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Someone from an HVAC duct cleaning company told me today that MERV-16 charcoal filters are harder on the furnace and that the charcoal is a gimmick that just costs more. He said he went to a class and everything, making it sound like MERV-16 was a bad idea. He said that air flow will suffer and threfore heating/cooling efficiency will to.

Ultimately he recommended MERV-13 because it will put less pressure on the blower motor.

Since my sensitive nose does notice the difference, I did some digging on the Lennox filters we have been using and found this pdf where, on page 5, Lennox provides the specs for a MERV-13 and the actual MERV-16 model we have been using (Lennox X6672).

Our filter is the 16x25x5" model, which is the center column on the two spec sheet images below. Note that the CFM rating for both MERV-13 and MERV-16 are the same. There is a small difference in "Initial" pressure drop but the final pressure drop for both is 0.40 in. w.c. as you can see below.

According to our furnace's installation document, "This furnace is not certified for operation [...] with an external static pressure which exceeds 0.8 inches w.c." and both filters shown below max out at 0.40 in. w.c. and page 42 of the furnace install document (page 48 of the PDF) indicates that CFM maxes out at about 1365 @ 0.50 in. w.c. (I think, don't actually know our blower size). So if I understand correctly the filters below should work fine since they spec a CFM of 800-1600.

In case you it matters: this furnace serves 8 vents upstairs with 2 returns, and downstairs there are 2 vents with 2 returns with a total space of ~2200 sqft and 8' ceilings.

Questions:

  • Given the two filters shown below, is there really a difference between the two that would affect furnace performance?
  • Other considerations?

MERV 13 Specs

Lennox MERV13 Filter Specs

MERV 16 Specs

Lennox MERV16 Filter Specs

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    The furnace's static pressure limit applies to all static pressures combined, not just that which derives from the filter.
    – Harper - Reinstate Monica
    Commented Sep 28, 2022 at 22:44
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    Someone from an HVAC duct cleaning company should've told you that if you use any type of media filter like that then you never have to call them for anything. And since your sensitive nose does notice the difference, then you're going to use 16s no matter what extra strain it puts on the blower, right? The filter on the unit is to protect the unit. If you want to abuse your air-handler as an air filter, go nuts (not that it really does) but if you're serious about air quality, get a TFP. hvac-talk.com/vbb/threads/97676-Lifebreath-air-cleaner
    – Mazura
    Commented Sep 29, 2022 at 2:45
  • Can you get your duct static pressures measured?
    – ThreePhaseEel
    Commented Sep 29, 2022 at 3:23
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    In my experience most HVAC people operate on rules of thumb. It's a good rule of thumb to not use a filter way outside the range of "normal" without careful design and measurement. Give your guy the benefit of the doubt ... he doesn't know how to properly install a MERV 16 filter on your furnace, and he knows well enough not to try it randomly. But YOU seem to know enough to do this properly. A manometer costs $50 and you can learn to use it on youtube. You don't have to follow rules of thumb, you can learn, and measure, and MAYBE you can use these filters! .......
    – jay613
    Commented Sep 29, 2022 at 14:48
  • It's also possible your guy has enough experience to know without measuring that THIS filter on THIS furnace won't work. You could find out by asking the right questions.
    – jay613
    Commented Sep 29, 2022 at 14:49

3 Answers 3

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More static pressure = Blower motor dies sooner

If it's worth it to you to replace your blower motor every few years then go ahead and do as you please. The tech just doesn't want to be at fault due to your negligence.

Your furnace has an acceptable static pressure range so get a reading before and after installing the MERV-16. Additionally, check the static pressure monthly so that you replace the filter before static pressure reaches damaging levels.

Another option could be to replace your blower with one that can handle greater static pressures.

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    Hey @KJ7LNW you seem to be enough into the technicalities of this to buy a $50 manometer and measure the actual pressure in your furnace return chamber with various filters installed. Try it with vents open or closed as they typically might be by occupants of their rooms. An early-warning indicator would be rapid cycling of the furnace (gas flame) or A/C compressor or freezing of the A/C coils in the blower. "Rapid" is relative to "normal" which you hopefully have some sense of.
    – jay613
    Commented Sep 29, 2022 at 14:38
  • If it is a variable speed blower, and the blower keeps the static pressure constant based on its own internal sensors, then wouldn't the motor have the same amount of load, and in fact, spin slower at the same static pressure?
    – KJ7LNW
    Commented May 27 at 16:22
  • @KJ7LNW That is a gratuitous amount of assumptions for a single sentence, I'll try to explain...
    – MonkeyZeus
    Commented May 28 at 13:43
  • @KJ7LNW A variable speed motor knows nothing about the CFM it's producing nor the static pressure it's encountering; if it did then it would be smart enough to fault when the static pressure is too high. You set a desired temperature and the motor operates on a pre-defined speed curve to achieve the requested temperature. If there is a bunch of static pressure then it has to operate for longer periods to achieve the desired temperature resulting in mechanical strain, increased electricity usage, and general comfort issues for you because it takes longer to hit a desired temperature.
    – MonkeyZeus
    Commented May 28 at 13:43
  • @KJ7LNW Google "why does static pressure kill furnaces" and try to wrap your head around how static pressure affects CFM which affects how long it takes to reach a desired temperature.
    – MonkeyZeus
    Commented May 28 at 13:45
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I am an a/c specialist who works for a large company designing and installing very complex HVAC systems. Thank you very much for supplying the specs on the filter. Having this knowledge allows me to help explain to everyone how their HVAC systems work and answer the MERV 13 is better than a MERV 16. When you talk about static pressure you’re on the right path. Static pressure is the pressure tolerance of your system. The static pressure is determined by the size of your ducting and filter. So yes, if you have incorrect sized ducting with a stronger filter you will increase the static pressure and the wear and tear on your system, you’re better off going with a weaker filter for better air flow. If your system is designed correctly with proper sized ducts and with a MERV 16 filter you will have good static pressure and no issues. Think about it for a minute and it starts to makes sense.

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    The furnace has a variable-speed blower that always keeps the static pressure in spec by adjusting the blower RPM, so the static pressure supported by the furnace is never exceeded. In this scenario, how could it increase wear?
    – KJ7LNW
    Commented Jan 14 at 21:53
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My trane is 19 years old this year with a Honeywell 20x25 media filter on it and I've been running a Lenox Merv 16 on it for 15 years. I've only changed the igniter. Two stage variable speed.

The company installed it with just a Merv 10. Then I went to 13 and then I went to 16. Just like anything else it's all about maintenance change your filter regularly don't let it get clogged.

Sure put a merv 8 in your furnace like the guy said instead of 16 and just clean your ducts more often.

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  • This is just anecdote and will not apply to all situations, so the implications are not accurate. In addition to making the motor work harder, you can also increase output temperatures to the point that thermal switches trip. Maybe your unit was oversized for the ductwork enough that the more resistive filter didn't have a detrimental effect, but that will be true for everyone. Also, we don't tolerate name-calling on this network.
    – isherwood
    Commented May 25 at 23:14

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