57 PM UTC-4, Bill Gill wrote:

Seems to me the question comes down to two choices. 1. Is it a push or a pull? 2. What are you going to check?
Push or pull means do I have to remember to go look at it and pull the info rmation, or does it push it to my attention somehow?
What are you going to check - really just two obvious choices: visual appe arance of dirt, or effect on the airflow. Effect on the airflow can be mea sured by the noise (air passing through, or some filters have a whistle bui lt in) or by pressure differential (liquid or other manometer). You can se t up a manometer to give you an alarm, in which case it is a push, or you c an remember to go look at it periodically.
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On 10/7/2015 9:49 AM, TimR wrote:

find an off-the-shelf pressure sensor that I can use. I recall from 50 or 60 years ago seeing advertised a sensor that switched over from white to red when the filter needed to be replaced, but I haven't seen anything like that in many years. As far as a whistle is concerned I probably wouldn't be able to hear it. My high frequency hearing is poor. That would also probably be in high priced filters. I prefer the lowest priced ones that will do the job. What I have been trying to get guidance on is what are the criteria that would mean I needed to replace the filter.
Bill
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On Thursday, October 8, 2015 at 9:08:50 AM UTC-4, Bill Gill wrote:

measured by the noise (air passing through, or some filters have a whistle built in) or by pressure differential (liquid or other manometer). You ca n set up a manometer to give you an alarm, in which case it is a push, or y ou can remember to go look at it periodically.

They start to look dirty compared to a new one, works for me.
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Don't be a tight ass and just buy the inexpensive ones and change them out every month or two. You are only talking less than $ 15 per year.
I have been doing that for years. I did have a man out after 8 years on a new unit becuse the heat pump quit. Bad starting capacitor. He cleaned the coils and said the inside ones did not need any cleaning, but did it anyway as it was part of the service. We do not open any windows so the inside air tends to stay clean.
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On 10/8/2015 7:02 AM, Ralph Mowery wrote:

+1
We do it monthly as it's easier to just *remember*, come the first of each month, to remove one of the NEW filters from the bag that is kept near the furnace and replace the one *in* the furnace.
OTOH, if we had purchased the fancy shmancy HEPA filters, we'd be less inclined to do so as they are pricey and, according to the sales folks, don't really make much of a PRACTICAL difference.
We had the ducts cleaned when we bought the house many years (decades) ago and they are still clean. So, likely that the filters are doing their job.

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On Thursday, October 8, 2015 at 10:24:17 AM UTC-4, Don Y wrote:

I find it rather fascinating that you change filters every month but believe some salesman that a better filter doesn't make a difference. Also, AFAIK, there isn't really a HEPA filter for furnaces, but there are lots of MERV filters that are way better than the 1" thick cheap ones. Those trap so little that I can't imagine how they could need changing in only a month, unless you're living in a dust storm. And those are there mostly to try to protect the eqpt from excessive crap, not to remove particulates from the air for air quality.
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On Thursday, October 8, 2015 at 10:24:17 AM UTC-4, Don Y wrote:

Okay, think about WHY you change the filter at all.
Does it filter more poorly as it gets dirty?
No, of course not. The dirt is on the side the air goes in. The more dirt there, the more efficiently it traps even more dirt. It doesn't start let ting more through. Dirty filters are better, UNTIL:
Dirty filters trap more dust, but they also have more resistance to the air flow. Usually there is plenty of air flow to spare, or those 4 inch filte rs wouldn't work. But it does make the fan work harder, if it has some way to keep the air flow constant. If the filter is really dirty, it could th eoretically slow down the air to where you wouldn't get cooling or heating. Maybe.
Then if the filter becomes completely blocked, sometimes it will rip and th en unfiltered air can hit your coils.
At any rate, changing too often is a bad idea.
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TimR wrote:

or cooling efficiency drops which will increase your energy bill. How about your car air filter? Dirty filter is bad, drop in MPG, hard on the engine not breathing well, etc. I replace filter every 6 months. Merv 10 rated 16 x 25 x 5" pleated one.

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On Friday, October 9, 2015 at 6:39:45 PM UTC-4, Tony Hwang wrote:

letting more through. Dirty filters are better, UNTIL:

ilters wouldn't work. But it does make the fan work harder, if it has some way to keep the air flow constant. If the filter is really dirty, it coul d theoretically slow down the air to where you wouldn't get cooling or heat ing. Maybe.

What he posted looks factual to me.
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On Friday, October 9, 2015 at 3:13:47 PM UTC-4, TimR wrote:

etting more through.
OK, so if it traps more "even more dirt" as it gets dirtier, then, by defau lt, it must be letting some dirt through while clean, right? If not, where did all that "even more dirt" that it is now trapping go while the filter was clean ?
The air in the house didn't get dirtier as the filter got dirtier, so based on your theory, all of that "even more dirt" must have gone through the fil ter until it got dirty enough to trap it.
Based on your theory, shouldn't we take our clean filters and load them up with dirt before we install them so they'll trap all that "even more dirt" right from the start.

ters wouldn't work. But it does make the fan work harder, if it has some w ay to keep the air flow constant. If the filter is really dirty, it could theoretically slow down the air to where you wouldn't get cooling or heatin g. Maybe.

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On Friday, October 9, 2015 at 11:21:39 PM UTC-4, DerbyDad03 wrote:

That seems reasonable. Furnace filters are rated by how small a particle they will trap. Smaller than the rating, they go through. But over time, as the filter gets more plugged up with particles, it seems likely that it will trap start trapping the finer particles.

He did point out the other important side of that equation, which is that as the filter plugs up with dirt, the resistance to airflow goes up and the closer it gets to needing to be replaced. So, I think if you pre-loaded a new filter with some dirt, instead of having a filter that will trap X micron particles, you'd have a filter that would trap some of the smaller particles too, but airflow would be less and the filter would be Y months closer to needing to be replaced.
That's why if you want a filter that traps smaller particles, they have them and they are 5" thick and cost a lot more money.
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On 10/8/2015 9:08 AM, Bill Gill wrote:

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You could always build a bracket that somehow would fit in the available op ening, but permit you to use a smaller filter that was thicker. But, you h ave to be careful not to restrict the airflow very much as the furnaces ar e designed to be at their maximum efficiency based on a certain airflow. A lso, too low a flow could also lead to blower motor overheating.
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