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Retrofit thicker, higher-MERV HVAC filter?
Posted by: deckeda
Date: March 03, 2021 01:21PM
An older house, so the HVAC setup we have is itself a retrofit, with one large and mostly centrally-located air return vent in a small space between 2 bedrooms, the kitchen, and living room. One step in any direction and you're in one of those rooms.

So this isn't the typical split design that places the filter at the air handler, evaporator and furnace. All of that, the entire HVAC except for the ductwork, is a packaged outside unit with long ductwork into the house.

Anyway it fits a standard 1" filter at that large return vent which does mostly nothing, filters mostly nothing if using a standard fiberglass, see-though filter. I think we've been using upgraded pleated types from 3M but I'm interested in boxing out the filter holder if possible to accept a thicker filter with more surface area, for reduced air restriction.

Lookin' also for a higher MERV or even a HEPA rating, if possible. But again, only if air restriction does NOT increase.

What I'm envisioning is something that snugs into the 1" gap but sticks out into the "room" (its not a room, OK) the thickness of the filter and doesn't use the existing metal vent grate. Like maybe a bare filter with just a little more protection on the front that doesn't quite look like a bare filter jerry rigged there.
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Re: Retrofit thicker, higher-MERV HVAC filter?
Posted by: anonymouse1
Date: March 03, 2021 02:40PM
Be aware that higher-MERV filters can put a significant strain on the blower motor. I guess the increased surface area would offset that?
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Re: Retrofit thicker, higher-MERV HVAC filter?
Posted by: deckeda
Date: March 03, 2021 02:55PM
Right.

My understanding is that if a filter is designed with more pleats, you get more surface area. It’s the reason why car air filters flow freely. Lots of pleats.

And so a larger filter offers an opportunity —- not a guarantee —- of more pleats.

It’s also why a MERV 13 filter that’s only 1” thick is a really bad idea. No way to have enough surface area to offset the inherent restriction that others comes with extra filtration.
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Re: Retrofit thicker, higher-MERV HVAC filter?
Posted by: Gareth
Date: March 03, 2021 03:52PM
Here's how someone adapted the inside of the frame to fit a 4" filter without it sticking into the room:
[youtu.be]

And from the comments on that video, apparently there are 5" "adapter" filters for 1" frames (1" deep frame, that narrows down for the 5" section to fit inside the recess of the frame)
[www.amazon.com]
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Re: Retrofit thicker, higher-MERV HVAC filter?
Posted by: deckeda
Date: March 03, 2021 04:25PM
Awesome, thanks! I'll rip off the cover this weekend and see what my options are.
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Re: Retrofit thicker, higher-MERV HVAC filter?
Posted by: MrNoBody
Date: March 03, 2021 06:51PM
Quote
anonymouse1
... I guess the increased surface area would offset that?

No, it would not. Filtered air handlers are designed with a certain spec'd.
motor to allow for air-flow with specified physical restrictions.

Filter info [youtu.be]



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Re: Retrofit thicker, higher-MERV HVAC filter?
Posted by: deckeda
Date: March 03, 2021 07:12PM
Quote
MrNoBody
Quote
anonymouse1
... I guess the increased surface area would offset that?

No, it would not. Filtered air handlers are designed with a certain spec'd.
motor to allow for air-flow with specified physical restrictions.

Filter info [youtu.be]

There's nothing in that video that contradicts what's been said in this thread. And he literally pulls out a large surface area filter at 3:45 like what we're talking about. smiling smiley

His only mistake is at the beginning, characterizing the "better" 1" filters as too-expensive, when the 4" media filters won't cost less. The real issue with pleated 1" filters is that you can't add enough pleats to a 1" filter to NOT add restriction.
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Re: Retrofit thicker, higher-MERV HVAC filter?
Posted by: RAMd®d
Date: March 04, 2021 03:14PM
I just watched a This Old House (or Ask This Old House) episode where they built a (slightly inelegant) pair purifier from a 25" box fan and four 25" MERV 13 (I think) filters.

They discussed how pleats add surface area and drop resistance.

There wasn't any mention of a pleated 1" filter not having enough pleats not to increase air resistance, though I can see how that would be a problem.

  • I'm not very familiar with anything but the cheap 1", stranded filters.
  • What MERV rating are they, and isn't there some MERV level of a pleated filter that wouldn't impede airflow, while increasing filtering?
  • Would it not be beneficial to move to the highest MERV rated 1" filter that doesn't impeded air flow?


Back to the DIY air purifier, it looks like a fun project though it won't make House Beautiful or Architectural Digest.

Typically I'm a form over function guy, favoring a graceful blend of style and substance, usually in that order.

But this looks like a useful functional DIY project, even if it becomes the elephant in the room.

Though the cheap 1" stranded filters don't really filter tiny stuff, as they age, their filtering ability increases somewhat as their efficiency drops.

LOL






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Re: Retrofit thicker, higher-MERV HVAC filter?
Posted by: deckeda
Date: March 04, 2021 06:01PM
That same video is what prompted this thread!

There wasn't any mention of a pleated 1" filter not having enough pleats not to increase air resistance, though I can see how that would be a problem.

Well, the "mention" was in early initial example of holding up a 1" filter that was an upgrade form the see-through basic fiberglas ones. That's where he said when you increase filtration you increase resistance.


isn't there some MERV level of a pleated filter that wouldn't impede airflow, while increasing filtering?

HVAC people refer to this as system headroom, i.e. how much resistance the system can withstand before bad things happen such as motor failure due to strain or whistling through the vents due to heavy negative pressure from improperly sized ductwork. So I imagine they design with a certain amount of headroom in case someone slaps in a restrictive filter or some such (up to a point.)

Would it not be beneficial to move to the highest MERV rated 1" filter that doesn't impeded air flow?

Given the above, yes, better than a filter that does nothing.

What's funny is you'll read some professionals who say things like, "I've been doing this for 20 years and I NEVER recommend anyone use anything higher than MERV 8 !!!" ... without any context that would allow for higher filtration if the rest of the system is designed for it and so on. I think those are the guys that only service 20 year old units and likely grumble at having to learn new text and procedures.
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Re: Retrofit thicker, higher-MERV HVAC filter?
Posted by: RAMd®d
Date: March 05, 2021 04:23AM
Well, the "mention" was in early initial example of holding up a 1" filter that was an upgrade form the see-through basic fiberglas ones. That's where he said when you increase filtration you increase resistance.

Actually, it's not 'mentioned' until a 2" MERV 13 filter is shown and Kevin says 'thank you for buying me this' and says he's going to go home and using in place of a M4 or 8 filter.

Then Ross says 'Not so fast' and shows a 4" M13 and says too much resistance is bad.

Their context was COVID related, ergo the use of M13, but mine was just improving general filtering of 1" filters.

So I guess what I need is some kind of comparison chart that shows if there's a pleated filter of some M-rating that passes as much air or even nearly as much as the MERV 1 I have now.

Maybe the difference between a 1" M1 and a 1" M2, 4 or 6 is significant.






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All you can do, is all you can do.

There’s trouble — it's time to play the sound of my people.

Your boos mean nothing to me, I've seen what you cheer for.

Insisting on your rights without acknowledging your responsibilities isn’t freedom, it’s adolescence.

I've been to the edge of the map, and there be monsters.

We are a government of laws, not men.

Everybody counts or nobody counts.

When a good man is hurt,
all who would be called good
must suffer with him.

You and I have memories longer than the road that stretches out ahead.

There is no safety for honest men except
by believing all possible evil of evil men.

We don’t do focus groups. They just ensure that you don’t offend anyone, and produce bland inoffensive products. —Sir Jonathan Ive

An armed society is a polite society.
And hope is a lousy defense.

You make me pull, I'll put you down.

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Re: Retrofit thicker, higher-MERV HVAC filter?
Posted by: deckeda
Date: March 05, 2021 09:17AM
I think we’re talking about the same thing. If you’re restricted (no pun intended) to using a 1” filter, any MERV rating that’s higher than bog stock will restrict more. At least, I’ve never heard of better filtration for a given thickness, especially at 1”, that would NOT be more restrictive.
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