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Old 10-06-2011, 09:43 PM   #1
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Ductwork handle smaller unit?


I recently had an energy auditor come to our home and perform an energy audit using blower door test, IR imaging, etc. He found some areas that could be improved, specifically improperly installed insulation. I am pretty sure that our AC/heat pump has been oversized, and my concern is that with continued energy decreasing measures, this will exacerbate the problem. That is fine, we can put in a smaller unit, but I am not sure that our ducts will handle a smaller unit.

Our hope is to begin finishing the basement in the next month or two, which will make replacing the ducts at a future date a REAL problem. We do not consider replacing our AC/heat pump as an option right now, so I am looking for some expertise on a way to determine the minimum CFM our ducts will handle to determine if we could downsize without duct changes in the future. All of the branches have balancing dampers, so I am more concerned with the main supply trunk. Please let me know if I am wrong in making the assumption that I could close off those dampers (or remove the ducts completely before finishing the basement) and end up with the same end result.

Our air handler unit is in the middle of the house, and the supply coming off each side is mirrored on the other side. Each side has an 8" x 18" rectangular run for 15', then a 2' restriction down to an additional 15' run of 8" x 14" rectangular duct. We currently have a 4 ton system (I think the manual for the air handler says ~1600 CFM), so I am wondering if this would be oversized for a 2 to 2 1/2 ton system...or would it handle 800-1000 CFM and still provide adequate circulation? Let me know what other information is important and I will be happy to provide. Thanks

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Old 10-07-2011, 05:38 AM   #2
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Ductwork handle smaller unit?


No problem. You would still have a velocity of 700FPM in the main trunks.

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Old 10-07-2011, 08:20 AM   #3
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Ductwork handle smaller unit?


beenthere - So the main supply trunk should be able to handle the fewer CFM's and still have adequate flow through the ducts? Does it sound like our current air handler is at the upper end, or perhaps even too big, for the current trunk size? I really appreciate your expertise.
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Old 10-07-2011, 10:46 AM   #4
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Ductwork handle smaller unit?


Your ductwork if it is galvanized metal it is rated for 1200 cfm and .1 friction rate if the equivelant length of you ductwork is under 100 ft. 2.5-ton should work fine. It should also save you a lot of money.
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Old 10-07-2011, 02:50 PM   #5
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Ductwork handle smaller unit?


Does "rated for 1200 CFM..." mean the maximum it can handle without exceeding the 0.1" WC static pressure? I guess my question would then be, what is the minimum it would handle at 0.05" WC? I'm just trying to determine a range to work within, especially if we decide to upgrade to a variable speed blower (i.e. like the Carrier Greenspeed) in the future.

It looks like the 2-ton Greenspeed operates between 700-900 CFM. Would this be too low, considering it will be split between both sides of the air handler?

The 3-ton system operates about 875-1200 CFM, which sounds like it might work better. But that might still be oversized (for BTUH) for our home when all is said and done.
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Old 10-07-2011, 02:57 PM   #6
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Ductwork handle smaller unit?


at .05" your ductwork should be delivering 1160 CFM. Without seeing the rest of your ductwork I would say that you should be fine. If the ductwork is tight your air should be ok.
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Old 10-07-2011, 02:59 PM   #7
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Ductwork handle smaller unit?


You also want to check what design temp he used for his calculations. there is a big difference between 2.5 and 4
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Old 10-07-2011, 03:43 PM   #8
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Ductwork handle smaller unit?


I have not yet had a Manual J done, I'm just trying to prepare for the future. Originally, I can tell you the heat load calculation was something like "about 2000 sq ft, 1 ton for 500 sq ft, that's a 4 ton system". Most of the HVAC guys are doing that in my area. I will have a Manual J performed when we change out the system, but just want to make sure we're on the right track before I box in the ducts when finishing our basement. I'm guessing our current system is oversized by at least 1 ton, and when I am done improving our insulation and air infiltration I'm thinking we'll get down to 2 or 2.5 ton. That is worst case though, just want to make sure the main supply will handle the minimum.

Here is a diagram of the main supply trunk. There are about 17 6" ducts coming off of it, but I have closed several of the balancing dampers due to comfort issues on one side of the house (one half has 5 registers, the other has 12). Would 700 CFM (worst case scenario) coming through this size duct at the location of the air handler still get up to 700fpm for adequate airflow?
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Old 10-08-2011, 06:58 AM   #9
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Ductwork handle smaller unit?


Don't confuse FR(pressure loss) with static pressure. They are 2 different things.

You have 2 8X18 trunk lines, each of them should easily and quietly flow 600 to 700 CFM. But, you need to also know the sizes of the supplies off of each trunk line, to kn ow how much air they will be able to move. As this will determine what size unit the whole duct system can handle.

Best to do a load calc, and find out what size you need, and then determine if your duct system is adequate.
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Old 10-08-2011, 09:14 AM   #10
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Thank you for the clarification, I am feeling more at ease. I am planning on trying some hvac heat load software, so I will work on that, see where I end up and post back. Thanks again

Last edited by kmachn; 10-08-2011 at 08:28 PM.
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Old 10-10-2011, 11:33 AM   #11
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Ductwork handle smaller unit?


I performed a heat gain/loss calculation using hvac-calc software. It gave me the following results at the standard conditions in St. Louis, MO.

Summer Design Temp. 95F, ID Temp 75F
Winter Design Temp. 8F, ID Temp 70F
Relative Humidity: 50%
Forced Ventilation: 100CFM
Sq. Ft. 1811

Sensible Gain: 15,757 (BTUH)
Latent Gain: 5,377 (BTUH)
Total Heat Gain: 21,134 (BTUH)
Total Heat Loss: 36,600

The Carrier Infinity w/ Greenspeed 2-ton system puts out 23,140 BTUH with 19,200 BTUH, sensible (or more, depending on Wet Bulb) at 900 CFM so I think that should be adequate cooling capacity. The 2-ton will only meet about 55-60% of heating needs at those design conditions, but we do have alternative heat sources to supplement and worst-case scenario would likely include auxiliary heat strips.

So, it looks like we're dealing between 500-900 CFM, if we were to go with the Infinity Greenspeed. I'm working on a duct layout, but still have to calculate the TEL and duct size then I'll post a diagram and see if a) I calculated TEL right and b) if I have calculated the duct sizes for proper CFM. It does look like this 2-ton system should work though.

I'm still having a hard time coming to grips with our current system trying to push >1600 CFM through ducts designed to carry a maximum of 1200 CFM at 0.1" WC. No wonder it is so noisy.
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Old 10-10-2011, 09:49 PM   #12
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Ductwork handle smaller unit?


I think I may be making some progress. I've eliminated some of the supply branches since most of them were really not well designed, and 17 (I have always thought) is just too much, at least for a smaller system. Maybe I am wrong.

Here is the layout I am thinking I will work with. I'm hoping to get some feedback before I start trying to calculate friction rate, get duct sizes, etc. Do the TEL lengths look correct? If they are, my longest supply length is 239' and my longest return (I think) is 230'. Does a total of 469' sound reasonable? Below is an example of how I calculated the length with TEL 239'.

Supply #2: 40' (plenum to trunk) + 24' (length of trunk to branch) + 45' (1 takeoff downstream) + 30' (round 90 degree takeoff) + 20' (length of branch) + 80' (90 degree register boot) = 239' TEL.

There are not any reducers, Y's or Tee's. All branches have 90-degree round takeoffs and all but one have 90-degree register boots (versus the straight register boots).
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Old 10-10-2011, 10:06 PM   #13
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Ductwork handle smaller unit?


Your method of TEL is correct. And 469 is not out of line.
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Old 10-10-2011, 10:19 PM   #14
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Ductwork handle smaller unit?


Great! Thanks for the quick feedback. This has been really interesting (and I admit, kind of fun) learning and working on all this. This all started for the sole reason that I didn't want to create problems when I finished our basement, but the more I'm learning about it the more I realize how really screwed up our current system is.

Should I be extremely concerned about making these changes while leaving our current system in place? I can leave a few of the branches for now and take them out later, but some of them really need to come out before finishing the basement. Sounds like static pressure is probably already too high, and once I finish sealing the ducts and eliminating some of them, it will likely make things worse.
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Old 10-11-2011, 05:09 AM   #15
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Ductwork handle smaller unit?


Since it will probably come out to larger duct then you already have, should be no problem.

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