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Return-Air Problems

What happens when the bedroom doors are closed

Posted on Oct 16 2009 by Martin Holladay, GBA Advisor

In homes with a single central return-air grille, return air often struggles to find its way back to the furnace. The result: room-to-room pressure imbalances that lead to uneven room temperatures, comfort complaints, higher energy costs, and even moisture problems in walls and ceilings.

When a furnace comes on, heated air is pushed through supply ducts to registers in each heated room in a house. If the forced-air system is properly designed, the house includes return-air ducts to convey air back to the furnace to be heated again, in a kind of continuous loop.

While most HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. contractors install ducts and registers to deliver conditioned air to every room in a house, they often neglect to provide an adequate return-air path from each room back to the furnace. Most rooms don’t have a return-air grille; instead, there’s often just a single large return-air grille in the living room or a central hallway to serve the whole house. That means that all of the air needed by the home’s forced-air system has to be pulled through that single grille before it can be heated by the furnace or cooled by the air-conditioning system.

Isn’t one big central return good enough?

Here’s what can happen when a forced-air system doesn’t have adequate return-air pathways:

  • When the furnace is operating, it pushes conditioned air into each bedroom.
  • If the bedroom doors are closed, there’s no easy way for the air to get back to the return-air grille in the hallway. As a result, each bedroom becomes pressurized, forcing air into cracks in the bedroom walls and ceiling. During the winter, this humid interior air can contact cold surfaces in the wall, leading to hidden condensation and even mold.
  • Meanwhile, the big return-air grille in the hallway is starved for air. Since the hallway and living room are now depressurized, air is pulled from the attic into the hallway through cracks in the ceiling.

Jim Cummings, a senior scientist at the Florida Solar Energy Center (FSEC), has measured pressure differences arising from unbalanced forced-air systems in over 200 Florida houses. “The typical positive pressure in a bedroom with a closed door is on the order of 15 pascals — ranging from a low of plus 5 pascals to over 45 pascals,” said Cummings. “That’s a tremendous amount of pressure. Air is being pushed out of these rooms into the attic and through the walls towards the exterior, while the central zone of the house is depressurized and pulls air in from the attic. … A lot of this uncontrolled airflow is actually an exchange between the house and the attic.”

During the winter, the air that is pulled from the attic is cold, and the cold air increases the heating loadRate at which heat must be added to a space to maintain a desired temperature. See cooling load.. During the summer, the air that is sucked indoors is hot and humid; this infiltration increases both the sensible and latent loadCooling load that results when moisture in the air changes from a vapor to a liquid (condensation). Latent load puts additional demand on cooling systems in hot-humid climates. on the air conditioner.

Best-practice guidelines

There are three possible ways to solve the pressurized-bedroom problem. Each bedroom needs either:

  • A return air grille ducted back to the furnace;
  • A through-the-wall transfer grille connecting the bedroom and the adjacent hallway; or
  • A crossover duct (a jumper duct) connecting a ceiling grille in the bedroom with a ceiling grille in the hallway.

The best of these three solutions is the most expensive: install a return grille ducted back to the furnace in every conditioned room of the house. Through-the-wall transfer grilles are a cheaper solution, but they have a major drawback: noise transmission. Homeowners don’t want people in the hallway to hear what goes on in the bedroom.

Most contractors who bother to address the pressurized-bedroom problem install crossover ducts. As typically installed, a crossover duct connects a ceiling grille in each bedroom with a nearby ceiling grille in the hall. Although a crossover duct will transmit sound, it provides more muffling than a through-the-wall transfer grille.

However, crossover ducts have all of the disadvantages of any attic duct. To prevent unacceptable heat loss or heat gainIncrease in the amount of heat in a space, including heat transferred from outside (in the form of solar radiation) and heat generated within by people, lights, mechanical systems, and other sources. See heat loss., it’s best to limit the installation of crossover ducts to homes with cathedralized attics — that is, homes with insulation that follows the roof slope.

Sizing crossover ducts

A variety of methods have been proposed for sizing transfer grilles and crossover ducts. According to Cummings, a transfer grille should provide 70 square inches of free area per 100 cfm of supply air ducted to the room; Cummings’ recommendation is sometimes called the “FSEC guideline.” Cummings says, “That guideline is based on calculations, confirmed by experiments in the field.”

Some engineers prefer larger grilles. According to Paul Raymer, the chief investigator at Heyoka Solutions and former president of Tamarack Technologies in West Wareham, Massachusetts, a manufacturer of through-wall transfer grilles, it’s best to add 40% to the FSEC guideline. “We figure that with two grilles, instead of 0.7 square inch per cfm of supply air, you need one square inch, to account for the resistance of the two grilles,” said Raymer.

Armin Rudd, an engineer at Building Science Corporation in Westford, Mass., advises that transfer grilles need only 54 square inches of free area per 100 cfm of supply air. While Cummings prefers to keep room-to-room pressure differences at 2.5 pascals or less, Rudd is willing to accept room-to-room pressure differences of 3 pascals. (The 2.5-pascal standard has been mandated by the Florida Building Code since March 1, 2002; in almost every other state, building codes are silent on the issue.) Rudd’s rule of thumb assumes that the average bedroom door has a 1/2-in. by 32.-in crack at the bottom — a crack that contributes to solving the pressurized-bedroom problem.

According to Rudd, a 6-in. round jumper duct is adequate for a bedroom with 75 cfm or less of supply air. An 8-in. duct can balance a room with 120 cfm of supply air; a 10-in. duct balances 175 cfm of supply air; a 12-in. duct balances 225 cfm of supply air; and a 14-in. duct balances 300 cfm of supply air.

Why can’t I just undercut the door?

Undercutting a bedroom door won’t solve the pressurized bedroom problem. Raymer points out that even a very large gap — a gap of 1 inch between the finish floor and a 30-inch-wide door — can handle only 47 cfm of return air at a maximum 2.5 Pascal pressure difference. That’s adequate for only a tiny room measuring about 75 square feet.

There’s another problem with door undercuts: they’re not under the control of the HVAC contractor. The final gap depends on the choice of finish flooring and the way the door is hung; these details are usually determined by the finish carpenter, long after the HVAC contractor has left the job site.

First, design the system; then install it; then commission it

Of course, even well-designed jumper ducts will only work properly if the forced-air system is set up properly. When the HVAC system is commissioned, the supply airflow to every register in the house should be verified and individual balancing dampers adjusted, to be sure that the system meets the design air flows shown in the Manual D calculations. Although residential HVAC system commissioningProcess of testing a home after a construction or renovation project to ensure that all of the home's systems are operating correctly and at maximum efficiency. should be routine, it is, alas, still rare.

More information

For further discussion of ways to solve room-to-room pressure imbalances, see Peter Yost's article, “Return to Sender – HVAC Return Pathway Options.”

Last week’s blog: “Hot-Climate Design.”


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  1. Martin Holladay
1.
Fri, 10/16/2009 - 21:56

Good Post
by Grant Dorris

Helpful? -1

It is information like this that should be used to educate consumers on what to look for in a house. Few builders will ever undertake this initiative on their own and the consumer needs to know what to demand.


2.
Sun, 10/18/2009 - 14:42

Return-Air Problems - A through-the-wall transfer grille option
by Armand C Magnelli

Helpful? 1

This is an important topic. The pressure imbalance created by closed bedroom doors can also create problematic negative pressures in other areas of the home as the air handler struggles to find sufficient return air. The 3 suggestions of (1) A return air grille ducted back to the furnace, (2) A through-the-wall transfer grille and (3) A crossover duct (a jumper duct) are all reasonable options. I'd suggest that the Tamarack Technologies "Return Air Pathways" (RAP) is a simple and effective through-the-wall transfer grille solution that significantly limits sound transfer. You can see the product on the tamtech.com web site under the "Products" link in the Pressure Balllancing category.


3.
Mon, 10/19/2009 - 19:59

Return air pressure relief
by Paul Raymer

Helpful? 0

It's great to see this information get out there. The studies we did at Florida Solar Energy Center arrived at three formulas for calculating the required openings to keep the pressure across the doorway at less than 2.5 Pascals:
For flexible jumper ducts the diameter of the flexible jumper duct should be equal to the square root of the delivered airflow: For example 100 cfm delivered would require a 10 inch diameter duct.
For the slot under the door: The area of the slot should be equal to one half the airflow delivered: For example 100 cfm delivered would require a 50 square inch opening - a 1.65 inch high by 30 inch wide cut.
For a grille covered opening through a wall, the area is equal to the airflow delivered divided by .83. For example 130 cfm delivered would need a 156 square inch opening perhaps 10 by 15 inches.
There is more about this stuff in my recently published "Residential Ventilation Handbook". (By the way, I am now with Heyoka Solutions in Falmouth, MA.)


4.
Wed, 10/21/2009 - 10:58

Comments by Russel Goyen
by Martin Holladay, GBA Advisor

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I just received the following e-mail from Russel Goyen, the Chief Mechanical Inspector for the city of Idaho Falls, Idaho. He writes:

"Thanks for weighing in on the side of reason. A vote from someone of your stature in the arena of energy and airflow is very welcome. I just read your recent blog on "Musings of an Energy Nerd," and I must say: "Rah, rah!" I couldn't agree more.

"My background is in the inspection and plan-review field of mechanical code enforcement. I work for the City of Idaho Falls, Idaho, a relatively small but active jurisdiction that sees a fair variety of construction methods from residential to industrial and medical installations. I get to be involved with all of it since the jurisdiction is small enough to need only 2 mechanical inspectors, of whom, I am the "chief inspector." We are one of the few jurisdictions in the country currently enforcing the IRC's requirement for residential HVAC designs. We started 6 years ago, very much "alone and without a leader," as Horace Rumplole would say. We've made mistakes along the way, but have reached a point where designs are routinely submitted, thoroughly and competently reviewed, and field-inspected. Just last year, we took the further step of requiring all new forced-air systems to be balanced and a balancing report submitted as part of the final commissioning of the the system.

"We have been very active as a jurisdiction and as an ICC chapter in sponsoring training of contractors to incrementally bring them up to speed in meeting these new requirements--requirements that were unthinkable 6 years ago.

"As your article correctly states, such measures are rare in the residential HVAC profession, and my personal belief is that, although the HVAC system is probably the most complex system of any house, the average HVAC worker is the poorest-trained of all the licensed tradesmen. So, I applaud your efforts to help spread information of the kind you do, and look forward to more."

Russel Goyen
Chief Plmg/Mech Inspector, City of Idaho Falls


5.
Wed, 10/21/2009 - 11:07

Thanks for contributing, Paul
by Martin Holladay, GBA Advisor

Helpful? -1

Paul Raymer,
Thanks for your helpful comments and guidelines. I'm sorry that I neglected to note that you have moved on from Tamarack Technologies and are now at Heyoka Solutions. I've corrected the text to note your current position.


6.
Wed, 10/28/2009 - 16:26

return size
by ashish

Helpful? -1

Hello,

I hope i am in the right place. I havea question to whihc i am having a problem getting an answer.

I am finishing my basement about 450 SqFt. I have three registers of 6" each and i was wanting to know what should be my size of the return that i put in.

Thanks
Ashish


7.
Wed, 10/28/2009 - 16:51

We need more information
by Martin Holladay, GBA Advisor

Helpful? 1

Ashish,
1. The best place to ask questions is at our Q&A page. You will find a greenish-yellow box or tab marked "Q&A" at the top of each page at the GreenBuildingAdvisor site. If you click that box, it will take you to the Q&A page.

2. You haven't provided enough information to answer your question. Proper duct sizing requires extensive calculations, using Manual D (a calculation method developed by ACCA). If you don't know how to perform a Manual D calculation, you should research the issue or hire a contractor who can perform the necessary calculations.

3. In general, return air ducts need to be at LEAST as large as supply ducts, and are often sized larger.


8.
Sat, 07/17/2010 - 10:59

crossover
by glenn

Helpful? 0

i have been told that my ac unit is a crossover, and not very efficiant. could you explain exactly what a crossover is and compare it to one that is not?


9.
Sun, 07/18/2010 - 04:33

Response to Glenn
by Martin Holladay, GBA Advisor

Helpful? -1

Glenn,
If you read the article, I explain what a crossover duct is. If someone told you that your AC unit is a "crossover unit," I'm not sure what that person meant. Perhaps you should ask the person who told you.


10.
Fri, 10/15/2010 - 00:38

Where to put a return?
by doggie

Helpful? -1

My boss wants me to put a 3-ton heat pump split system to control more cooling where their offices are. They used to have a 15-ton unit that supplied their building, but it has been removed and replaced with three 5-ton units to cool and heat their rooms. They want to add a 3-ton in the back for their offices but their large kitchen is in the middle of the offices. But they have an open back door. The air handler is above those rooms. The condenser is less than 20 ft away. The old supply and return outlets we took off are where the condensing units are located. The total rooms they want to cool are 6. The measurements are: 3 - 13 x 7 x 8 ft high, 1 - 12 x 10 x 9 ft high, kitchen is 14 x 16 x 10 ft high, 2 - 20 x 6 x 7 ft high bathrooms.

I really don't know where to put a return grille.


11.
Fri, 10/15/2010 - 04:30

Response to Doggie
by Martin Holladay, GBA Advisor

Helpful? 0

Doggie,
I'm sorry, but it's impossible to design a duct layout based on your description. Someone would need to have good plans or visit the site.

If you have no idea where the return grille belongs, perhaps a different HVAC contractor should design the system.

As a general principle, the best HVAC systems include at least one return grille in every conditioned room.


12.
Mon, 01/14/2013 - 21:58

return duct size and lenght of run
by Joe Li

Helpful? 0

I have the same question as ashish, so what did he missed to provide? the length from the return grille to the furnance? if so please provide size per foot?


13.
Tue, 01/15/2013 - 05:49

Response to Joe Li
by Martin Holladay, GBA Advisor

Helpful? 0

Joe,
To design a duct system, you need to know your calculated heat loss and cooling load. Once you have chosen your register and grille locations, you will know the length of each duct and the number of required elbows. At that point you can begin designing your duct system.

For more information, see Saving Energy With Manual J and Manual D. Most homeowners find it easier to consult with an HVAC contractor or engineer rather than to do their own duct design.


14.
Sun, 12/29/2013 - 14:45

Edited Sun, 12/29/2013 - 22:26.

Returns for Low Volume Systems (e.g., HRV)
by Len Moskowitz

Helpful? 0

Do the same guidelines hold for low volume ventilation systems like HRVs?

Our PassivHaus has a Zehnder ERV that has a max airflow of around 210 CFM. That's for the entire home. It has dedicated returns only in the three bathrooms, kitchen and open-plan third floor. Will door undercuts be sufficient as returns in the other rooms?


15.
Sun, 12/29/2013 - 16:34

Response to Len Moskowitz
by Martin Holladay, GBA Advisor

Helpful? 0

Len,
The advice in this article does not apply to ventilation systems with dedicated ventilation ductwork (like your Zehnder ERV). It only applies to furnaces with forced-air ductwork or whole-house air conditioners with air handlers and ductwork.

The airflows needed for ventilation are so small that the concerns raised in this article don't apply.


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