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Ductwork handle smaller unit?

7K views 24 replies 3 participants last post by  beenthere 
#1 ·
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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#3 ·
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.
 
#5 ·
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.
 
#8 ·
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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#9 ·
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.
 
#11 ·
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.
 
#12 ·
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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#14 ·
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.
 
#16 ·
Great. beenthere, I really appreciate your help and expertise. I guess the last thing I've been struggling with is trying to accurately determine the friction rate to use for sizing the branches.

I've been planning on using a Carrier fe4 fan coil, which it looks like has a maximum static pressure of 1.0" w.c. This is referenced on page 8, "Maximum Static Table". It also makes reference to 0.8" and 0.6" w.c. but it looks like these are probably higher CFM than I would be using. Although I confess, with a variable speed blower I do not fully understand what numbers I should be working with.

http://www.xpedio.carrier.com/idc/groups/public/documents/techlit/fe4a-3pd.pdf

Page 11 also reference static pressure drops and pressure correction, although I do not completely understand how to use the data. I think a 10kw auxiliary unit would suffice, meaning no correction is needed. If I am not using a factory installed filter, then I would not use the corrections listed on the second table. I think I would need to use a "Air Delivery Performance Correction Component Pressure Drop" value (which I think would be subtracted from the 1.0" w.c.) but I am not sure which value to use. Maybe the 0.028 for the 2-ton air handler?

Using the data that I think is accurate, I end up with the following.

1.0 (Max Static Pressure) - 0.028 (Component Pressure Drop Correction) - 0.1 (filter) - 0.03 (supply register) - 0.03 (return grille) - 0.03 (balancing damper) = 0.782 ASP

With a TEL of 469', that gives a friction rate of 0.17" w.c. Does that look correct, or is that high?
 
#17 ·
Using the 1" of ASP, would mean that you have little load up ability on the air filter. Meaning you would need to change it very often.

Best to use .8" ASP or less. That gives your air filter the ability to catch dirt, and not have to be changed every 2 to 3 weeks. A .17" FR will tend to make a noisy duct system.

Remember, the higher the static pressure, the more electric an ECM blower uses. They use less electric at .5" TESP and lower.
 
#18 ·
Does that mean that since it has a maximum of 1.0" wc, I can choose to use a lower number? I just have to look up the value to know that I can not use a higher number, is that correct?

So with the ECM blower and using 0.5", a better calculation would be.

0.5" (Static Pressure) - 0.028 (Component Pressure Drop Correction) - 0.1 (filter) - 0.03 (supply register) - 0.03 (return grille) - 0.03 (balancing damper) = 0.282 ASP

With a TEL of 469', that would give a friction rate of 0.06". That is much lower, and more in line with what I have seen. Does that make more sense?

That is also with a basic filter, a better filter would give more of a pressure drop I think. Would it make more sense to start with a higher static pressure (maybe 0.65") and use a filter that has a pressure drop ~2.5?
 
#19 ·
looks better in that calc.

Works best to use the PD for a better air filter, even though you only intend to currently use a standard air filter now. And see if you can still use .5" as your ESP. Go to .6 if needed, but try to avoid if possible.
 
#20 · (Edited)
Wow, very informative, thank you. That sounds like quite a task though. Just by doing the math, I would have to decrease my TEL to ~235' for using a filter with a PD of 0.24. That is basically eliminating my return side, which isn't realistic. There are not a whole lot of other variables to change.

Another option would be to eliminate the 10kw auxiliary heat strips. But even that with a 0.24 PD from a filter would only give a static pressure of 0.04" w.c.

What seems to me to be the most realistic option is the Honeywell (or similar) pleated filter. The Honeywell looks like it lists a 0.23 PD at 492 fpm It would appear that (if I follow the curve down) the 20x25 filter has a pressure drop of only ~0.09" w.c.-0.1" w.c. at 900 CFM. If I need 2 CFM/sq in. for the return, this would slightly exceed that and give me a lower velocity (meaning less pressure drop) and this filter would make the situation work. Does that sound realistic or am I missing a variable somewhere.
 
#21 ·
Turning vanes in the return drop can reduce its TEL, along with a scoop in the supply plenum at each trunk line attachment.

Often, you do have to use .6 or .7", in order to meet space confinements.
 
#22 ·
Turning vanes and scoops on the supply side sounds like it should be easy enough to have done during the installation of the new unit. Right now the plenum to the supply trunks is just a 90-degree Tee (TEL of 40') and the return basically has 90-degree bend. How much would these changes reduce the TEL?

Also, what about my idea on the 20x25 Honeywell air filter at 900 CFM?

http://customer.honeywell.com/techlit/pdf/PackedLit/68-0239EF.pdf Page 11, "Capacity and Pressure Drop" table

Does this sound reasonable? Realistic?
 
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