Recommended Merv rating for filter

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Old 04-17-19, 04:10 PM
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Recommended Merv rating for filter

Hello, I have owned my current home for 2 years, looking for recommended Merv rating under my circumstances. I believe my a/c system is as old as the home, late 90's.
The condenser sets right behind my bedroom wall, the compressor was very noisy so I covered it in roofing material, second day owning the home the unit started blowing hot air, I changed the capacitor and contactor, while those parts were on the way, I used a dremel to clean up the contactor and it fired right up, but I changed the parts anyway.

I had read that a lighter air filter will actual prolong the life of the air handler as you will put less stress.
I currently am using a Merv 4 rated filter. I just went up in the attic and confirmed there is no filter inside my Ruud Silhouette II. Since there is no secondary filter, am I ok with only a Merv 4? the furnace is pretty dirty inside now.

 
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Old 04-17-19, 04:30 PM
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Merv 4 filters are good for allowing airflow, but aren't able to filter out the smallest particulates. You really need to find out what your furnace specs recommend. Higher Merv filters restrict airflow, so higher Merv is not necessarily better.
 
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Old 02-26-20, 10:44 AM
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Might as well jump on this old thread rather than start a new one, as I have the same question.

I have a Carrier Weathermaker 9300TS furnace. I can't find any info about this on the Carrier site or anywhere else, I cannot find a better model number on the inside furnace panels anywhere. So I have no "furnace specs" telling me what level of filtration I should use.

After last year's CA wildfires, I installed MERV 11 to help filter smoke out of the indoor air. (The fires were distant, but the smoke blew in our direction.)

Air flow through the 4" pleated filter seems fine, but recently I read somewhere on the interwebs that 11 is too restrictive for residential use and I should use MERV 8 instead. How to determine the optimal filtration in the absence of truly reliable documentation?
 
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Old 02-26-20, 10:59 AM
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Under most circumstances a residential HVAC system is not designed to filter air, just heat and/or cool it. The primary reason is that the entire system is not well sealed allowing unfiltered air to enter. This becomes clear when recognizing that the filter is at the return to the air handler rather than at the point of supply to the living area.

The reason for locating the filter at the air handler is to protect the air handler from damaging particles. That being the case, the filter should be sufficient to protect the equipment and more filtration is both unnecessary and inefficient.

If air filtration is desired for health or other reasons, the best course is to purchase an air purifier.
 
clancy voted this post useful.

Last edited by Tony P.; 02-26-20 at 11:27 AM.
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Old 02-26-20, 12:44 PM
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"Air flow through the 4" pleated filter seems fine, but recently I read somewhere on the interwebs that 11 is too restrictive for residential use and I should use MERV 8 instead."

From my reading about this, is that those MERV recommendations apply mostly to 1" filters, which most people, including myself, use. With a 4' filter, there is much more (at least 4X) surface area of the filter, and therefore is less restrictive, so you can get away with a higher MERV than I can with my 1" filter. Also, these thick filters "last" a lot longer, in some cases requiring changing every 6 moths or so -- it takes longer for filtered particles to clog up the filter with a large surface area than it does a thinner filter (1") with a lot less surface area that may clog up in a month. But at what point do you know that any good filter (at any thickness) has become too clogged and, therefore, too restrictive? Probably you don't know, and really, cannot know, so those recommendations to change out a 1' filter every month is a safe way to go, regardless of MERV; I'm guessing that a 4" filter could "safely" be used for 3 or 4 months...?? I have a bunch of 1" 3M pleated filters I bought at Sam's Club when they were on sale.; I'm now changing these out every month. I am seeking advice about poly filters on another thread in this section of the forums; will change over soon as my 3M supply is getting low.
 
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Old 02-26-20, 01:15 PM
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I've been changing my 4" pleated filters every 6 months and they come out pretty clean. I could probably go 9 months if we don't get any more smoke.

My returns are in the ceiling, so floor dirt, pet hair, etc. never enters the system, which is a brilliant setup. Airflow through the furnace is downward, with registers in the floors. This is also brilliant, as gravity makes sure no particles fall out of the filter as they might with upward air flow. Just gotta not drop things down the floor registers.

I understand the main purpose of the filter is to protect the furnace. But it seems logical that a high enough MERV will also help clean the air - unless, as mentioned, the duct system is not airtight. Ugh, I don't feel like crawling under the house to check.

I suppose I could experiment with different MERVs and measure airflow, but that sounds like work. Can I presume that if the airflow "feels" good that my filter's not too restrictive?
 
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Old 02-26-20, 01:16 PM
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big.ron, I don't believe there is a typical replacement time for HVAC filters. One reason is that filters get more use during the hottest or colder periods so they are most likely to require replacement at those times. There are other times when the HVAC system is used less so filters need not be replaced as often. A second reason is that individual homes have different levels of dust and other particles. For example, homes with pets or in the desert may require replacement more often than others.

In terms of when to replace, the filter needs replacement when dirty. Check the filter by holding it up to the light and replace it when limited light passes through.
 
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Old 02-26-20, 06:34 PM
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I ran across this YouTube video a few days ago. It's pretty long, about 25 minutes, but it is interesting. Take a look if you can.

https://www.youtube.com/watch?v=GoOKjINPgpY
 
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Old 02-27-20, 03:54 AM
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My unit uses 4 inch Merv 11 and I replace them every 6 months.
That is what my furnace guy recommended when my high efficiency furnace was installed.

He said the minimum he would recommend is Merv 8 as new furnaces are more susceptible to problems caused by small particles and the 4 inch also gives good air flow.

I use 11's because they are easier to buy than 8's which are hard to find in the size I need.

The next time I change it I will do a similar test between the new and old to see if perhaps I need to replace them more often.



 
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Old 02-28-20, 08:51 AM
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I've done a lot of reading and thinking, and I think I will try 2-in. pleated filters, MERV 11. (My unit will accept 1- or 2-inch filters only.) I've always used 3M; currently the only filtering spec available at Sam's Cub is their rating 1200D, which should be around MERV 11. I've been using something like this for over 25 years, and haven't had a motor/blower problem. The 2 inches will provide less resistance yet give me the filtering I've been using.. The cost is about the same, as these Sam's 3M 1-inch, MERV 11 filters are $10 each when not on sale. A lower MERV rating and/or poly filter might be safer for my equipment, but I want the best of both that as well as some health benefit.

I stumbled across this web site, and they offer their own branded filters as well as Glasfloss filters. Both filters seem to have identical properties, yet the branded ones cost more,,,, I'll have to ask them why. Both do not absorb moisture and do not support microbial growth, which is important to me (as I'm sure I have a mold problem already on my 10-month old system... I'll probably start a thread about that as I work towards correcting that problem).

Furnace Filters 2" Pleated

2" Pleated Filters

 
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