Is 14" duct and 20x20 filter sufficient for air return (intake)? 3 ton unit


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Old 06-25-17, 06:20 PM
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Is 14" duct and 20x20 filter sufficient for air return (intake)? 3 ton unit

We have a recently installed (late 2014) Heil 3 ton gas/AC pack. The original contractor that installed it never said anything to me about a 20X20 being a bit too small for a 3 ton running an X13 blower. The X13 blower has died twice under warranty.

I've been told by 3 different residential HVAC contractors that my 20X20 is about half of what I really need. These are guys I know personally (wish I had known them in 2014) who aren't trying to make money off me. They know I will DIY anything I can.

My 20X20 is right beside a closet. In fact, it's partially in that closet (the metal box), and the filter/grill is in the hallway. I could very easily build a box to add a second (or third) filter inside that closet and modify the door to handle air flow.


The duct that is in the current air return seems to be 14". At least, that's the best i can tell using the tape. It's over 12" in most places inside but it's not perfectly round in the place I can reach it to measure. It's possible it's 12". Is this sufficient?

I guess I'm asking so that I'll know whether I need to add another return duct (of whatever proper size) to another side of the house........ or if my current return duct is sufficient diameter and I just need to add a box for another filter or two.


I do know that my family (6 people, VERY active) is a dusty bunch. We plug up the Filtrete Basic air filters in 2-3 weeks. I'm thinking I really need to do this.
 
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Old 06-26-17, 07:21 AM
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Here's some information on basic duct sizing. Remember you'll want ~400 CFM per ton. The lowest I'd go is 350 per ton for very humid conditions.

http://hvac-talk.com/vbb/attachment....1&d=1484928646
 
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Old 06-26-17, 04:03 PM
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m,
You have a 3 ton unit which is 1200 cfm's. As r mentioned 1 ton = 400 cfm's. If that 14" round return is all you have it is undersized for your unit. A 14" round pipe is equivalent to 750 cfm's. If it's flex it's 700. Your return trunk should at least match your supply or even 10-20 percent larger.

You are only going to get as much air out as you have coming back.

You can look up duct conversion charts for the proper size or your friends should have a chart.

If you only have 1 truck line going out I'm guessing it's at least a 22 x 8 which is good for 1200 cfm's. 24 x 8 is good for 1320. Your return should be at least that.

Another 12" round will give you 525 cfm's. Flex will give you 480.

This might be helpful with your filter sizing which according to the formula is close.

http://www.dnr.louisiana.gov/assets/...P/hvac/g/g.htm

Hope this helps a little.
 

Last edited by spott; 06-26-17 at 04:22 PM.
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Old 06-26-17, 05:10 PM
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Yeah, thanks for confirming my suspicions here. I'm going to have to add a second return line. Being a carpenter this shouldn't be too hard but I have to get under the house and plan how to split the return.

I'm going to run a return to the other side of the house. My kitchen/dining room don't get sufficient air flow anyhow, and pulling the warmer air from there will be helpful in keeping the house more balanced, I'm sure. I'll eventually probably need to add another couple supply lines/vents, I really don't think there's sufficient ones.


Meanwhile I am going to go ahead and modify the current filter and put another 20X20 or two in the closet. It will be a good "quick patch" sort of thing until I can get the extra return line ran.
 
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Old 06-27-17, 09:40 AM
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m,
Just a thought. The filter you have now is sufficient for the amount of return air you have coming in. The filter is only going to deliver as much air as the return ducts can provide. Also without increasing your return capacity you may end up compounding your problem by adding more filters as they become more restrictions the blower must overcome.

Another thought, when you add return lines you must make sure the main return trunk is large enough to handle added air flow. Return capacity all starts with the proper size trunk line.

Just my thoughts.
 
 

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