An Easy Retrofit for Return Air From Bedrooms

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Perfect Balance in-door return air pathway

Your bedroom really doesn't aspire to be a balloon. Yet, because of the way your heating and air conditioning system was installed, it may be acting like one. At least to an extent. It doesn't expand the way a balloon does, but it does get blown up. Think about it. If your bedroom has a supply vent from your HVAC system but no return vent or other pathway for the air to make its way back to the unit, what happens to that air blowing into the room when you close the door?

The problem with bedrooms

Forced-air heating and cooling systems use ducts to move conditioned air from the furnace, heat pump, or air conditioner back into the living space inside the house. The air that it's conditioning, though, isn't just whatever it can get. Ideally, it's a closed system with as little duct leakage as possible, and the air that it's conditioning comes from inside the house. The furnace or air handler (AHU in the drawing below) pulls air through the return ducts, conditions that air, and then sends it through the supply ducts back into the house.

HVAC system supply and return vents

The problem with bedrooms occurs in houses where the HVAC design did not include return vents in each bedroom. Well, I should say it's only a potential problem because a good installer will be working from an actual HVAC design, which should specify exactly how air gets back to the air handler. If a home has a central return vent for all the air going to the air handler, a good HVAC design will include return air pathways from the bedrooms. That could be jumper ducts (shown below) or transfer grilles. Door undercuts typically won't allow enough air to get out of the bedroom unless you leave a gap bigger than most people want under their doors.

Jumper duct as return air pathway (image from Building Science Corp.)

The consequences of inadequate return air pathways

What happens if you don't make provisions for the return air from the bedrooms? When the system is running and the bedroom door is closed, the pressure in the bedroom goes up. It becomes positive relative to the main part of the house and probably also to the outdoors. Some of that air will find its way under the door and through interior leaks back to the central return vent. The rest of it, though, will push through openings and leak to the outdoors and into interstitial spaces. It leaks out. You lose air that you've paid to heat or cool.

The other side of that pressure equation is what happens in the main part of the house. If the bedroom doors are closed and the bedrooms become positively pressurized, the main part of the house sees negative pressure. The central return is pulling air but can't get everything back from the bedrooms. That negative pressure can be a big problem.

Negative pressure in the house can cause a number of problems:

  • Infiltration - Since the central return can't pull back all the air the AHU sent to the bedrooms, it pulls it from wherever it can, including unconditioned air from outside.
  • Bad air from outside - Some of that air pulled in from outside may be nasty air full of fumes from the garage or moldy air from the crawl space.
  • Backdrafting - If that negative pressure extends to the location of a natural draft gas water heater, you may end up with air coming down the flue instead of exhaust gases going up the flue. This backdrafting can result in carbon monoxide coming into the living space. What's worse, that central return pulls the carbon monoxide into the air handler and distributes it to the bedrooms.

Return air pathways are important.

An easy retrofit

If you live in a house that doesn't have dedicated return vents in the bedrooms and there's also no hint of jumper ducts or transfer grilles, you have another option. Tamarack Technologies makes something they call the Perfect Balance in-door return air pathway. (Disclosure: Tamarack is an advertiser here in the Energy Vanguard Blog. We like their stuff and think you will, too.) They're inexpensive, easy to install, and they work. Here's a video showing the pressure difference with and without the Perfect Balance.

I installed these return air pathways in my condo three years ago, and that video shows the one in our guest bedroom, where the positive pressure created by the lack of a return air pathway was the worst. It made a huge difference, dropping the pressure in the bedroom (relative to the hallway) from about 6.7 Pascals (Pa) to about 1.7 Pa. The ENERGY STAR new homes program requires pressure balancing of bedrooms with a maximum pressure difference of 3.0 Pa.

It took me about a half hour to install these things in all three bedroom doors. All I had to do was remove the door from the hinges, place it on sawhorses, trace the outline of the piece using the template provided, cut out the section of the door, screw the piece to the bottom of the door, and then put the door back on the hinges. Bam!

If you're worried about privacy, the Perfect Balance has a cardboard baffle built in to reduce sound transmission. I've lived with them in my condo for three years now and have never had a problem. By getting down on the floor and putting your face up against it, you can see through into the bedroom, but you get a limited, narrow view of the other side.

The best use of the Perfect Balance is in existing homes where you don't have returns in the bedrooms and can't (or don't want to) install jumper ducts or transfer grilles. If the pressure in the bedroom with the door closed and the air handler running is higher than 3 Pa, this is a great way to retrofit a return air pathway. If you don't like the look of the Perfect Balance, Tamarack also sells transfer grilles, which you install in a wall.

The next time your heating and air conditioning system comes on, think about that pressure building up in the bedroom. Imagine where that air might be going. Imagine the kind of air that's coming in from outside.

It's time to pop that balloon.

 

Related Articles

4 Ways a Bad Duct System Can Lead to Poor Indoor Air Quality

The Basic Principles of Duct Design, Part 1

The Sucking and the Blowing — A Lesson in Duct Leakage

Can You Save Money by Closing HVAC Vents in Unused Rooms?

 

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Comments

I would go the extra mile and put in the jump duct whenever possible.

Yes , it is better to stay well balanced, than under pressure.

Allison
Bailes

RJ, that's easy to do in new construction or remodeling (or easier anyway). In my condo, it wasn't an option. I agree in general, though.

Tamarack makes baffled transfer grilles in several sizes and shapes. Alternatively, a transfer grille can be installed over the door or in a closet if the doors are louvered or loose-fitting.

Allison
Bailes

Yes, transfer grilles are also a good option and, as you say, allow you to use other locations for the return air pathway. Whichever way you go, it's best not to use a straight-though transfer with no baffle. I've seen those installed and wouldn't want them in my house.

No, you don't want a straight-through transfer with no baffle. We have them and it's like there is no wall there. I hate them and so glad we found a better alternative-I hope!

Very well written- nice job. I agree that in order of preference return, jumper, transfer, door cut grill. Trouble with cutting most doors is that you violate the joinery of a wood door or cut through the solid blocking of a hollow core door. It should be a method of last resort and in the case of a solid wood door- maybe never.
I applaud your concise treatment of the issue- Thanks.

Allison
Bailes

Rob, I may have stated them in that order (return, jumper, transfer, door cut grill), but that's not my order of preference. Individual returns increase duct area, duct leakage/sealing, and total effective length of the duct system so I'd rather avoid them when I can. I do agree that going through the door should be the last resort, though. And in a lot of existing homes, they're just what's needed.

Rob, agree totally in the case of solid wood doors. Just a note regarding the use of the Tamarack Perfect Balance in a hollow core door, Masonite tested and approved the Perfect Balance for use in their "Safe N' Sound" interior door.

My circa 2000 townhome has individual returns leading from each bedroom. It never made any sense to me since the main return for the second floor is only 15 ft away in a hallway ceiling. I suspect jump ducts leading into the hallway would've been much easier.

Transfer grilles transfer sound. A couple years ago I toured a tract McMansion bought by a friend and his family. The master suite is right off the family room and right over the door is a nice fat through-wall direct transfer.

As far as I know, it transfers return air...and every word uttered.

Allison
Bailes

Curt, I'd never use a transfer grille without a baffle or some other sound-dampening method. I've seen them done, too - but only in small, affordable housing units. I haven't seen them in larger houses like the one you describe.

"Door undercuts typically won't allow enough air to get out of the bedroom unless you leave a gap bigger than mostly people want under their doors."

This all depends on the amount of air being delivered to the bedrooms. For existing houses with oversized and unbalanced systems, the quote above is probably true most of the time. However, we design and balance systems all the time with central returns and WITHOUT transfer grilles or jumper ducts for most bedrooms. We find that with a standard door undercut of about 1/2", we can reliably keep bedroom pressurization to https://www.dropbox.com/s/ojuvtpock2p53sg/Screen%20Shot%202017-10-17%20a...

The Tamarack transfer grilles ARE a great product, though...excellent solution for existing houses with inadequate return air pathways. We spec their standard transfer grilles when room design airflow gets up over 75cfm (especially typical in today's master bedroom suites).

Allison
Bailes

John, thanks for calling me on that. I should have emphasized in the article (and will go back & add it shortly) that this product is most useful for existing homes. Well, I did put "retrofit" in the title. I agree with you on new homes.

Spot on again Allison. Light and noise transfer are legitimate concerns where baffles aren't used in transfer grills or the jump duct is too short. Unfortunately too often the solutions for those concerns run to the extreme, door undercuts or fully ducted returns. The inadequacies of door undercuts has been discussed at length but I for one am anxiously awaiting that next post I'm sure you have cooking in your brain where you'll enlighten us all on the performance impacts of restricted return ducts. Especially when done seat of the pants with that flexy bendy stuff.

Allison
Bailes

Arn, I hadn't thought of what my next post might be yet, but who knows? Maybe it will be restricted, flexy, bendy return ducts. As I mentioned in one of my replies above, individual returns aren't our first choice when designing duct systems.

One of the best ways during design to figure out if you've oversized the HVAC system is whether you can get the air back out of the bedrooms (with 3 Pa or less pressure across the doors). Standard door undercut plus a small jumper duct (always with fully extended flexible duct) to the return zone can only handle up to about 100 CFM. So putting in smaller systems that deliver more thermal capacity to the intended targets using best installation practices means that systemic problems are avoided. This is a problem in every home I've ever tested and I haven't ever found anyone who would allow me to cut their door more than 1/2" or who liked the look of these. But in a rental or apartment, the owner can solve some problems with these (only other concern would be kicking and door durability as renters can be pretty rough sometimes- have you ever kicked on yours Allison?)

Allison
Bailes

Mike, your statement about installing "smaller systems that deliver more thermal capacity" sounds like you want to go to lower cfm/ton, which I know is the opposite of what you do with air conditioning in California. You might want to explain that statement a bit more.

Yes, I have kicked grilles in our bedroom doors a few times. One time, I broke one of the slats but only at one end. I was able to move it back into place and I'd have to look hard to find it again. If I had broken both ends, though, I'd have had to glue it back together. But my anger management classes have been working lately and I haven't kicked anything in a while. ;~)

Smaller systems and CFM/ton are two different things. You can install one with no effect on the other, meaning I can install a 3 ton at 1500 CFM and a 3/4 ton at 375 CFM and be doing the same sensible and latent work with very different volumes. In my CA dry climate, we always aim for 500+ CFM/ton so we do very little drying of already dry air, using ventilation instead to dry homes (year-round more dry outside than inside).

Allison
Bailes

Mike, yes, that part is clear. But you didn't address the other part of your statement about "smaller systems that deliver more thermal capacity." Please explain the enhanced thermal capacity of said smaller systems.

Allison you simply got too excited and ended your quoted passage too early, as you cut off "using best installation practices" which explained the "delivering more thermal capacity" question. Essentially it's paying careful attention to the distribution system design and installation so that you don't waste half of the efficiency between the cool and destination. At least 7 key pieces to pay very close attention to, and hundreds of details- a tiny bit too much to address here (but mostly done in various topics on your blog over the years). :-)

Allison
Bailes

Thanks, Mike. I am a bit excitable at times.

In a house with no cooling and radiant heat, will an HRV-only system create enough pressure difference to warrant the same considerations? (I understand that the HRV system as a whole needs to be balanced, I'm just wondering about room to room differentials.)

Allison
Bailes

Benjamin, you probably don't need to worry about excessive pressures if your only air flow into the bedroom is from the ventilation system. That's something that should be checked during commissioning, though. As long as you have a half-inch undercut at the door, your ventilation air should be able to make it back to the return. See also John Semmelhack's comment above.

Does the location supply air grill with respect to the return air grill make a difference? For example, I've seen 20x17 master bedrooms where the the return air is a few feet away from the supply air. Wouldn't it make sense to separate them further apart say opposite end of the room?

Peter, yes and no. Yes it would be better to distance the supply and return if the supply is a typical stamped Louvered register that simply dumps air into the room. If, however, the system is a high performance installation using supply grilles that were selected for their throw, the location near the return is optimal. The reason is that the return is a sink, with not much measurable velocity a foot away, and the supply is throwing the air at high velocity to the distant and opposite wall. So the air will travel across the room, down the outside wall, across the floor, then back up the near wall to the return- totally "spent" of thermal capacity for change- and ready to be replenished. So you can see in this instance, the nearest physical location is actually the longest possible throw- which is best for system efficiency.

Didn't think about that. Thanks for the excellent response!

Hey Allison! My coworkers and I have been having the discussion about zonal pressures caused by imbalanced return. Here in Iowa we put a returns in almost all bedrooms and all other living areas in the house. My question is can you cause a zonal pressure problem or any other comfort issue by having a strong, over sized return in one room and and a return in another room that is sized only to handle that room? Will an over sized return effect the return or comfort in another room? What are your thoughts on over sized or unbalanced returns?

@Austin, you pose an interesting question. My initial reaction is that an oversized return in one room would only have a meaningful impact on another room IF the return path from the other room is undersized relative to its supply.

Ignoring any interaction with envelope leakage, keep in mind that return side imbalance in of itself is only an issue to the extent that it impacts supply side balance. Said differently, as long as the return path for a given room doesn't impede the amount supply air to that room, I don't think it matters how large the other returns are.

The reason is that a properly designed duct system will have much smaller resistance on the return side than the supply side. I like to see no more than 15% of TESP (total external static pressure) on the return side.

This would be easy to test by partially covering the oversized return grille and measuring any difference in the zonal pressure in the other room (as a proxy for airflow to that room). If this results in a meaningful change, I would argue that the problem is not the oversized return, but rather that the other return is undersized. In any case, restricting a return in one room to force more air to another room is a bad idea.

I'm just a homeowner, I'm not in the business. Much of what you write is over my head, but I have also found articles I actually understand! We put in tilting transom windows over our bedroom doors. I like the way they look and I suspect they are serving the purpose you are discussing. However, our HVAC vents are in the ceiling. Are we working against our heating and cooling system by having the return up high? We shut our doors to keep our cats out. I think the cats would rip up any lower door insert. They destroyed the bottom of our bedroom doors; we added metal plates. I found your column and started following your articles after I remodeled and sold my mom's home and then prior to the sale was told the HVAC unit my mom had installed was too large for the space. I thought that was hogwash, and then found out it was true, by reading your information. I continue to find your articles interesting. Now we are redoing a small duplex for retirement. The cats are coming too. I guess I better look at adding transoms or something to the remodel plan. Is it better to have the return air ventilation low?

@Cyndy, what a great idea! My grandmother's home had interior door transoms. It's not a problem having the return path at the same level as the supply outlet. The supply air is at its greatest velocity as it exits the diffuser, so the location of the return path is largely irrelevant as long as the supply air is not directed toward or in close proximity to the return path. Conservatively, you want at least three feet of separation, and make sure the vanes on the grille are not aimed toward the transom.

I don't see anything regarding smoke dampers in jump ducts in the IRC. NFPA now recommends that people sleep with doors closed to slow fire smoke. Heat and smoke rise and will travel right through jump ducts. I would like to see smoke dampers tied into fire alarm systems or transfer grills at the bottom of doors.

I live in Central Texas and 'Dust Is A Country Collectable.' I was wondering how would one clean the cardboard baffel ro would it need to be replaced?
Thank You for your time and expertise. Emelee

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