Furnace filter air resistance

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Old 02-18-16, 10:47 AM
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Furnace filter air resistance

I am choosing between 3 furnace filters. They are MERV 10 (current standard) and otherwise equivalent except the initial resistance at 300 FPM is .18, .21, and .24. How do I figure out which to choose? BTW, all IR seem high to me.
 
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Old 02-18-16, 11:27 AM
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It's a trade off. A higher level of filtration means more flow resistance which cuts your air flow.

Personally I don't view my HVAC as a filtration system and I don't expect it to be doing anything more than heating and cooling my house. I use a filter to prevent the coils of my HVAC system from becoming clogged with dirt/dust. So, I use a low grade filter. If I install a higher grade filter it will remove more, smaller particles from the air but my system will have to work harder and be less efficient. How much harder it has to work I have no clue. In the end on my system I don't think it's a big difference. If your system is undersized, poorly installed or has airflow problems a super air filter could noticeable affect it's performance.
 
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Old 02-18-16, 12:10 PM
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I pretty much agree with PD.

When customers ask me what filter to buy at the store I ask these questions.

1. Do you have a dust problem?

2. Do you have allergy problems?

If no to both, then I tell them to buy the cheapest they can. Usually that means the cardboard flimsy fiberglass that just barely catches large particulate such as dog and cat hair. If they answer yes to either question then I tell them to start with the low end Filtrite filters that will begin to eliminate smaller particulates. Keep moving up in filtration capacity until a comfortable range is achieved.

True, higher capacity filters will restrict air flow to some degree but the units are designed for it and I would not worry about it. The loss of efficiency is minimal compared to the relief you may get from allergens in the air. Usually this is a bigger concern in the winter than summer. But nowadays with more and more A/C, summer will see a rise in filter replacement and allergy suppression.
 
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Old 02-18-16, 12:50 PM
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One more vote for the lower resistance filters and keep in mind the ones which are supposed to trap such fine particles typically cost more and have to be replaced more often due to the resistance increasing faster with the greater trapping of particles.
 
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Old 02-18-16, 01:08 PM
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Thanks for the responses. Everyone makes sense on the air quality side. But, isn't there an offsetting issue with keeping dust from the blower area. If that wasn't the case, there wouldn't be a need for a filter at all. Very low cost filters don't look like they would stop sand. Is this not a concern?
 
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Old 02-18-16, 01:10 PM
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Dane said it - the cheap ones protect your furnace while the more expensive ones actually affect the air you breathe. I see no reason to pay for the second one but Norm then explained why one would/should.
 
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Old 02-18-16, 01:15 PM
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Did I miss something? Are you saying you have a very dusty (sandy?) atmosphere? The filter filters the cold air return from the house, not the air in the motor section. Allowing for lots of holes and such its a closed system that gets circulated and goes through the filter. The motor area should not see any filtered air going through the house. Most furnaces will be located in the basement and the motor and drive will see only that surrounding air, not the air being heated or cooled that gets circulated throughout the house.
 
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Old 02-18-16, 01:27 PM
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Norm, I suspect I understand this even less than I thought. My understanding is that the air from the intake part of the system is filtered by the air filter. The filtered air goes through the blower and is pushed (blown) out. Is that not the case? If not, where's the heated are coming from?
 
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Old 02-18-16, 01:44 PM
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You have a heat exchanger. The gas heats this up. The motor has a pulley with a fan belt attached that goes to a squirrel cage type fan that is either in front of or on top of heat exchanger. This all contained in the main plenum. The heated air is circulated through the ducts and the return air comes back to the heat plenum, but will go through the filter media first.
I'm keeping this very simplified. The actual physical layout can very between various manufacturers.

You don't have an air intake per say. The air being used to be heated or cooled is the surrounding air in your house that you breath. Again the filter is located at the cold air return as it enters the main plenum. So if you have a dust problem or allergy problem you want that air filtered before it gets heated up and circulated. Will dirt and dust build up on the motor, fan and other parts of the unit? Yes. But that is just normal and a yearly tune up by your furnace company will be happy to charge you $$ for a yearly cleaning. Not needed on a yearly basis. Perhaps every 3 to 5 years.
 
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Old 02-18-16, 01:57 PM
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All of the responses are true IF you limit yourself to the one-inch thick filters. Use a three or four inch pleated filter and you most certainly can use a higher MERV rating and catch more dust without straining the blower motor or damaging other parts of the system.
 
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Old 02-18-16, 02:31 PM
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All of the responses are true IF you limit yourself to the one-inch thick filters. Use a three or four inch pleated filter and you most certainly can use a higher MERV rating and catch more dust without straining the blower motor or damaging other parts of the system.
True. But most furnaces are only built to take the one inch filters. 16 x 20 being the most popular. Those 4" pleated filters cost an arm and a leg to boot. And I some times if they really make that much of difference.

One other note concerning filters. Its the only item I know of that the manufactures are not trying to sell more product than necessary. They say good up to 3 months. Seriously? I tell my customers to check the filter every four weeks and most likely replace after six to eight weeks. Now if only I would follow mu own advice!
 
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Old 02-18-16, 02:50 PM
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They say good up to 3 months. Seriously? I tell my customers to check the filter every four weeks and most likely replace after six to eight weeks.
Basing the necessity of a filter change on appearance is wasteful. In a former life I used to change filters as part of my job. I have seen filters that were absolutely black yet when checked with a pressure drop manometer they showed either no more drop than a new filter or just slightly more than a new filter. ALL filters should have a pressure drop gauge and then change the filter according to the pressure drop, not the appearance.

If I am using the cheap one-inch filters I regularly get three months service and when I use the three-inch I get about a year.
 
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Old 02-18-16, 02:57 PM
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Furd,
I know of what you speak. Every year at our product show the filter people set up a manometer pressure drop demo. Very convincing but to very little avail. People being people will use looks to determine if filter is good or bad. And who's going to pay for a manometer gauge on a new furnace? All in all filters are cheap.
 
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Old 02-18-16, 03:13 PM
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I've decided on a new policy in responding. I'm not going to argue any more. I'll make one rebuttal and that's it.
 
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Old 02-19-16, 09:41 AM
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OP 13 years old!!!!!! lol
 
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Old 02-19-16, 02:31 PM
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Thanks to all. I fully understand your thoughts and am about ready to put this to bed. But not quite yet. In looking at non-pleated filters, I see virtually all are made of a spun material, often glass. There is no covering over the material in the event pieces tear off. Should I avoid these products or am I over-analyzing?
 
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Old 02-19-16, 04:50 PM
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In my opinion you are over analyzing. But no harm done. If you feel more comfortable with pleated filter then by all means use them. The cost difference is minimal.
 
 

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