# what is best furnace filter to buy?



## yuri (Nov 29, 2008)

Filters except 3m are rated in MERV (google that for more info). The higher the merv the more air is restricted and the harder it is for your furnace fan to move it. This can result in an overheated and damaged furnace or frozen and damaged AC. Only way to know for sure is to try it and check the temp rise thru the furnace after is has run for 5 minutes. Stick a thermometer in the supply duct downstream a foot away from the plenum and subtract the house temp from it and check the temp rise specs on the model # sticker inside the furnace. ie:35-65degF. I recommend a Merv 8 filter and unless you have a newer furnace with an ECM motor and large ductwork you can have problems with the more restrictive types. AC needs checking by a Pro to see if it has enough airflow.

http://www.furnacefiltercare.com/merv-ratings.php


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## dinosaur1 (Nov 22, 2009)

yuri said:


> Filters except 3m are rated in MERV (google that for more info). The higher the merv the more air is restricted and the harder it is for your furnace fan to move it. This can result in an overheated and damaged furnace or frozen and damaged AC. Only way to know for sure is to try it and check the temp rise thru the furnace after is has run for 5 minutes. Stick a thermometer in the supply duct downstream a foot away from the plenum and subtract the house temp from it and check the temp rise specs on the model # sticker inside the furnace. ie:35-65degF. I recommend a Merv 8 filter and unless you have a newer furnace with an ECM motor and large ductwork you can have problems with the more restrictive types. AC needs checking by a Pro to see if it has enough airflow.
> 
> http://www.furnacefiltercare.com/merv-ratings.php


But would you stick to a metal or paper filter overall?


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## yuri (Nov 29, 2008)

Post a pic of the metal filter. IMO none of them and the so called "electrostatic" membrane ones are worth a darn. MOST people don't clean them properly and they tend to gum up and plug up internally with scum. I have seen lots of frozen and damaged ACs from those filters not been cleaned properly and I don't recommend them at all. The pleated MERV filters are guaranteed/rated to do what they are supposed to vs. the "buy guess and by golly" aspect of those other filters. For under $20 for a 3 pack of Merv 7-8s how can you go wrong. ACs and furnaces are over $4000 so the price of the filter is hardly an issue IMO.


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## dinosaur1 (Nov 22, 2009)

yuri said:


> Post a pic of the metal filter. IMO none of them and the so called "electrostatic" membrane ones are worth a darn. MOST people don't clean them properly and they tend to gum up and plug up internally with scum. I have seen lots of frozen and damaged ACs from those filters not been cleaned properly and I don't recommend them at all. The pleated MERV filters are guaranteed/rated to do what they are supposed to vs. the "buy guess and by golly" aspect of those other filters. For under $20 for a 3 pack of Merv 7-8s how can you go wrong. ACs and furnaces are over $4000 so the price of the filter is hardly an issue IMO.


My furnace was installed when the home was built. 1997.


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## dinosaur1 (Nov 22, 2009)

yuri said:


> Post a pic of the metal filter. IMO none of them and the so called "electrostatic" membrane ones are worth a darn. MOST people don't clean them properly and they tend to gum up and plug up internally with scum. I have seen lots of frozen and damaged ACs from those filters not been cleaned properly and I don't recommend them at all. The pleated MERV filters are guaranteed/rated to do what they are supposed to vs. the "buy guess and by golly" aspect of those other filters. For under $20 for a 3 pack of Merv 7-8s how can you go wrong. ACs and furnaces are over $4000 so the price of the filter is hardly an issue IMO.


I found these at Sam's Club. What do you think?

Filtrete

Allergen reduction filter
1200 performance rating
Lasts up to 3 months
Approved by the American Lung Association
2 twin packs


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## Jackofall1 (Dec 5, 2010)

dinosaur1 said:


> I found these at Sam's Club. What do you think?
> 
> Filtrete
> 
> ...


You may experience problems with that one

Mark


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## dinosaur1 (Nov 22, 2009)

Jackofall1 said:


> You may experience problems with that one
> 
> Mark


Why is that


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## Jackofall1 (Dec 5, 2010)

Yuri mentioned it already, you could try it, but be weary that you cause flame rollout, higher temp rise through heat exchanger than max design and AC "A" coil freezing in the summer.

Basically the better the filter the higher the resistance the higher the resistance the less air flow.

Go ahead try em, if they work for ya then cool, I would check the temp rise as a min, just to make sure you aren't cause heat exchanger problems.

Mark


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## dinosaur1 (Nov 22, 2009)

Jackofall1 said:


> Yuri mentioned it already, you could try it, but be weary that you cause flame rollout, higher temp rise through heat exchanger than max design and AC "A" coil freezing in the summer.
> 
> Basically the better the filter the higher the resistance the higher the resistance the less air flow.
> 
> ...


Where did you find a 3 pack for under $20?


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## Jackofall1 (Dec 5, 2010)

dinosaur1 said:


> Where did you find a 3 pack for under $20?


I didn't I believe what Yuri was getting at was, the filters you can use without fear of causing problems with your systems sell for about $20 for a 3-pack.

It would be like the Filtrete with a rating of 1000, I believe.

Mark


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## dinosaur1 (Nov 22, 2009)

Jackofall1 said:


> I didn't I believe what Yuri was getting at was, the filters you can use without fear of causing problems with your systems sell for about $20 for a 3-pack.
> 
> It would be like the Filtrete with a rating of 1000, I believe.
> 
> Mark


The ones I found have a 1200 rating.


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## Michael Thomas (Jan 27, 2008)

If you use a HVAC contractor who is into "measured performance" and actually checks pressures, they will generally find that ANY of the high MERV filters are creating excessive pressure on the return side and a reduced air flow across the heat exchanger / A-Coil to the point where system efficiency is significantly reduced.

One clue that this is occurring is "oil canning" of the ducting - when the blower comes on sides of plenum are bent inwards, and then pop back out, - often with an easily audible sound - when the blower goes off.

When I see that (unless it's because the return side of the systems his horribly mis sized) I know I will find either a badly obstructed (dirty) filter, or one of the high MERV types.


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## dinosaur1 (Nov 22, 2009)

Michael Thomas said:


> If you use a HVAC contractor who is into "measured performance" and actually checks pressures, they will generally find that ANY of the high MERV filters are creating excessive pressure on the return side and a reduced air flow across the heat exchanger / A-Coil to the point where system efficiency is significantly reduced.
> 
> One clue that this is occurring is "oil canning" of the ducting - when the blower comes on sides of plenum are bent inwards, and then pop back out, - often with an easily audible sound - when the blower goes off.
> 
> When I see that (unless it's because the return side of the systems his horribly mis sized) I know I will find either a badly obstructed (dirty) filter, or one of the high MERV types.


So I should stick to a 1000 rating instead of 1,200?


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## Jackofall1 (Dec 5, 2010)

In a nut shell, yes

Mark


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## dinosaur1 (Nov 22, 2009)

Jackofall1 said:


> In a nut shell, yes
> 
> Mark


So that's like a Merv 11 

Pleat Spacing on Filtrete® Micro Allergen Reduction Filter
The Filtrete® Micro Allergen Reduction Filter carries a 3M Microparticle Performance Rating (MPR) of 1000 - roughly equivalent to a MERV 11 rating


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## dinosaur1 (Nov 22, 2009)

yuri said:


> Filters except 3m are rated in MERV (google that for more info). The higher the merv the more air is restricted and the harder it is for your furnace fan to move it. This can result in an overheated and damaged furnace or frozen and damaged AC. Only way to know for sure is to try it and check the temp rise thru the furnace after is has run for 5 minutes. Stick a thermometer in the supply duct downstream a foot away from the plenum and subtract the house temp from it and check the temp rise specs on the model # sticker inside the furnace. ie:35-65degF. I recommend a Merv 8 filter and unless you have a newer furnace with an ECM motor and large ductwork you can have problems with the more restrictive types. AC needs checking by a Pro to see if it has enough airflow.
> 
> http://www.furnacefiltercare.com/merv-ratings.php


So I should get this one?

http://www.filtrete.com/wps/portal/...1011_nid=3JNKR6364PgsQHZF8K475Hgl85WZKW2WB2bl


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## Jackofall1 (Dec 5, 2010)

You could try that, but, again do you have a meat thermometer? With one of those filter in place and the furnace running for about 5 min put the thermometer in the plenum above the heat exchanger and see what temp you are getting, then look at the thermostat and subtract that temp from the one you measured.

Take that and look at the tag in the furnace, are you in spec? If you are no problem if you aren't then change the filter to a lesser grade.

Heres a complete list that I have found, I mispoke I though a rating of 1000 was closer to MERV 8 it not and MERV 8 is what you furnace heat exchanger was designed for.

Here are some of the Filtrete filters offered by 3M: 
Dust & Pollen Filter, Microparticle Performance Rating 600, 
MERV rating of 8 
Micro Allergen Filter, Microparticle Performance Rating 1000, 
MERV rating of 11 
Ultra Allergen Filter, Microparticle Performance Rating 1250, 
MERV rating of 11 
Advanced Allegen Filter, Microparticle Performance Rating 1500, 
MERV rating not available 

Good Luck,

Mark


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## yuri (Nov 29, 2008)

Those red ones are not too bad and don't seem to cause problems where I am but for some reason we have larger ductwork than a lot of what is commonly used in the US. I prefer filters with a Merv rating stamped on them like the True Blue kind from HDepot. The other guys are a bit vague in their info. Irregardless of what you use the temp rise should be checked thru the furnace and airflow and freon pressures check in the summer by a Pro.


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## yuri (Nov 29, 2008)

When checking temp rise it is important the thermometer be a foot downstream from the plenum/bonnet and not be able to "see" the heat exchanger as radiant heat from it will give a false reading.:yes:


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## dinosaur1 (Nov 22, 2009)

yuri said:


> Those red ones are not too bad and don't seem to cause problems where I am but for some reason we have larger ductwork than a lot of what is commonly used in the US. I prefer filters with a Merv rating stamped on them like the True Blue kind from HDepot. The other guys are a bit vague in their info. Irregardless of what you use the temp rise should be checked thru the furnace and airflow and freon pressures check in the summer by a Pro.


The 700-1085? I think 600 sounds like the way to go.


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## yuri (Nov 29, 2008)

I would start with it. May want to buy one of the higher grade but get the temps and freon pressures checked normal preventative maintenance that needs to be done) and ask the tech whether it is safe to use. No way to verify that over the net.:no:


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## dinosaur1 (Nov 22, 2009)

yuri said:


> I would start with it. May want to buy one of the higher grade but get the temps and freon pressures checked normal preventative maintenance that needs to be done) and ask the tech whether it is safe to use. No way to verify that over the net.:no:


You would start with 700-1085 vs 600? Wouldn't 600 be safer to start with?


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## yuri (Nov 29, 2008)

Start with the lowest grade (600)which is safer, and work your way up one grade if you find it is not doing as good a job as you like. If you want a REALLY good filter system which has very little resistance/pressure drop we install these on 95% of our jobs. 5" pleated filter. Need a contractor to install it if you have room in your ductwork or when you get a new furnace.
http://www.lennox.com/products/indoor-air-quality-systems/HC10/


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## Jackofall1 (Dec 5, 2010)

Its all based a heat exchanger performance, if you are not going to check that performance than yes for sure stick with the 600. 

If you are going to measure the perfomance then you can experiment.

Mark


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## dinosaur1 (Nov 22, 2009)

Weird thing is consumer reports rates the 3M 2200 the highest. 600 the lowest.


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## Michael Thomas (Jan 27, 2008)

yuri said:


> temps and freon pressures checked normal preventative maintenance that needs to be done) and ask the tech....


That's really the key: find an HVAC contractor who knows how to _measure_ actually performance, and understands _what the measurements mean_. This is common in the commercial HVAC world, but at least here in Chicago such contractors are far in the minority at residential work - a few percent of the total. 

(I know this because these tests leave physical evidence they have been performed, and I almost never see such evidence at home inspections).

There are reasons why this is so: such testing it takes time, it's hard to get customers to pay for this sort of investigation, it's hard to get customers to understand the result, and it's _*really*_ hard to get customers to pay to fix the problems that are found - both because it's often expensive to do so, and because "it seems to be running fine as it is...".

But... these are the contractors who will find the furnaces that are running at far below potential efficiency because of poor design and incorrect installation, and the ones who can achieve additional efficiency at existing installations.


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## Michael Thomas (Jan 27, 2008)

dinosaur1 said:


> Weird thing is consumer reports rates the 3M 2200 the highest. 600 the lowest.


They are testing in a very narrow context, not in terms of overall system performance.

Also, the situation is complicated.

For example, a filter that traps a greater percentage of the airborne particulate matter should in theory decrease the amount of material trapped on the AC coil, which should slow the rate at which such material decreases system performance - however the rate at which material sticks at the coil also depends on particulate size, and as a filter traps increasingly smaller particles, it has decreasingly less effect on how material is deposited at the coil... it makes a difference whether you are filtering out cat hair or pollen.

____________

As noted above, one way to _both_ increase capture _and_ reduce back pressure at the filter is to use a thicker pleated filter, which has greater surface area.


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## beenthere (Oct 11, 2008)

Use a standard pleated air filter. And a bypass HEPA filter.


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## yuri (Nov 29, 2008)

Like this one:

http://www.lennox.com/products/indoor-air-quality-systems/HEPA/

There are other brands (Lifebreath makes one). They can be expensive to maintain as a HEPA filter kit may be $200 or more. Great for people with allergies, asthma etc.


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## AClogston (Jan 23, 2011)

Very informative thread, I've learned quite a bit.

In my apartment I've got an old furnace (manufactured in '86 installed in '91) and just yesterday the ancient washable filter disintegrated on me (cheapskate landlord)when I was washing it, so I had to replace it. 

Before reading all of this I headed to the local hardware store to pick up a new one. They had three choices, one which was rated at MERV 11, which I bought and installed. After reading this I'm thinking 11 might be a little high for the old girl. I went back to the store and the filters that were more expensive were the Filtrete ones rated at 1000 (MERV 10 if I understand correctly), and the only other option were the True Blue ones rated at MERV 2. Which of these should I get? Is MERV 10 probably still too high for an older furnace, and will one with a rating as low as 2 present any problems?

Any help appreciated.


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## yuri (Nov 29, 2008)

I would never use any filter higher than a MERV 8 on any furnace unless it has a powerful ECM motor or large ductwork which lots of people don't have. (tests needing doing to make sure if a higher grade is used). True Blue makes Merv 7 and 8 at HDepot. I buy them there for my neighbor. Also at Canadian Tire in Canada. Merv 7 is good for yer ol gal. 2 is better than nothin but not much better.


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## AClogston (Jan 23, 2011)

Thanks yuri, I will try to find something around 7-8 ASAP. The 2 won't harm the furnace in the meantime will it?


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## yuri (Nov 29, 2008)

No.....


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## AirstarFilters (Jul 5, 2010)

I couldn't agree more...MERV8 unless there is an ECM motor of sorts to overcome the static pressure


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## beenthere (Oct 11, 2008)

Those see through blue fiberglass air filters were used for a long long time before those 3M came out.

It won't hurt the furnace.


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## ALPINEHVAC (Sep 17, 2012)

*Not all Metal (Electrostatic) Filters are created equal*

I realize this is an old post but I have had a lot of experience with electrostatic filters. Keep in mind that several manufacturers make electrostatic filters and each has it's own specs and design. Like any filter, there is a balance between dust capture and airflow. I used dusteater brand at home and did not like it...to much resistance. I have also learned to ask my customers if they are good at remembering to change their furnace filters. If they forget for months at a time, I do not recommend any permanent filter. Most of my older customers have a good routine for checking their filter every month. In this case I have found that the boair brand electrostatic filter works very well. It has very low resistance to airflow and traps dust well. The unique thing about boair is it contains 5 layers af thin media and no polyester so it is easy to wah clean. I've been using them in my units at home for about 10 years.


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