If a single filter is used, the fan might fail. Using additional filters decreases the load on the fan, because there are more ways for the air to come in, so the torque required for a certain CFM value (which stays fixed for a certain fan speed) is lower.
Filter's airflow restriction is primarily a function of surface area and filtration medium layers. More surface area results in a better airflow and more filtration medium layers results in a worse airflow.
To maximize surface area of a filter, you can get a thicker accordion filter. This will generally provide good airflow and filtration characreristics.
Sorry about video links below, I liked them at chart times, so you don't have to watch the video to check out the summary.
Here is an example of airflow restriction vs filter. Note that 4 inch Honeywell filter only provides small static pressure rise over no filter option: https://youtu.be/RkjRKIRva58?t=456
Thicker filters cost more, but also last longer. General rule seems to be that when you double the filter thickness, time between replacements doubles as well.
In a previous house I rented, we swapped the furnace filter for a much finer one (from 8 to 13 on whatever scale they market these things in), and it made the HVAC system run noticeably worse. Lower airflow, noisier, etc. It seemed to me that the intake system couldn't keep up with the reduced flow of the denser filter.
Could moving to a coarser filter but adding room filters be useful in that context?
Efficiency in terms of airflow goes down, and static pressure difference goes up - but the filters trap more of the particles going through them. Changing filters on furnaces and AC units is really important, since lack of airflow hurts efficiency.
Wirecutter (which is usually trash) confirmed this in testing; versions of units that had been running for months or longer (ie one they bought a year ago and used in someone's home, vs a new unit bought new with little run time) tended to perform better in terms of how many particles they completely removed.
The unit starts using more electricity to do the same work, has to run at higher fan speeds, etc. So it's a tradeoff between that and the cost and waste generated by buying more filters.
It’s a trade off though, because good filters will keep dust out of the HVAC systems innards, which according to the techs I’ve talked to makes a difference in longevity.
It "may" reduce efficiency, but as long as you don't restrict airflow too much, it seems unlikely to damage anything. It's not like restricted airflow puts stress on the AC blower. In fact, less airflow creates more of a vacuum and causes the fan to draw less energy.
The one thing it can do is cause your equipment to run longer (because of the lower efficiency) thus wearing things out quicker. But, most HVAC systems are not nearly as "fine tuned" to the size of your house as many might think. Most traditional HVAC systems basically have 2-3 speed settings for the fan depending on home size. There is a lot of play between those rough settings where a bit more filtration is not going to put it out of recommended ranges.
I have a very high Merv rating filter on mine plus a charcoal filter, but I change it out every 3 months or so.
Compared to the standard, cheap, barely there filters, the run time and energy usage difference is not enough to notice.
I've had HVAC techs tell me not to use those thick filters unless your system is designed for it. They make the fan work harder to pull less air. This reduces how much air it can move.
We were getting a new system and the installers recommended against using 3M filters because ‘they can harm the system’ etc. They recommended an extremely porous ‘filter’ with little to no pressure drop.
Turns out, if you use a filter that actually works, you have to be more diligent about changing it, or else the pressure drop gets high enough to put pressure on the HVAC. There are new filters that monitor the integrated pressure drop to track when the filter needs to be changed.
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