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  1. #1
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    Dec 2019
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    Need Help selecting Merv Rating

    Hello everyone -

    First post here! I need some help determining the max MERV filter rating my furnace can handle. I've been told for the past year that I should not use a MERV filter due to the restriction, pressure drop and damage it causes to my system and compressor. Unfortunately, my boy suffers from pollen and dust allergies - so using a cheap (able to see your hand - not just the shadow - through the filter) filter is just not doable. My HVAC tech strongly pushes that I use only the thin pre-filter provided with my system.

    I cannot find anywhere in my owners manual a filter spec other than "Low Velocity". Can an expert please look at my systems and tell me max MERV I can use before any real damage occurs? Specs are below:

    Home is 4,270sq ft. with an unfinished basement - additional ~ 300sq. ft.

    Condensor #1
    Trane XL 15i
    4TWX5030A1000AA

    Furnace #1
    Trane
    4TEE3F31B1000A


    Condensor #2
    Trane XL 15i
    4TWX5036A1000AA

    Furnace #2
    Trane
    4TEE3F37B1000AA

  2. #2
    Join Date
    Mar 2018
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    Chico, Ca #StateofJefferson
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    Your equipment has nothing to do with what kind of filter you can run, that's entirely on your duct systems ability to move air. Since oversized equipment on undersized ducting seems to be the rule and not the exception, many people cannot even use pleated filters without causing the harm you speak of. You need to have your duct static pressure measured, this tells you how much resistance the airflow is encountering going thru the filter and return and supply ducting, I consider a 0.5" of water column to be the max allowable even though some systems may show up to 0.8" as max. You'll want some static pressure on the supply side, bit as little as possible on the return side which may mean adding another return or enlarging the current one. Also filters work best when the airflow going through them is slow, 300 feet per second or less, this is achieved by having 200 square inches of free face filter per ton. Looks like you have a 3 ton and a 2.5 ton heatpump systems. Let's look at the 3 ton, you would need at a minimum 600 square inches of filter to accomplish the 300 fps speed thru the filter, a 20x30x1 meets that minimum, but most filters have a cardboard or metal frame that blocks 30% or more of the filters face so youd need closer to 900 square inches of filter to get the 600 square inches of free face filter. None of this matters of course if its connected to undersized or sagging ducting, airflow doesnt like making turns. Your return ducting will need to be oversized if you intend to heavily filter the air, as airflow does drop like a rock the higher up the MERV scale you go, a way around that(partially) is a filter media 4-6" thick like a honeywell F100 filter media box or a honeywell fc40r if it's in a return grille. It all goes back to knowing what your total external static pressure (TESP) is. I'm in the same boat as your kids with allergies so I've got (2) 20x30x2 return grilles on my 3 ton, and I leave the fan switch on, 24/7 filtering and mixing the air. Hope this made sense

  3. #3
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    Dec 2019
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    Thread Starter
    Makeitcold -

    What you said makes sense. I believe the first step would be to measure/calculate the TESP? Is this done with or without a filter in place? I am guessing without, but wanted to confirm before I ask my HVAC tech to perform this test.

  4. #4
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    Mar 2018
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    Chico, Ca #StateofJefferson
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    Always with the filter. Once you know the TESP you can usually just compare that to the blower performance chart to find out the actual airflow. If this cant be found there are various other ways to measure the airflow that can be used. I have to stress that all registers have to be fully open, the blower wheel and evap coil are clean, if the blower has a run cap this must be measured while in use to ensure the measurement is accurate. As airflow decreases so will static pressure, and if your losing a bunch of CFM because the blower wheel is dirty or run cap is half dead, then its a garbage in, garbage out measurement

  5. #5
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    Dec 2019
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    Thread Starter
    Makeitcold -

    Understood. I have an HVAC tech coming out later this week.

    Another sort of on topic question - I'm going to ask my Trane HVAC tech to quote me the price of installing a deep media filter housing to replace my CleanEffects system. I understand that the CleanEffects is arguably a better filtration system, but the previous homeowners destroyed that thing before I bought this house (and to be perfectly honest, I won't clean it once a month as I should). I had so many problems with that power door when I bought this place that I'm done with it - it's not worth the trouble.

    Is it *ALWAYS* better for my equipment to run a deep media filter (like a 4" or 5" box filter) than it is to run the 1" filters? Are the pressure drops across the deep media filters really low compared to a 1" MERV filter? Can I assume that a 4" box filter installation would be safe for my unit?

    I'm still going to ask for a TESP reading so I know.

  6. #6
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    May 2000
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    Indianapolis, IN, USA
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    I like the Aprilaire media air cleaners personally. You can get MERV13 medias with a decent static drop even on big units. A clean 1" 20x25 was over twice as restrictive as a clean MERV13 20x25 Aprilaire media on our furnace running 1600 FM. So I would say yes, 4-5" filter will always be preferable to a 1". The Aprilaire was very nasty when I went to change it and even then is was just under 1/2 as restrictive as the clean 1". If you saw what it takes to get to our furnace you'd understand why the media doesn't get changed often

  7. #7
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    Aug 2009
    Location
    Beatrice, NE
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    I have some good news and some thing for you to check into. the good news is that your inside unit has VS blower motor which makes it a bit less forgiving on air flow restriction but that does not mean that it will overcome any restriction. It is just that instead of being limited to.5 static you can go up to .75 maybe a bit more and the blower will still move the right amount of air. The other thing is that Trane has a media filter that will fit the clean affects cabinet. It is the same media as fits their perfect fit air cleaner as I understand. I do not know what MERV rating it has or how restrictive it is. Just saying it may be a cheaper option than going to the Aprilaire although I do love the Aprilaire and never put in the Perfect fit which is why I don't know a lot about them except they are available.

  8. #8
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    Nov 2004
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    SW FL
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    Quote Originally Posted by putrid View Post
    Hello everyone -

    Unfortunately, my boy suffers from pollen and dust allergies - so using a cheap (able to see your hand - not just the shadow - through the filter) filter is just not doable.

    Specs : Home is 4,270sq ft.
    _______ with an unfinished basement - additional ~ 300sq. ft.
    Hunter Residential
    HEPA UNITS
    ___ 500 CFM

    https://www.hunterpureair.com

    https://hunterpureair.com/collection...r-type_console


    https://cdn.shopify.com/s/files/1/02...Manual.pdf?330
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  9. #9
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    Dec 2019
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    Thread Starter
    @BNME8 - I have not found that replacement and I've been looking for (2) days now. Trane rep will quote an AprilAir system.

  10. #10
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    Nov 2004
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    SW FL
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    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  11. #11
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    Mar 2018
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    Chico, Ca #StateofJefferson
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    Making assumptions cost money in my experience. Always verify, but yes a deep pleat filter will have a much lower pressure drop than a 1" thick filter for the same MERV rating and usually even for a higher rating, but that doesnt mean your pulling enough air, gotta know your static. I'd measure it before you spend money with and without the filter in place to get a baseline of what your working with. As BNME8EZ said some furnaces allow up to 0.8" wc, but they will eat the motor every couple years doing this which gets expensive quick

  12. #12
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    Aug 2009
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    Quote Originally Posted by Makeitcold View Post
    Making assumptions cost money in my experience. Always verify, but yes a deep pleat filter will have a much lower pressure drop than a 1" thick filter for the same MERV rating and usually even for a higher rating, but that doesnt mean your pulling enough air, gotta know your static. I'd measure it before you spend money with and without the filter in place to get a baseline of what your working with. As BNME8EZ said some furnaces allow up to 0.8" wc, but they will eat the motor every couple years doing this which gets expensive quick
    Actually I said .75 maybe a bit more. It is actually rated to almost 1.0. I have seen several of these 4TEE3F air handlers that will only ramp so high then the amps start to drop off, from dirty filters. They were running well over 1.2 with the dirty filter. I have yet to have any motor fail in less than 10 years with most going over 15 so I feel fairly comfortable with .75 static if it has to be that high. I would prefer it .5 or under but as the Rolling Stones said, "You can't always get what you want".

  13. #13
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    Mar 2018
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    Chico, Ca #StateofJefferson
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    I've seen furnaces list 0.8" max on the data plate and usually the blower performance charts will go up to 0.9-1.0. I've had a ton of customers who went through the modules every other year or so because sales guys did the "It cures bad ductwork" thing. Sagging, undersized flex duct is king in these parts and I'll guess it's just super hard on the boards capacitors, every time the customer had us fix their ducting the issues stopped though. If the ducting is reasonably free flowing they will outlast PSC motors though, had a fair number of clients still on their first motor and module 15 years down the road if the TESP was right around the 0.5" ballpark. I'd say 80% of the time around here, the problem is undersized returns and people using merv 13, 1" filters

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