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Increasing Comfort, Energy Efficiency and Revenue by Properly Sizing Return Duct Systems

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Studies have shown that 80% of homes have undersized return duct systems.  An undersized return duct system reduces the amount of air flow through the system and may cause reduced comfort and efficiency as well as poor supply airflow, duct sweating, improper refrigerant charge, and excessive air noise. The problem can lie in the sizing of the main return trunks, return grills or both.  Proper return duct sizing is an often overlooked issue in solving customer’s comfort issues and assuring they get the most out of their HVAC system.  A return duct survey should be a part of every Home Performance Call, Equipment Sales Call and even every Maintenance Check.  Basic return duct sizing takes less than 10 minutes per system.  Sizing charts, apps and software are readily available.  A quick footnote:  This article is meant as a first step to help people get better at delivering a more comfortable and efficient system to their customers.  It is important to take existing and prospective system static pressure into account in duct sizing.  Sadly, this is a tool which is not used often enough on the frontline of diagnostics, maintenance, and replacement system sales.  There are numerous teaching aids available to learn how to calculate existing and prospective system static and I highly recommend obtaining that knowledge.  

Uncovering customer’s PAIN upfront makes it easier to sell the solution.

It is a lot easier to sell a solution if the customer understands how it will fix their existing “PAIN”.  PAIN=complaints customers have with their indoor environment.  As soon as you walk in the door, the first thing you should do is ask the customer if you may ask them some questions to make sure they get the most out of today’s visit.

Questions to ask customers to uncover potential return issues:

*Do you have any rooms which are hard to heat or cool or where you feel there is poor airflow?

(an undersized return duct system will reduce delivered cfm below design parameters)

*Are any rooms or areas of your home stagnant or stuffy?

(undersized or missing returns cause lack of air movement)

*Rooms or areas of your home which have high humidity in the summer?

(missing or undersized returns can allow humidity and heat to build up)

*Bedrooms which become uncomfortable if the door is closed?

(lack of return in bedrooms can cause massive comfort issues when the door is closed.  Hint:  This change normally takes place as children become older and want to close their door for privacy)

*High utility bills?

(restricted return airflow means the system cannot not provide its full capacity and will need to run longer to cool the home)



After asking your customer questions so you know the problems you will address, you now need to size the return duct system.  Be sure to record the model number of the air conditioner so you can determine the required system air flow.  Even if you plan to increase or decrease the capacity of the system, this process will tell you if you need to increase the return size to match the new system.  Although some systems will work properly at lower cfm per ton (325 to 350 cfm), always size the return duct work at 400cfm per ton and no more than .08 static (iwc).  However, you will be safest sizing the return at .05 static (iwc).  Another footnote:  This is where understanding static pressure and using that knowledge is important.  The .05 and .08 iwc will generally be fine but it won’t work for all duct systems.  In addition, if there are still issues, other duct work may be required to reach the system static target.  It is absolutely a best practice to fine tune your numbers by determining proper static pressure for your system and sizing the return and supply ducts to that number.  Don’t forget, a high MERV, dirty or undersized filter can add a lot of static to the system.   Start by sizing the main return trunk(s).  This will often take multiple measurements starting with point where the return connects to the fan coil/furnace.  From there, work your way back to every duct trunk as well as any returns which connect directly to the return plenum (often seen with basement returns).

Next, measure the return grills for each duct trunk to determine the cfm which can flow through the grill.  This is important as we often find that while the trunk is large enough, the return air becomes restricted at the grill.  If the proper amount of return air can’t get through the grill and into the trunk, it certainly won’t get back to the indoor unit to be conditioned and supplied to the home.

How to explain this to customers and get your Customer to say, “Can you fix it?”

Hopefully, the potential to solve your customer’s PAIN will get them to come with you while you size the return duct system.  The duct sizing process provides a great opportunity to show how different you are from your competition.  Your customer will also be able to see the problems (and solutions) as you go through the sizing process.

As you are doing the sizing, it’s a good idea to point out what you are doing and why to your customer.  “Ms Customer, to deliver proper comfort and efficient cooling your system needs to move a certain amount of air.  An undersized return duct restricts the amount of air which can be delivered to your home.  That is why you have the problems you mentioned earlier.  For example, this duct is undersized and restricting about 1/3 of the air your system can deliver.   This means you are getting less cooling and airflow than your system is designed to produce.  Increasing the size of the duct will allow more air to get to your system and more air and cooling into your home.  Does that make sense?”  The customer will usually say something along the lines of, “Can you fix it”.  At that point, you know you’ve made the sale and you will be allowed to make your customer’s home more comfortable and energy efficient.

Tools you can use to show/explain duct sizing issues

How to explain the effect of undersized return on airflow:  “Mr. Customer, try breathing in a half breath and blowing out a full breath…your system can’t do that either!!!”

Closed door test for bedrooms without returns:  If a customer complains about a stuffy or hot bedroom which does not have a return, you can easily show them the need for a return in the bedroom.  With the system fan running, close the bedroom door for a minute or two.  This will allow the bedroom to pressurize.  Assuming the door opens into the bedroom, while pulling the door towards you (closed) turn the handle to the open position and very slowly open the door.  As you open the door, you will feel pressure pushing the door closed.  Once the pressure stops, hold the door at that spot.  The amount of air needed to balance the room is the width times height of the opening (give or take).  Point out to your customer how much air is needed to balance the room (let the air get back out of the room and to their system).  They will often ask if they can just further undercut the door.  Of course they can, but note, if you opened the door just two inches, you need to undercut the door another 5” which they may not want to do.  You can then explain how easy it is to add a ceiling return (in most cases).  You can also perform this test using a magnehelic gauge which will show the pressure difference between the room and the hallway.  However, it’s not as visual and relatable of a demonstration.  If the customer doesn’t get it, close the door and let them open it.

Show how adding return will improve airflow: If the return system is noticeable undersized, it’s easy to show the customer how adding return capacity will improve airflow.  Step 1, with the fan on, have the customer show you the registers which they feel don’t move enough air.  You should both feel the airflow coming from the register(s).  Hint, don’t put your hand on the register, put it about 8” to 12” back (if the register throws to the side, make sure you put your hand in the air stream and not the middle of the register).   Step 2, making sure pets and kiddies are not around, open the door to the furnace and put a piece of tape over the safety switch.  You have now effectively allowed the system to pull as much return air as possible.  Now, go back and feel the airflow from the register.  You will usually find a noticeable increase in airflow and the customer will again ask, “Can you fix it?”  Please note, if the customer has an ECM motor you may need to choke down the opening you created so the motor gets enough pressure to operate.

Fixing problems

Often the fix is as simple as adding a return to a finished basement.  In the case of an undersized return grill, you have the choice of either adding another return off the same trunk (this is where you tap an attic or high wall return to add return to a bedroom) or increasing the size of the return grill by opening the wall and return duct so you may install a larger return grill.

Hopefully, this information will set you apart from your competition while generating additional revenue and helping make your customer’s home more comfortable and energy efficient.  Please use this as a first step.  If this interests you, please take the time to learn more about duct sizing and static pressure.  Comfort Institute/Aeroseal and NCI provide outstanding training on duct design and static pressure.

Andrew Oser

Posted In: Building Performance

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