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The filter in our central air/heat is directly under the unit and very difficult to replace filters. Even though we use 3 month filters, we replace them monthly due to pet hair/dander. It's a pain to replace them and pretty costly to replace them so frequently.

I was wondering if it would restrict air flow too much if I were to use cheap fiber filters in the return air vent to catch the big stuff and the better pleated filter in the unit for the fine stuff so I don't have to replace the more expensive ones as often?

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In my area most duct systems are too restrictive as it is. Adding an extra filter would make them more so unless the other filter was removed and the new one was less restrictive. Problems could include AC icing and furnace overheating. As always you can try it and see.

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Entirely subjective.

But yes, you can do that, and yes it will decrease flow, and yes that's ok.

It's definitely not going to improve flow.

Test it out and compare to single filter to see what heats/cools your house faster. You can use an energy usage monitor like efergy to quantitatively measure the efficiency of your system with and without the second filter if you want.

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There is a common misconception about restriction and the load on the motor and the restricting flow. The squirrel cage blower is designed to move a column of air, "restriction and if taken out of the unit and set in the open, then it would overload the motor. Restricting the airflow reduces the load on the motor until a point of diminishing returns at which the motor is at the rated speed and moving the optimum amount of air. Further restricting beyond this point will actually reduced air flow but the load on the motor will continue to be the minimum. Once enough air is restricted, being almost completely, then the motor can overheat as it relies on air flowing around the motor to keep it cool. The more vents are open, and the larger and shorter the ducts the more load is on the motor and the less it flows after a point.
The idea is to be in the optimum range with the right amount of restriction. Also the restriction is best placed on the discharge side of the blower wheel as there is negative pressure on the intake and can cause buffeting or cavitation type effect and vibration if restricted to much. Also the purpose of the air filter is to keep the unit clean, not to make the air clean for breathing. Usually there is not enough volume to keep the dust in the air long enough for the intake of the air conditioner to act like an air purifier. Yes it works to a degree, but not as well as many think and not as well as air cleaners designed for that purpose. So it is backwards from what most people think. Retired Electric Motor Person with over 30 years experience.

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[Addressing updated question:]
It is possible that a cheap fiber filter will not reduce airflow (CFM) too much if there is one regular one on the unit and one fiber on the return air vent. It entirely depends on the filter. It will definitely have some measurable effect on CFM. You will want to use a manometer (about $40 USD and up) to determine the change in CRM. Measure with on in the unit before the coil in the supply plenum and one outside the unit before and after you replace. It also wouldn't hurt to take a reading before and after on the return side before the coil in the supply plenum. (see below for illustration) For the latter, you'll need to look up your blower unit's specs to determine its rating. You will still likely want to have your ducts cleaned. See this link for more details.

If you don't want to go through the expence and trouble then I would recommend erring on the side of caution and not install two filters. (Keep reading to see why.)

Manometer

[Addressing Original question: Use of two filters. (As well as other helpful info, assumes two regular filters at unit. )]

The reasons to replace the filters in central air systems is because they either become too restrictive on airflow (CFM) or they begin to not work as well by letting in the very things they are trying to filter if not replaced overtime. Putting in two filters might temporarily address the 2nd factor but it will have a negative impact on CFM and may, in fact, require that you replace the filters more often.

Assuming it is a PSC motor in your unit, the actual amp draw on the motor will be reduced with a reduction in CFM (1), the net effect of a reduction in CFM from the return air will cause your Central Air Unit more wear and tear as the motor will have to spin more RPM's to move less air throughout the house putting more wear on the brushes (if it has any) and bearings, and using, in total, more power because it is running longer. Also most blowers run the risk of overheating if CFM is reduced too much. The lower the CFM the more the heat the blower unit generates and this reduces life on the blower as well (1). ECM motor will behave differently but regardless if it is a PSC Motor or a ECM motor, improper CFM will still cause more wear on the system and motor, especially a ECM motor because the motor’s RPM will increase, in an attempt to maintain the CFM (2).


Bottom Line: Putting two filters will not make it last longer but may make you have to replace it earlier, assuming it doesn't already reduce air flow too much with just the filters.

Air Flow Chart

See picture above: Reducing CRM on the return air, will reduce the force of the air coming out of vents as well, making your entire system have to work longer to heat or cool your home to the desired temperatures. Overtime this will put more strain on your entire central air unit including your blower, cost you more money on your utilities, and may cause premature failure.

AC Diagram

Assuming your home also has an A/C condenser, there is also the risk of destroying the compressor. When there is not enough airflow through the evaporator the suction pressure is below normal because the refrigerant flowing through the evaporator picks up less heat than normal resulting in lower pressures. Liquid could be returning to the compressor through the coolant lines which overtime will eventually cause the compressor to fail (3). These compressors are not cheap and neither will the labor to replace! Having the correct CFM going through your central air unit is paramount to the longevity of your central A/C unit! CFM will make or break a Central AC!
Bottom Line: Putting two air filters on a central air system will only have the net effect of producing a higher utility bill and reducing the lifespan of your central air unit.

In short, the use of two air filters is just a BAD idea! I would strongly recommend against using two filters at once, especially if it is just by reason of convenience. You'd be much better off resolving the reason why you have Pet hair in you unit. I'd recommend having your ducts cleaned and if your return air is near the floor, then make sure to keep that area a clean as possible. In any case, replacing these filters is going to be much better than premature failure of any part of your system.

Sources:
(1) http://hvacrfundamentals.blogspot.com/2009/08/understanding-centrifugal-fan-motor.html
(2) https://yorkcentraltechtalk.wordpress.com/2012/09/11/what-is-an-ecm-motor/
(3) https://yorkcentraltechtalk.wordpress.com/2012/07/20/air-flow-effects-on-air-conditioning-revisited/

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    Agreed. If you want more filtration two stacked filters might make sense. If you want longer life for the same amount of dust, you'd need to find a way to spread the airflow over more surface area, which from your description sounds difficult unless you build some sort of outboard filter holder. – keshlam Feb 27 '16 at 1:03
  • I wish I had a dime for every person who fails to understand centrifugal fan loading and posts foolishness like this. Both "strain" and power consumption are reduced as airflow is reduced. It's a simple matter of physics and engineering... – Ecnerwal Jul 16 '17 at 13:11
  • If I understand you, you're trying to say that what I wrote was somehow wrong? Well, if airflow is restricted to the system, then less air flows throughout the house, therefore less cool/hot air is circulated through the system. If the system in question is regulated by a thermostat, then it logically follows, a priori & confirmed posteriori by you're truly, that the system will will have to work longer to achieve the same effect in a given environment to have the effect desired, i.e. lowering/raising the ambient temperature. Hence, the system is, working harder (strain) and using more energy. – JaredW82 Jul 16 '17 at 17:53
  • Jared - no I believe he is referring to my answer and the discussion we have on the comments there. I suspect we all agree that restricting the airflow increases system runtime and that system runtime decreases system life, including and especially motors. Where I am disagreeing with both of you is that it draws more current when restricted. It does - effectively. Throw A/C into a coil of wire and let it sit on a table, it gets hot, this is a KILN or a HAIR dryer. Only difference is it is a motor coil. When the motor moves, it effectively uses less current that the psuedo dead short equivalent – noybman Jul 20 '17 at 0:49
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    Noybman - This is off topic to the question asked, nevertheless, I'll bite. People often forget that air is matter & it has volume. Imagine a propeller in water, then imagine taking it out of the water. Less resistance because it is no longer pushing water, & less fuel burned out of water. It will also spin faster. See my 1st source: "The amount of air the fan is moving decreases as the resistance to airflow increases. If the fan blades are moving less air, they can actually spin easier because there is less air to sling. This causes [...] RPM to increase & the motor amp draw to decrease." – JaredW82 Jul 20 '17 at 5:36
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I have to disagree with Electric Motor Man. Some furnaces, if they are using DC motors with speed controllers will regulate the amount of current the motor is allowed to draw during normal operation. A/C motors are the MOST COMMON in low & mid range furnaces. If your furnace has speed control, it is likely a D/C motor.

In either case, the furnaces with SPEED CONTROL, and A/C motors, are going to spin open ended (meaning no back pressure) at the highest rate of speed that the coils or controller is designed to operate at from the MOTOR manufacturers factory, until a controller steps this down *(if equipped as noted momentarily). Then when this is placed into a furnace by a furnace manufacturer, they are expected to (by good engineering practices) to rate the furnaces load capacity taking into consideration the MTBF (mean time between failure) rated (usually in hours) rating of the motor manufacturers blower hours. Placing more and more and more back pressure on a motor absolutely draws more current, as noted in the previous post, heat is generated! Conversion of A/C to magnetic motion, or to D/C and then Magnetic motion is NOT a 100% efficient process. Heat is a by product of conversions, and physically restricting airflow to a motor is actually equivalent to holding the motors fan blades, or the shaft from turning at all (that is an extreme condition, but "slowing it down" is not). So the point here is, the more the motors operation is hindered, the more current is drawn, up to a point, because eventually the ever increasing heat from aforementioned HIGHER current draw causes the motor to burn out (or a protection circuit, either thermistor or speed controller safety) to shut down.

Some motors have brushes, some don't, some have RPM controls, some dont, but in all cases, if a motor shaft is restricted from turning, it amounts to more current drawn, and as previously noted, less cooling too, in both cases, shortening the life of the motor and raising your electric bill. Now lastly - placing a few filters in a air cleaner is not likely going to be significantly noticeable in electric usage, or strain on the motor, but there is a mathematical impact (its not free).

My thoughts on the OP question,try it, but it might be useful to pay for a duct cleaning. Unless you try it, you wont know if the content truly shortening the filters lifespan is "big" particulates or high density of more microscopic particulates. If the latter, then it wont help. Multi stage filtering is not a bad idea, and is done in many environments (especially commercially) for the same reason you asked, bringing only the pre-filtered air with whatever remains to the more expensive staged filter. (mind you, the expensive filter you are buying is likely already doing this internally)so you might be able to build a 4 stage system (4 1x's) if you have a 4" plenum. Etc.

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    Yet another student fails fan engineering 101. A restricted centrifugal blower fan spins FASTER than one that's "wide open..." - moving the mass of air is what "loads the motor down" and "restricts the shaft from turning faster" thus causing it to burn more power...when less mass of air is moved, the motor sees less loading. – Ecnerwal Jul 16 '17 at 13:08
  • I don't disagree that moving the mass of air slows the blower down, e.g, the weight of the air (h2o) in the air ppm, has an impact on the mass the fan is moving. It is heavier.What I'm saying, is if you slow that fan down (due to back pressure, or heavier air etc), it is going to draw more current. Shortening the motors life. It certainly is not going to spin faster, with a heavier load, regardless of how that load is parlayed to the coils of the motor. – noybman Jul 17 '17 at 23:37
  • Restricting the airflow reduces the mass of air moved, which reduces the motor load. Put an ammeter on the motor and try it. You can also put a tachometer on it and watch it go faster. – Ecnerwal Jul 18 '17 at 13:21
  • Regardless of mass of air, Ecnerwal, you are simply wrong. It seems you have a better understanding of the theory and not the practical applications and you have failed to understand the simple thermodynamics of a central air system. If the volume of airflow through the motor is restricted the it follows that the volume the system is also restricted, therefore, the goal of a central air system is hindered and the house or structure in-question ultimately doesn't achieve the desired temperature as efficiently. Therefore, it must work longer to move more air and achieve the desired temp. – JaredW82 Jul 18 '17 at 14:48
  • If I apply NO power to a motor, and I crank it, it will excite a current in the windings and act as a generator. If I hold the shaft still, the coils are acting as a transformer into ... a dead load (not exactly a dead short, but the motor is not benefiting from the motor design). It will absolutely draw more current than in the normal operation the motor was designed for, the coils heat up, eventually the motor burns out. – noybman Jul 19 '17 at 3:11
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There's another approach that doesn't require multiple filters or replacing filters, and that's to use a permanent filter. The options are initially more expensive than replaceable filters, but they last many years.

You do trade the convenience of simple replacement for the need to wash them regularly, typically on the order of every month to every few months. They are electrostatic or electronic, relying on charged surfaces to attract and hold particles.

There are a number of grades, and you pay for effectiveness. The first several grades are passive filters. They're based on layers of charged plastic fabric or foam that physically block big stuff, like pet hair, and attract and hold small stuff.

The bottom of the line filters won't meet your needs, since you're using a cheap filter to keep big stuff out of a pleated filter. The bottom of the line permanent filters are roughly replacements for your cheap filter, they filter out birds and small children. They cost in the range of one to several decent pleated filters. An example would be something like this.

The middle grade passive filters are much better than a cheap disposable filter, but not as good as a pleated filter. The upper end of the passive filters includes some that are claimed to be roughly equivalent to a pleated filter, but not in the range of a HEPA filter or good microparticle filter. These generally run in roughly the $50-$100 range and have a 10 yr to lifetime warranty. An example would be something like this. I have no personal experience with these and can't vouch for them, but that example seems to generally be well rated.

At the top is an electronic filter. These are roughly equivalent to a high-end pleated filter. An example would be something like this. Honeywell seems to own the market.

The drawback is that they are expensive to put in. They require professional installation because the air handler generally needs duct modification to accommodate it (they're 6 to 7" wide and don't fit a standard filter compartment). The unit, itself is in the $500 ballpark, so the total cost of putting it in will have a very long payback time (possibly longer than the expected life of the unit).

In your situation, a good quality passive filter is likely to be satisfactory unless you have allergies and need a microparticle filter. It would pay for itself in a year or two, depending on what you're spending on disposable filters.

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I replaced two trane 20 year old system s with dual speed trane 18 seer. The unit with the longest travel had a terrible odor. What had changed was the tech recommends I run just filters in house and eliminate the return on the air handlers. I added that filter in the odor unit thus causing the air handler to run longer and in this case more efficient

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    Hello, and welcome to Home Improvement. It isn't clear what you mean here (e.g. was the odor before or after? what was the result of your adding the second filter?); you should add some elucidation to your answer. And, you should probably take our tour so you'll know how best to contribute here. – Daniel Griscom Jun 30 at 0:00

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