Search the forums

Loading

A/C Return Air

Don's picture

Daughter's house has two A/C systems.  One has a return air plenum, the other does not.  The one w/ the return air plenum serves the master bedroom.  Unfortunately, the plenum merely collects air from a single point & moves it via a torturous route to the underside of the A/C unit,  (I know, stupid, but that's the way it works.) meaning that all the return air has to slither under the door to get out of the BR when the door is closed. 


We want to install return ducts in the BR - it's rather large & does not get or stay comfortable, year round- believing it will solve the gomfort problem. Currently it is serviced by two inlet ducts feeding about 400CFM of conditioned air.  To properly return that much air, seems to me you'd need about the same number of square inches of space under the door therefore the door has to be cut off about 3 inches from the floor.


Does such a move make sense? There is plenty of space on the surface of the return air plenum to splice the new return air ducting into.


When we had our house built, the HVAC contractor chose to place the inlet air openings along the outer walls & the return openings at the inner walls of every room.  Makes sense to me.  Said the outer walls were the source of hot air in summer & cold in winter.


I'd like to put in two like sized (compared to the inlet ducts) return air ducts in the room.  Does this sound rational?


Don


Don Reinhard
The Glass Masterworks
"If it scratches, I etch it!"
Don Reinhard The Glass Masterworks "If it scratches, I etch it!"

(post #115913, reply #1 of 26)

Bimpity, bump, bump, bump!

Don Reinhard
The Glass Masterworks
"If it scratches, I etch it!"

Don Reinhard The Glass Masterworks "If it scratches, I etch it!"

(post #115913, reply #2 of 26)

Just my two cents.  If you put in the return air in the partition stud bays.  Do yourself a favor and pound a piece of scrap above the return air cutout and seal it up.  Around here the HVAC guys don't block off the return airs and the furnace draws air down through the walls from the attic or along wire runs from the outside walls.


Personally had frozen pipes in the middle of full depth basement because of unblocked return air vents that found the outside air.  I blocked all of mine and sealed them with caulk.  Of course, if you have a two story or story 1/2 you have to be careful you aren't blocking a return from above they intend to be in the same bay.


 

(post #115913, reply #3 of 26)

More on using stud bays for a return air path:


You will need at least two full-width stud bays (14" wide by 3-1/2" deep) to return the 400 CFM. I would locate them high on wall of the bedroom side, and low on the wall of the hallway side. This offsetting will help reduce "cross-talk" that would result from straight through grilles.


Another possibility would be to run a "jumper duct" connected to a ceiling grille in the bedroom to a ceiling grille in the hallway. For 400 CFM, the duct (R-8 insulated flex duct) should be at least 12" inside diameter. This approach assumes that there is an attic above the ceiling, and that you have access to it.


In either of these methods, the air gets returned to the hallway, from where it floats to the central return. Providing an unrestricted return pathway like this will greatly enhance the comfort of the master bedroom whenever the door is closed.

(post #115913, reply #4 of 26)

Screwball floor plan.  BR doesn't share a wall w/ rooom where return duct is located. BR is off kitchen. w/ a wall only wide enough to house a 3'0" X 6'8" door. 


To me, best solution is to connect to return air plenum in attic.  BUT... the "Jumper Duct" idea has some merit.  The big return duct opening is but 6 ft from the BR door in the kitchen.


Is there any reason why (other than money) we should not relocate the inlet air ducts to the outer wall of the BR & use the current inlet duct openings in the ceiling for return air connected to the return air plenum in the attic?  Seems to me it would gurantee return air out of the BR.  Right now, there is almost no flow out of the room w/ the door closed.


Don


Don Reinhard
The Glass Masterworks
"If it scratches, I etch it!"
Don Reinhard The Glass Masterworks "If it scratches, I etch it!"

(post #115913, reply #5 of 26)

>>Is there any reason why (other than money) we should not relocate the inlet air ducts to the outer wall of the BR & use the current inlet duct openings in the ceiling for return air connected to the return air plenum in the attic?  Seems to me it would gurantee return air out of the BR.  Right now, there is almost no flow out of the room w/ the door closed.


Sounds like the HVAC system is located in the attic.


Your above plan is good. The more direct path fpr either supply or return ait is better. If you use Flex duct ask for a cut sheet when you buy it. Heep the runs as short as you can, stretch it tight, and support it properly.


While you are up there look at any other flex duct they used. Compare the installation to the instructions you got with your new stuff, and correct the old install as best you can. My guess is that the old return air, if run in flex, isn't pulling much more than 50-60% of the design requirements for the system. Fix it and you will improve the comfort level throughout the house and save them some $$ on heating/cooling cost.

(post #115913, reply #10 of 26)

Personally ... IMO ... generally say NO to flex duct. Use it ONLY where you need to. Your tips are good, though if you use it. But if you can stretch it tight like you say, you likely have room for hard duct ... so you should use it. People IMO overuse flex because it is a little easier to install. But the air flow suffers regardless if it is tight or not.

There ain't NO free lunch. Not no how, not no where!

(post #115913, reply #21 of 26)

Amen !


Your preaching to the choir here. I've ranted about the stuff for years, but that doesn't changed the fact that is used a lot.


Since I can't change that, I try to get the information out that there is at least a correct way to install the stuff, as well as correct uses for it.

(post #115913, reply #8 of 26)

First, it's important to realize that return air is only a means of relieving pressure that is created by the supply ducts.


Basic physics: air only flows from an area of greater pressure to an area of lesser pressure. The supplies drive the system. So, any unrestricted opening from the bedroom (even the door, when it's open) will allow the bedroom supplies to do their job. They do their job best when they can throw the air out in a pattern that adequately mixes and entrains room air. Any resistance created by the return air path, e.g., lengthy ducts with a number of bends, will use up the air handler's capacity that would be better used to keep up good velocity and airflow on the supplies.


So, to get to your question: make the returns as large and short as you can. This  means not using returns located any farther from the air handler than absolutely necessary. Size of returns, in general, should be at least 50% larger than the supplies.


Also, don't fret about the location of the returns in relation to the supplies, even though many HVAC contractors routinely try to locate them opposite the supplies. To understand this, look up at the duct system at the next Wal-Mart or other big box store you enter. You will see the exposed duct system with supplies distributed throughout the space, but with only one large return at the point directly below the air handler on the roof. There will be supplies very close to it and supplies far from it, but the system works, summer and winter. It is designed to use almost all the air handler's capacity for the supplies--nothing is wasted on return duct runs.


 


 


 

(post #115913, reply #13 of 26)

I don't think the system at your Walmarche' is the same animal as in your house or in the office. RA grilles too close to SA registers can be a significant problem and you shouldn't treat their proximity lightly. I think the reason it works at Walmarche' is due to the MUCH higher SA velocities ... mainly due to the high ceiling. The high velocity prevents short circuiting that can be very common in low ceiling applications where velocity at the register is relatively MUCH lower than at the local big box store. The high velocity is necessary to get similar mixing in the volume of space below at the big box.

There ain't NO free lunch. Not no how, not no where!

(post #115913, reply #22 of 26)

If you can get hold of the Air Conditioning Contractor's of America's (ACCA) Manuals D and T, which are the industry standards for designing air distribution systems, you will find that return air location is the least of the problems in proper air distribution.


Sizing of return air is big (bad pun), but location is not.


The reason is that the only function of R/A is to relieve pressure created by the supplies. If the Wal-Mart example seems non-applicable, then think of hotel rooms you've stayed in:  the through-the-wall unit is located at the far end of the room (20 to 25 feet long) under the only window, and the return is located directly below the supply. The system works just fine (except for noise), and the velocity is below 500 feet/minute, which is the standard for house systems. (Actually, the Wal-Mart systems also operate at static pressures below 1" water column, similar to house systems, which operate at 0.5" up to 1")


If you have an unrestricted R/A path in any room, no matter where it's located, the really important things all have to do with the supplies--high or low? ceiling, wall or floor? velocity? size of branch duct? throw pattern of register? temperature of delivered air?

(post #115913, reply #23 of 26)

Not disagreeing, but I have seen short circuiting and the comfort problems it can cause. If the air doesn't mix in the room, it doesn't get a chance to do its thing.


The hotel unit vent .... at least the supply is pointed up and away and the return is down low. I didn't say proximity was the only consideration ... proximity AND configuration. Drapes aside, it's not a bad configuration ... all things considered.


I'm thinking the supply on the Big Box is higher velocity than your run of the mill residence or office ... maybe even twice as high at the discharge from the register. While my residency may run ... what is it ... say a few hundred feet a minute (I forget the discharge velocity) ... I'll bet Wallmarche' may run 600+ They can have some serious wind coming from those things. This in spite of relatively low static pressures.


IMO the return isn't as passive as you make it out. On residential style systems, the fan is sucking the return as much as it is blowing the supply. There is a negative pressure at the return, not a positive. On larger systems, they often have a separate return fan and they ventilate ... so you have some other dynamics going on there ... but still a negative on the return. This is a VERY abbreviated discussion of all this ... it gets more complex fast ... I'm leaving a lot out for simplicity.

There ain't NO free lunch. Not no how, not no where!

(post #115913, reply #24 of 26)

I'll bet Wallmarche' may run 600+ They can have some serious wind coming from those things.


I don't know about  Wallmarche', but our system here can put out from 200 to 1600 cfm per vav zone at a static pressure of around 1" wc. Actual cfm for each zone is predicated on supply duct size and staying within the static pressure limits. We use a plenum return that is the open area above the drop ceilings and there is no static pressure on that side of the system.


Commercial buildings are also reguire to have a minimum of 10% outside or make up air mixed into the supply side. With that amount of outside air coming in you end up with a pressurized building so they add relief dampers or exhaust dampers to keep them from over pressureizing the building. (it would be hard to keep the doors closed if the inside pressure was a great deal higher the ambeiant outside pressure)


System design for residential should be based on load requirements for each space in the building. An equal amount of return air should be included for each space, but is seldom included in the specifications because it cost as much to run return ducts as it does supplies. Short cycling of the air flow loop is seldom a problem because there likely isn't enough return ducting to begin with. What is called short cycling is more likely a form of half cycling because there is not enough return air and the air mixing is limited to some percentage of what it should be.


The historical solution is to just oversize the system. That covers all the design flaws from not running load calculations to proper supply and return ducts, and poor installation.


Heck, you only need to shuck and jive the customer for a year or so before your off the hook, right?


:~)

(post #115913, reply #26 of 26)

BTW    I was referring to air velocity, not volume. 600+ fpm.


You are right on re: the RA issue, usually sorely inadequate. I didn't mean to sound like short circuiting is commonplace or always a problem. I've seen it happen in commercial applications. But it can be easy to do if you aren't careful about the layouts of ducts, etc.

There ain't NO free lunch. Not no how, not no where!

(post #115913, reply #25 of 26)

Airflow at the the supply registers is what drives any forced-air system. Restricted returns affect the performance by limiting the flow of the supplies; if the return is restricted in a bedroom with the door closed, for example, the supplies will soon pressurize the room, and the air will find an easier path somewhere else in the system. The result will be an uncomfortable bedroom, partly due to low air volume and partly due to poor air mixing.


You can locate the return anywhere in the room you'd like if the return is unrestricted. But you must have supplies with the right registers to throw the air and entrain the air--supplies will entrain, or move and mix, air volume 10-20 times more than the actual register outlet if they have proper velocity and the register has the proper throw pattern.


If room airflow appears to be "short-circuiting", it is far more likely that the return is restricted, or that the supply is weak. Easy to test with a flow hood, if you have access to one: test the flow at the supply register first with the door closed, then with it open. If the readings are the same, the return is OK, and the supply is weak. If no flow hood is available, use a 35-gallon trash bag to cover the supply register; squash it down over the register, let it go and check the time it takes to fill. You can even calculate the actual CFM with this method if you want. (35 gallons = 4.7 cubic feet.)


Strictly speaking, you can never "suck" air. It is basic physics that air only moves from an area of greater pressure to an area of lower pressure. When you "suck" in your breath, you are simply lowering the pressure in you lungs so that air pushes into your lungs from an area of greater pressure, i.e., the atmosphere.


Again, I can't do better than to recommend ACCA Manual T for a very thorough and clear explanation of how all this applies to a forced-air system. It is full of excellent diagrams and easy-to-understand text.

(post #115913, reply #6 of 26)

Is there enough room on the plenum to add flex ducts to it and run them to the individual room ceilings for a direct return? 


Any $$$ to even entertain having an HVAC company come out to take a look at the entire house/system? 

(post #115913, reply #7 of 26)

Hi Don,


two inlet ducts feeding about 400CFM of conditioned air


I'm going to approach this completely out of the box which is normal for me.....


How big or how poorly insulated is this room that you actually need that much flow?


My bedroom is 14'x20' with a cathedral ceiling which peaks in the middle at 14'. It's on the outside north west corner of the house in central NC and has two 38"x64" windows and only has a single 4"x10" inlet. Insulation is for the most part typical...maybe a grade up......but it's sealed tighter 'n' a tick. I would have to go back and review my plans but the airflow is gentle enough that you can only feel air movement within 16" of the register. The door is 36" wide with 1" clearance under it. The room has never been more than 1 degree difference than the center of the house and that's with the door closed and at the extremes....100 d. days of summer and teen d. nights of winter. By the way that opening under the door serves the single register routing it's way from the master bath also.


My point is why not look at some means of better thermal efficiency of the room rather than moving more air? Start in that room and work your way through the house to provide for greater long term savings.


How high is the ceiling? If 9' could you put 2" of styrofoam on the bottom side with another layer of drywall? How about the same on the outer walls but use 1/2" or 1" instead. How about air sealing all the outlets in the room. And what about the windows......are they double pane "&" low-e?


Yes this will cost a bit up front but energy costs are going to continue to rise so why not spend money up front to reduce energy costs forever....electricity is not deductable and any money spent on energy is post taxed so any money saved long term is pocket money not to mention the room will actually be more comfortable and as an added benefit you'll save some of Al Gore's carbon footprints to boot....goodness knows he's already got enough.


Pedro the Mule - Always kicking my way out of the box

  

"What you leave behind is not what is engraved in stone monuments, but what is woven into the lives of others."

(post #115913, reply #11 of 26)

His windows and orientation may be much less favorable than yours. Are your windows facing west? Are they tinted/mirrored glass?


If he has the same windows with less favorable conditions, his cooling load could easily be double yours. Point is ... you can't translate your situation to his w/out knowing more particulars.

There ain't NO free lunch. Not no how, not no where!

(post #115913, reply #15 of 26)

Hi Clewless1,


His windows and orientation may be much less favorable than yours. Are your windows facing west? Are they tinted/mirrored glass?


If he has the same windows with less favorable conditions, his cooling load could easily be double yours. Point is ... you can't translate your situation to his w/out knowing more particulars.


I agree I left out a few details as did the original poster.......mine face west and as I recommended to the poster for him to consider low-e glazing.....it was based on using low-e myself.


In general terms low-e will help in both heating & cooling in his location with the right deflection percentage.


My original "INTENT" wasn't to provide a completed technical solution.....because like you I didn't have enough information.....the "INTENT" was to get him to consider other things that he may not have looked at yet. He seemed to be completely focused on return air flow as his only solution, I simply wanted to make certain he had looked at the problem that affects the vast majority of home owners....poor hidden construction conditions. A million cfm won't heat or cool the blue ridge parkway. His system should already have sufficient flow unless he's far exceeding a 9520 cu. ft. bedroom......as such that's the size of my great room with 3 - 42x64 low-e glazed windows facing west and two supply vents with about 1/3 the flow he has and it's awesomely comfortable.


According to your feedback, I recon I should have asked specific engineering questions rather than making a generalize self surprised statement; however, I am still surprised that his stifled conditions aren't sufficient even without all of the specifics.


To the original poster. Rather than encourage you to consider other options at this point; will you please post specific details of complete room dimensions, glazing dimensions, all insulation details including blower door tests, any factors that may affect operations including special heat reflective paints, floor coverings, drapery materials and sizes, number of fabric covered items and any other items needed to be able to engineer a solution. At that point Clewless1 will provide you with a detailed solution.


Pedro the Mule - Misconstruing the obvious

  

"What you leave behind is not what is engraved in stone monuments, but what is woven into the lives of others."

(post #115913, reply #16 of 26)

Just goes to prove that the highest dollar HVAC system won't work worth a d a m n in a home built with a low end envelope.


:)

(post #115913, reply #17 of 26)

Good points. I was just trying to round out the conversation.


You forgot landscaping. If your windows face west w/ good landscaping (i.e. shading) the exposure means much less. If he is asking us for help, my guess is he doesn't have a blower door test to share w/ us.

There ain't NO free lunch. Not no how, not no where!

(post #115913, reply #18 of 26)

Hi Clewless1,

Good points. I was just trying to round out the conversation.


You forgot landscaping. If your windows face west w/ good landscaping (i.e. shading) the exposure means much less. If he is asking us for help, my guess is he doesn't have a blower door test to share w/ us.


No Problem, I simply live outside the box with 98.3417% of everything I come across in life....not only does it keep things interesting....it actually gets people to think beyond the problem on the surface.


My personal favorite line in any movie ever made is from Patch Adams..... Arthur Mendelson: "You're focusing on the problem. If you focus on the problem, you can't see the solution. Never focus on the problem!"


I will forever live this way....it's my foundation...my second favorite line happens to be in the same movie by the same person......Arthur Mendelson: "See what no one else sees. See what everyone chooses not to see... out of fear, conformity or laziness. See the whole world anew each day!"


I'm constantly seeking a new perspective....comes from complete disgust of seeing us continually repeat history.....if the same problem returns.....it's not an indication that it was resolved correctly or incorrectly the first time. Ex. the caulk that was last used on a product may have only lasted 10 yrs. whereas another caulk would last 30...well if the 30 year caulk had not yet been invented the rist time around, then the 10 wasn't the wrong decision, in another situation what if you had reason to take it apart in 10 years, then you'd only want the 10 year caulk.....and probably another 100+ reasons to use one or the other.


I like your observation that trees could make a difference....could be they've always wanted a few nice trees near the window......since it takes trees so long to grow, it could be worth the money planting larger trees if they want instant shade "and" mature landscaping. Personally I'd cheap out and plant young trees and close metallic blinds until they grew up.


I simply hope he'll look beyond the problem to find his solution. I tend to answer general problems with general solutions. Others often ask for more detail prior to offering a suggestion. When someone is general, I hope my general answer is to invoke further thought.....not an immediate dash to the diy supercenter for supplies.


Best wishes to you Clewless1 and may everyone at least step out of the box every chance you get.....just be careful what you step in!


Pedro the Mule - Oooo...what's that smell.....Taco Bell....

  

"What you leave behind is not what is engraved in stone monuments, but what is woven into the lives of others."

(post #115913, reply #19 of 26)

I read a book once that taught me to identify problems before suggesting solutions (many people get them mixed up). Also most 'problems' are really only symptoms caused by problems. If you focus on the symptom you may never identify the problems that cause them. Identifying the problem is often the difficult task because you have to separate it from symptoms and apparent solutions.


Interesting stuff.


Back to windows ... what is your SHGC of the windows in your room? Beyond low-e, the SHGC will be probably the biggest controller of solar gain. Two low-e windows will perform dramatically different if the one has a SHGC of 0.68 and the  other 0.20.

There ain't NO free lunch. Not no how, not no where!

(post #115913, reply #20 of 26)

Hi Clewless1,


Back to windows ... what is your SHGC of the windows in your room? Beyond low-e, the SHGC will be probably the biggest controller of solar gain. Two low-e windows will perform dramatically different if the one has a SHGC of 0.68 and the  other 0.20.


I do recall having researched that.....ummmm about 5 years ago now.....so it's buried pretty deap.....but the best I recall it was 0.32 on the west windows with mature deciduous trees........something like 0.55 on the southern facing sunroom with deep overhangs and 0.40 on the eastern side with a deep porch.


I clearly remember there was a method to the madness and a great deal of time was spent in considering the windows since they are the week link in the chain. You don't want windows because they are inefficient but you do want them to keep from running lights 24/7 and of course the cross breezes and views that we have.


I've learned so much more since then....would have been some pretty significant changes in solar and insulation design if I were starting over. Of course it's hard to complain when comparing to most other construction.....4600 sq. ft. with 5 servers running 24/7, 70 in winter, 74 in summer.....util bills are under $200 /month and the house appears as traditional as any. Neighbor has a typical style of house at 3500 sq. ft. and none of the computer equipment running like mine......he rarely has a util bill under $400...normally around $600. Yes we have a few well planned items such as windows and radiant barriers but the biggest difference in the two homes is that his style of heat exchanger is nothing more than the air that infiltrates every crack and crevice the contractor could get away with. Its amazing....my Manual JCalcs call for 2.89 tons.....ended up with 3.5 due to equipment size availability......my neighbors should be roughly 2.8 tons built right but he's running 7.0 tons and struggles to cool it in July/August. His equipment runs great but it's hard to cool the outdoors.


Pedro the Mule - No sunburns through my windows

  

"What you leave behind is not what is engraved in stone monuments, but what is woven into the lives of others."

(post #115913, reply #9 of 26)

Right ... generally an equal amount of return air (RA) area to supply area. I've always thought that if the static pressure on the RA is low, you get away with a little less area, but my expertise isn't there. Ultimately, the RA plenum is equal the SA main duct.


Returns on interior sides of the room and supplies on exterior is fairly standard. Modern well built houses can deviate from that a little, but the concept is the same.

There ain't NO free lunch. Not no how, not no where!

(post #115913, reply #12 of 26)

I'm going to take another approach based on what some other guys said. We/I need to know sqft of the room. Orientation of the room in the house (exposure to the exterior). Window size and orientation. Is the room/ceiling otherwise well insulated?


I fully agree w/ the other poster's thought ... think reducing the load first ... if it is possible. If that load is from windows (which it could easily be), you could consider some options (e.g. more efficient glass, same frames; shading the glass on the exterior).


You may still want/need to reconfigure your SA/RA situation, but energy wise, you can improve comfort w/out increasing energy if you control the load. Your solution will likely at best break even energy wise (or increase it) while increasing your comfort.

There ain't NO free lunch. Not no how, not no where!

(post #115913, reply #14 of 26)

The simplest answer to your question is yes on all fronts.


Don't know how you arrived at the 400 cfm number, nor does it really matter. Adding an equal number of like sized return air ducts will help.


Will it fix the problem? Well, there is no way to truly answer that without performing a load calculation on the space in question. Adding an equal amount of return may do the trick. Nothing lost in trying.


Make sure that any added ducting is sealed and all duct plenum connections are sealed. Flex is an easy way out, but the number of joints is lower- especially if the install isn't a atraight line.


You are describing a residential central return system. That is a system the HVAC contractors are stuck installing to meet the price builders are willing to pay. Unfortunately, they do not work so well, and the person making the mortgage payment is stuck with the problems while the builder laughs all the way to the bank.