Reducing size of return air filter?


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Old 05-02-21, 01:54 AM
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Reducing size of return air filter?

My house is a bit on the small side, about 1250sq ft, and I have a heat pump that I believe is 3 tons. Very annoyingly, my main return vent and the filter behind it is 20x30. These filters are NEVER in stock anywhere! I have to special order them and even then it's hit and miss.

I don't see any reason it needs to be this large. My house isn't huge. I've had larger houses that use standard 20x20 filters. I'm wondering if I could block up the bottom 5-10" on the filter opening? 20x20 and 20x25 filters are usually easy to get. I know that reduces air flow but shouldn't that still be large enough for a 3 ton system?
 
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Old 05-02-21, 04:03 AM
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It's not such a simple answer especially with restrictive "high efficiency" filters becoming so common these days. First, you need to KNOW the size of your system. Not just believe it's size. Once you KNOW the size of your system then you can do the math to figure out how much filter area is needed.

Another option is to go to your local hardware store and buy a case of filters. If they don't stock the size you need have them get a case for you with their next filter order. They are really cheap when you buy them by the case and a case could last for years depending on how often you change your filters.
 
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Old 05-02-21, 04:31 AM
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I do not use high efficiency filters, maybe slightly better than bottom of the line. I don't know the exact rating on the heat pump because I don't have the numbers in front of me now, but it is definitely no larger than 3 ton, possibly 2 ton. (looking at electrical usage on my power company website it uses about 2.5kw)

If anything I would add a secondary 20" filter and have 2, but that seems excessive. I have had some stocked up before, and they still run out and when they do it is really annoying...
 
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Old 05-02-21, 10:47 PM
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OK, according to the info on the heat pump it is 3 ton. It is not the original system for the house, but all the ducting is. I have no idea what was originally installed. Am I correct that a 20x30 filter is excessively large for a small house with a 3 ton heat pump?

I understand the larger filter gives more surface area so it can last longer and not be as restrictive, but I really want something every box store has in stock. Eventually this house is going to become a rental and I don't want tenants having the same problem.
 
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Old 05-04-21, 02:49 AM
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My A/C system is a 3 ton unit and utilizes a 16" X 25" air filter so reducing your 20" X 30" filter to a 20" X 25" filter should be OK.
 
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Old 05-04-21, 03:53 AM
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And I have a 3 ton system that was installed in this home in 2016. The old system had a 16X25 filter... but when we bought the house, it wasn't cooling like it should. I added a 14X14 & it cools as it should now.

I am assuming their old system worked well with just the 16X25, but the new system just couldn't get enough return air.
Maybe every system is a bit different... requires different ventilation?
 
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Old 05-04-21, 02:20 PM
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Maybe every system is a bit different... requires different ventilation?
So how then does an HVAC installer know what size hole to cut? Pilot Dane seems to think it's an easy math problem but he has been quiet for days...

Directly behind and below the 20x30 vent is the main flexible return duct, I'm not going down there to measure it but it has to be smaller than a 20" circle. Looking at Home Depot 20" seems to be the biggest flexible duct they sell.
 
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Old 05-04-21, 04:13 PM
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I am no AC guy, but I agree with PD that there is a formula that you use to determine the size of ventilation required.
I'm not sure how my AC guy figured it but it was quick for him to come up with the answer.

I know mine was a replacement so I don't know what size the old one was or if efficiency has anything to do with how you get that number.
 
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Old 05-04-21, 05:48 PM
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Seems kind of odd this quick easy formula exists but after days in the duct forum nobody can find it. I would hazard a guess that most spec house installs the HVAC guy just wings it.

I'll just tape some cardboard over the bottom and see how it works.
 
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Old 05-05-21, 02:38 AM
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Am I correct that a 20x30 filter is excessively large
A 20x30 filter is not unique, in fact my GEO unit uses (2).

Go to HD or Lowes and order any MERV rating filter and buy a couple of dozen and be done with it!
 
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Old 05-09-21, 04:10 PM
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A 20x30 filter is not unique
Incorrect. Do you think I'm stupid? Neither Home Depot, Lowes, or Walmart keep these in stock. That was before all the shortages. No, I'm not going to order a case, and I don't use MERV filters (didn't I clearly say I didn't want to do that?) Why do people try to answer a question if they aren't going to even attempt answering the question?

The only filter I want to be used in currently my house and in the future a rental house is only one which is READILY AVAILABLE at every box store without ordering it. That's not an extreme request.

PS - it works absolutely fine covered down to 20x25, and the larger filters have always taken 6+ month to get dirty so there is plenty of surface area. Still larger than most houses my size.
 
 

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