Why Phoenix Is Called "The Land Of No Return"

 Maricopa County is the fastest growing county in the nation with the population expected to grow by 80,000 in the next five years.  Driving down the street, it’s not surprising to see Canadian licenses plates or meet people from IL, CA, NY or WA.  With all these new transplants you would think some of the good home building qualities would rub off on the way Phoenix home builders build houses...unfortunately home builders are slow to catch on which is why  I've been asked so many times, "where we are from we have returns in every room, why does my home only have one return?"  Good question.  On average Phoenix homes have only one return for each AC unit or for every ten supply vents there is typically only one return and this ratio is way off. 

The Land Of No Return, Why Is Having Only One Return A Bad Thing
During our energy audits we check and measure if the ductwork is properly sized and one common finding is that the returns are undersized and need to be larger or a second return added. If your home has only one return per AC system there is a good chance your house needs another return. 

Imagine breathing through one straw, it doesn't matter how hard you breathe and suck air in, you won't be able to get more air in your lungs until you add more straws to breathe through.  The same is true for an HVAC system that is trying to suck a lot of air through too small a return.  The AC unit is noisy and it has to work a lot harder because it’s “starved” for air.  That means if you have a 4 ton AC system, you are probably only getting 3 tons of air into, and out of the unit because the return duct is too small.  More returns need to be added to give your AC system 10 straws to breathe through and from a full 4 tons of air in.

Why Is Having More Than One Return Good?
We’ve listing the benefits of adding a second return and increasing the size of ductwork going into the AC system below.

- Better air circulation in the home
- Increased air into the unit means more capacity of the unit (your 4 ton AC system will be pulling a full 4 tons of air)
- Quieter every time the unit comes on
- Lower energy bills and better operating efficiency

Not All Returns Are Equal
Just like insulation contractors that "blow and go" and don't think twice about things that compromise its effectiveness like misalignments and air leakage, how a new return is installed is just as important as having an additional return. If the return is not enlarged at the closest location to the AC unit as possible, it will do no good to add another return. The way to fix undersized returns is to remove the bottleneck at its source. 

Think of a dam that restricts the water supply downstream, it doesn't matter how big you enlargen the reservoir upstream, until the flood gates of the dam open, you won't be able to release more water into the spillways. 

This is a point most AC contractors miss and takes some planning, training and inter-company communication to get right. Most contractors make the mistake of keeping the same size ductwork into the AC unit and splitting it into two smaller ducts, one for each return.  In most cases, that doesn't help because the volume of air is still the same, it's just split into two different rooms now so the room that had the existing return will suck less air in and be weaker than before. 
Package Units On The Roof
If your AC is on the roof and you had an old Geottl unit from the 1980s you likely have a restrictive elbow. This means that no matter how many returns are added to the house won't help because the bottleneck is the elbow on the roof. Goettl and Chas Roberts used twist elbows in all their installs and even if the AC unit was replaced, 99% of AC contractors re-use the existing roof jack that goes into the roof and put the same style elbow back on. So you probably got a new elbow but it's likely a twist elbow because that is what was installed before. The way to fix the problem is to install a side by side elbow with a return plenum.  This type of elbow does not restrict the airflow. It requires more labor and skill to install and have yet to see a typical AC contractor even recommend this be done because it’s a laborious process most AC contractors see a package unit change out as an easy day with no attic work.

We train out staff to recognize the correct bottlenecks with roof mount AC systems and how to spot restrictions in the attic. That needs to be translated to proper measurements, fabrication and communication to the install team.  Making the change from a twist elbow to a side by side elbow adds about 5 hours to a job that would only take 2 hours if the elbow style was remaining the same, which is why most contractors don’t dare, but the end product means a is definitely worth it for the consumer, our customer.

Spilt Air Conditioner and Furnace Units

If you have a condenser outside and a furnace and coil in the attic, closet or garage, chances are you only have one return for each HVAC system.  In the 1980’s Arizona homes started going away from using sheet metal trunk ductwork to using only flex ductwork.  Flex ductwork is very easy to install, almost anyone can do it compared to the skill required to install sheet metal ductwork.  With the ease of flex duct installation came at the cost of following best practices for ductwork installation. AC contractors made mistakes and got sloppy by using smaller ducts than the plans called for, using too many wye splits, not sealing the ductwork properly and using the cheapest registers possible.  We have a saying that building a home to code is the worst you can legally build a home for a reason.  The code is the minimum standard and for ductwork design and installation, we still have a lot of room for improvement. 

If you take a look at your air handler/ furnace in the attic, you want to see two large metal boxes attached to each end of the AC system.  Those boxes are called plenums and you want two of them, one for the supply side and one for the return.  50% of the time, we find air handlers/ furnaces only have 1 plenum on the supply side and nothing on the return.  This means that the return flex duct goes straight into the unit, which is a red flag for our inspectors. 

When performing an energy audit, we are always looking for a return plenum first.  If an AC system does not have a return plenum, we typically want to add one before adding new returns unless there is no room in the attic.  The size of the new return will depend on our readings and how restrictive the airflow is.

In general we want to see the following sizes of return ductwork.

22” return on a 5 ton AC unit

20” return for a 4 ton AC unit

18” return for a 3 ton AC unit

16” return for a 2 ton AC unit

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Comment by Tyson Pischel on July 22, 2019 at 4:32pm

Couple of thoughts. Building in the size range of 1,800-2,000 sq ft, we have one return in the main body of the home at or close to the highest ceiling.  We run a dedicated return to the master bedroom as it usually has a bath and larger closet with supply vents (undercutting the door isn't enough).  We have standardized installing passive 8" x 16" vents in the wall directly over the other bedroom doors without objection from buyers/customers. our testing has shown undercutting doors to be less than adequate for our return air target for secondary bedrooms.

Lastly, hopefully the HVAC contractor has experience with commissioning the air handler.  This is the only way we have seen to guarantee that the supply and return are balanced.

Comment by Walter Ahlgrim on July 22, 2019 at 12:32pm

I do not understand your sizing chart you say “22” return on a 5 ton AC unit” is that a 22x22 square or 22” round or any shape with 22 square inches.

I do like the chart

https://www.hamiltonhomeproducts.com/vb/Repository/InstallationManu...

My contractor accidently conducted an experiment in my house. He forgot to install 2 pans in my floor joists so all the return air was coming from the basement.  After 18 months I noticed and he corrected the fault. The change made zero difference in the upstairs comfort, but we generally do not close interior doors.

Walta

Comment by Jan Green on July 22, 2019 at 11:18am

David:  Actually, over 70,000 moved to Phoenix last year alone.  We will be back up to 100,000 moving here per year this year so LOTS of people moving here.  We're the fastest growing county in the US.

My rental property tested as needing a larger return to reduce static pressure as it was too small. This 2 story 1819 sq ft was built in 2007.  After the energy audit, the only thing it needed was a larger return, or open the grates farther. 

Comment by Mark Furst on July 22, 2019 at 11:11am

Good stuff.

Comment by Ed Minch on July 22, 2019 at 11:03am

The blanket statement that one return is no good is no good.  The reason your second return finally worked is because your first return was not big enough.  We see this all the time and if you added the same second return near the first one instead of on a different floor, then you would have solved the problem to the same extent.

We have worked with many HVAC contractors throughout the mid-Atlantic to get them to make one, big, 110% of design, well sealed return in a house, then allow for pressure relief at every room door.  Generally with one supply in a room, a 5/8" undercut at the bottom of the door relieves pressure to meet standards, and in the case of more than one supply, like a master bedroom, a transfer grill of jump duct will suffice.  

My own 11 year old, 2 story house at 2800 ft2 has a single return at the bottom of the stairs at 120% of design (cannot be too big), there are 4 supply trunks each at about 150% of calculated capacity, all fittings are designed for good airflow, my 2-speed heat pump on the 4 ton setting moves 1590 CFM, and all closed doors build no more than 3 pa pressure difference inside the room. The 4 trunks serve first floor front and back, and second floor front and back, and each has a large damper near the plenum.  Adding more A/C to the second floor and adding more heat to the first floor by closing off the various trunks so the correct total goes where it is needed each season - once a year, move 4 dampers to predetermined spots.

Even on the 100° day we had yesterday (actual temp), the house was within one degree throughout.  Single returns are our preference.  

Although harder to do, flex duct and attic systems can be made to work.  25 years ago we documented for a regional major builder corrections to his attic mounted heat mupms sytems in 5 houses.  We disconnected the flex at the trunks, cut the hanging straps, re-routed the ducts to be straight from the outlet back to the trunk, sealed the system, and blew insulation over the top of the ducts.  We got (IIRC) a 15% increase in flow on the average, and a 7° colder air temp on the average.  The returns were treated along with the supplies.  The 2018 IECC allows for this approach.

A better approach to sizing returns is to allow 80 in2/ton to meet man. specs, but then go to 100 in2 just to make sure.

In short, correct size total return and correct delivery volume by season and you are successful

Comment by Will johnson on July 22, 2019 at 10:50am

Great information.  I need to review Calif minimum returns requirements.

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