Adding Aux Return Air to Plenum

The general rule of thumb in HVAC is 400 cfm per ton of return air, so 1200 cfm for a 3 ton unit, but in humid areas like the Deep South/BR, 350 cfm per ton or 1050 cfm for 3 ton unit, is often recommended for increased latent heat (humidity) removal.

Your 20 x 20 return plenum with a 1 inch filter provides 800 cfm of return air (2 cfm return air flow per square inch of return plenum, so 20 x 20 x 2), so you have a return air deficit of between 250 to 400 cfm of return air for each 3 ton unit. A 12 inch diameter flex duct will provide you with 330 cfm of return air (see table below) so that’s just about right.

You might need a slightly larger grill box than 12 x 12 to attach a 12 inch flex duct, not sure, but I’d suggest you go to Colburns and check with them as to what size grill and types collars you’ll need to accommodate a 12 inch flex duct.

Your tech is right, you have 3 ton units but current you are only getting between 2 and 2 1/4 tons of cooling capacity based on the size of your return air plenums.

Now a question for another day is your supply air ducts properly sized to delivered 3 tons of cooling capacity?

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So how would I go about figuring this number out? Count the number of ducts (and their respective diameters) and add them up from the chart you show? I’m thinking the system will have enough supply ducts but I guess I have some research to do!

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The easiest method for determining whether you are getting the 1050 to 1200 cfm out of your supply ducts, and actually the return ducts as well, is to have total static external pressure test done on each system (TESP). TESP measures resistance to air flow. They drill 2 ports in the air handler or return and supply plenums, and use a manometer to take pressure readings. It literally takes about 10 minutes to drill the holes and take the reading with the manometer, then they plug the holes with a cap or tape.

Ideally for a single stage system with a PSC blower motor you are looking a TESP of near 0.5 inches of water column with 0.25 static pressure on both the return side and the supply side, but there is some leeway in those numbers. The air handler manual for your system will have a blower chart that you compare your static pressure readings to and it provides you the air flow in cfm of your system. A HVAC tech should know if your TESP is out of wack after taking readings.

If for example you have very high static pressure exceeding 0.25 on the return side then you likely have insufficient return air, and if you have excessively high static pressure on the supply size you might have restricted or undersized supply ducts. Here is an article to read. LINK and tons more info and videos on the subject on the web.

I had my HVAC serviced in April, the company sent a new tech out and that was the first thing he did - drilled holes and measured static pressure before doing anything else. The result were better than I expected. Like you, I thought I was under sized on my return plenum size based on the calculations I provided you in my earlier post, but based on the static pressure readings on my system I am under-sized on return air but not nearly as bad as I thought. The static pressure on supply side was showed my supply air flow was nearly perfect for a 4 ton unit at near 1600 cfm.

There are other methods for determining if your supply ducts are delivering 3 tons of air, manual D calculations is one, measuring air flow at each vent with an anemometer or air flow hood, etc. but measuring TESP is first thing HVAC techs on the forums I peruse recommend before doing anything else - it’s the quickest and cheapest method for estimating air flow.

Anyway, it’s just food for thought for you to consider looking into down the road. Experienced HVAC techs say the most common issue they see with clients that have home comfort issues is oversized (tonnage) HVACs with undersized ducts, either return or supply or both.

Bapple, before you get into all of that, the first thing I would do is to get someone with a vacuum gauge to tap your return air duct to see if you have actually got too much restriction. No point in relying on theoretical data and “rules of thumb” to make that decision.

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before you get into all of that, the first thing I would do is to get someone with a vacuum gauge to tap your return air duct to see if you have actually got too much restriction.

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That is the TESP test I mentioned in my earlier post and I certainly recommend that - just need to find a HVAC tech trained to do it - seems it is not commonly done around here and elsewhere around the country according to professional HVAC forums I follow - I don’t know why. A manometer is not an expensive tool, the tech just needs to make sure he doesn’t damage a component when drilling the test ports.

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No point in relying on theoretical data and “rules of thumb” to make that decision.

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The 400 cfm per ton of HVAC flow, 350 cfm for humid climates, even 450 cfm for arid climates, are not theoretical or even much of a rule of thumb - they are engineering criteria from the engineers that designed HVAC systems and the manufacturers that build them - it’s why air handler blowers motors have speed taps to adjust air flow from the blower motor to meet air flow criteria for a region or even peculiarities of an individual house.

Having return air volume even in reasonable excess of that specified by HVAC manufacturers is not a bad thing.

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I have 2 main units a 3 ton and a 4 ton, 3 ton has 2 returns bc its split by the living room (20x30); 4 ton has one return same size (20x30) I don't feel this is enough, and we have balance issues, some rooms very cold, others stuffy!

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Your 4 ton should have between 1400 and 1600 cfm of return air, a 20 x30 return plenum should be supplying 1200 cfm, so you are a little short of return air flow but not by much. You certainly seem to have enough return air for the 3 ton unit.

Usually the ducts, in the attic I assume, will have a damper to allow you adjust the air flow to the vents that supply each room. If so, you might want to see if you can adjust them and improve air flow to the stuffy rooms and reduce air to the cold rooms.

60% RH is too high. May need to reduce air handler blower fan speed to reduce air flow in hopes humidity is reduced. Might even need to consider having a blower door test done on the house to see how much air infiltration you have into the house.

Perfectly timed thread...

I am installing server rack for my gear in closet which is directly behind air return. Any harm in me running a vent into the return? Should take care of heat in the closet and not be much effort to do.

Just for kicks, 4 ton unit, 24 x 24 opening, so I am low for this setup a bit. Attic ducts have lots of hard bends, plan to address that in the fall.