return duct adequacy and plenum size?

gitrdoneAugust 7, 2010

Our evaporator coil is shot for our first floor system. New system to be installed soon.

Old system: Carrier 3 ton AC with 3.5 ton (1400 cfm's) Carrier 80% eff furnace. In the unfinished basement.

New system: Trane 3 ton XL15i with 3 ton (1200 cfm's) 60K XV95 2 stage (58K/37.7K) with 5" media filter cabinet.

During the bidding process, I asked sales folks about the adequacy of our ducts (all insulated and flexible). The consensus has been that our supply side seems ok, but the return side might be undersized None of the sales guys could say for sure. They want a technician to evaluate at the time of the install. Admittedly, I'm new to the wonders of the HVAC world and probably didn't ask enough questions. But, here's what I've been able to piece together so far. I really want to understand this situation better before a technician arrives and does an evaluation and offers advice.

SUPPLY plenum - insulated, sits on top of the evaporator coil (about 20"). It has two 12" (I think) ducts attached at opposite ends of the plenum. These ducts have a variety of splits.

RETURN plenum - not insulated, 24" wide. It has two ducts exiting the top of it. One has a starting collar of 12" (measured by me) and one does NOT have a starting collar.

Return duct #1 - This collarless duct appears to be 12" (per sales & my measurement) and runs 22 feet before splitting into three 10" ducts (I think). These 3 go another 6' max and exit at the baseboards on the sides of our center staircase in our foyer. I took the grills off these return vents and found 14" wide boots with 8" diameter nipples. Hope I'm using the right terminology.

Return duct #2 - Has a starting collar AND a split 6" above the top of the plenum. The primary part of this duct continues at the 12" width for maybe 15' and then splits into two 10" ducts (I think) which exit in the baseboards in our family room. Took the grills off these vents and found 14" wide boots with no round nipples. The secondary return duct that splits off from the main one (6" above the top of the plenum) appears to be 10". It creates the 4th return vent in our center staircase baseboards. Like the others, it has a 14" boot with a nipple 8" in diameter.


1. Is our return plenum too narrow (24") if the 2 main return ducts are 12"?

2. As I understand it, with a 3 ton AC, we need 1200 cfm's (minimum?) in our return duct work. Do we have that or are we undersized? I realize that a lot more information is probably needed to answer this question, but any guidance would be appreciated.

3. If the primary return ducts are each 12", what is the range of cfm's that they might count for? Does the 10" duct that splits off one of the main return ducts (near the top of the plenum) count for anything?

4. Should that 10" duct mentioned in question 3 be detached from the main return duct and have its own entry point into the plenum?

5. If we get a new return plenum, does that necessarily mean we should have a new supply plenum? The plenums are are back to back.

Thanks for any insights.

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It is impossible for me to determine if your supplies are adequate for the space to be conditioned without doing a heat load calculation and measuring the existing supply ducts. However, there is one given....the return should be sized equal to or larger than the total of the supplies.

The equipment must be sized according to the requirements of the structure. Then the duct work needs to be looked at to make certain it falls into the specifications the manufacturer requires for efficient operation of the equipment. This can and should be done before a final decision is made to purchase the equipment from the supplier. If the duct system needs upgrading, that should be determined before ordering any equipment.

In other words, the company you are using should be able to design the system you need to condition your building which would include proper duct work sizing and air movement assessments and the labor equipment costs before you sign a contract. You should not allow them to add a system if they don't even know what ducting is required and if yours is sufficient. You should know everything right up front before you sign a contract!!!

Don't allow them to fly on the seat of their pants, adding costs as they go and not professionally and competently designing and installing the right system. If your ductwork is not the right size, they should tell you that up front and let you know exactly what the ramifications are to not upgrading the duct work if needed. I personally would not install a system unless the customer agreed to all the work that must be done for that system to work properly and efficiently. To be able to do that, all design specifications would have to be done up front before a go ahead to do the work is given. You can then hold them to the warranties and expectations of the equipment. To piece meal the system as they go is not professional and makes for a lot of gray areas that not only can cost you more money but, also can lead to a dis-satisfying performance of the install.

    Bookmark   August 7, 2010 at 6:01PM
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Thank you, maryland irisman. I appreciate your candid feedback. I haven't signed a contract yet, just a verbal acknowledgemnet that I would like to use a certain company. Honestly, I feel a bit beaten down by this process, but your words are a good reminder of what I already knew. The Hotlanta heat and cranky kids are getting the better of me. I'll call the sales person and tell him that I need a more technically qualifed person to clearly define the scope and expense of the project before I sign a contract or any work is started. Manual J was done, but obviously we need more diagnositcs. Thanks, again.

    Bookmark   August 7, 2010 at 7:30PM
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* BOWER PERFORMANCE - Before designing/evauluation a duct system, a designer will look at the manufacturer blower performance data of the FAU (forced air unit, the proposed furnace/air handler) to determine at what ESP (External Static Pressure) can this FAU perform under to deliver the desired CFM (cubic feet per minute of air flow). This ESP will become the SYSTEM design target. Generally speaking, the manufacturers design standard blowers to achieve 400 CFM per TON of "rated air flow" (3.0 Ton furnace = 1200 CFM) at .5 inches of water column. The standard blowers have a performance curve with a steep drop off in performance. For example, going from .5 to .7 ESP (higher the number the more resistance) you might see a 170 CFM drop in performance, while going from .7 to .9, still just a .2 change, you might see a 270 drop. Variable speed blowers performance curve drop is more gentle, though often under estimated by uninformed designers.

* DEDUCT THE KNOWN PD OF THE DEVICES THAT WILL BE IN THE AIR STREAM FROM THE DESIGN ESP. - A designer will now look at everything in the air stream that creates resistance, except for the actual ductwork. We refer to the resistance as PD (pressure drop). The list of devices would include the indoor cooling coil, electric heaters, filter, grilles, registers, dampers, etc., everything external to the FAU, but again, not the ductwork. Every one of these devices have manufacturer data to draw from to get the rated PD at the design CFM. Coils and filters have the greatest PD and need to be carefully selected. (The PD through that 5" media that is being proposed could be .14 or as high as .38 depending on the actual size and amount of CFM that will be moving through the filter. Keep in mind the rated PD is when the filter is clean. The PD goes up as the filter loads up.) The designer calculates the total PD of all of the devices in the air stream then subtracts the PD from the system design ESP. This will give the designer the amount of availabe ESP to design the duct system to. At this point his notes might would look something like this...

Design Blower ESP (Typically at .5)
Minus Coil PD
Minus Filter PD
Minus Register PD
Minus Return Grill PD
Minus Damper PD
Equals available ESP to design the duct system

All the numbers would come from the manufactuer product data - they are not guesses.

* CALCULATING TOTAL EQUIVALENT LENGHT - in a residential duct systems, it is not the running footage, but the equivalent footage of all the fittings and transitions that create the majority of the resistance (PD). ACCA Manual D (duct design) has EL (equivalent lenght) ratings for every type of connection, fitting and transition a duct run will take. The designer needs to look at your duct system layout (size does not matter at this point) and determine the TOTAL EQUIVALENT LENGHT (EL) of the entire duct system. For example, if I understand your Return Air #1 duct, the running lenght is around 28'. I estimate the TEL to be in the 120'-200' range. For example, depending on the type of fitting that was used to create the "split", each "split" would have a rating between 15 EL to 50 EL. Two "splits" would equal 30 EL to 100 EL. If the "splits" are connected directly to each other the resistance increases.

* TIME TO SIZE THE DUCTS - Unless he is using software like Wrightsoft or Elite, a designer will use a "ductulator" to size the ducts. The ductulator looks like a slide rule. There are ductulators for metal ducts and ductulators for flex duct - the data is different. All of the numbers on the ductulator with regards to CFM are based on 100 EQUIVALENT LENGHT. BEFORE looking at the ductulator to begin sizing the ducts, the designer must perform a simple calulation to determine which number on the ductulator scale to use based on the TOTAL EQUIVALENT LENGHT of YOUR duct system. (Hint: Asking the contractor to show you this simple formula is a way to quickly determine if he knows duct design or not. If you don't KNOW the formula, you do not KNOW duct design.) He will then select/confirm duct sizing based on CFM and FPM (velocity) requirements.

* TIME TO GO - I need to get on with my day. I hope you found this helpful. This is a very, very short and over simplification of the design process, which is dynamic in nature as opposed to linear. My estimate is finding a residential HVAC contractor who KNOWS duct design is maybe 1 in 5 at best. Good luck.

    Bookmark   August 8, 2010 at 1:33PM
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Wow, oneuglymother! Thank you so much for the ductwork for dummies lesson. Your information gives me some basics that will help me in my discussions with the professionals. I hope I can find that 1 in 5 that knows their stuff! I appreciate the encouragement and the time you took to write your very useful notes.

    Bookmark   August 8, 2010 at 6:28PM
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