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Old 12-30-2008, 08:35 AM   #1
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Return Air Vents


All-

We recently moved into a home that had been flipped... some of the work was done OK, the rest not so well. I have spent the past couple of months reworking, repairing and finishing the work the flippers did.

One of the issues has to do with our return air vents. We have central air conditioning and forced air heat in a two story plus basement (its heated and air conditioned) row house. There are only two return air vents in the whole home (not uncommon from what I hear in older row homes where it is difficult to run ductwork), one return is on the main level of the home and the other is on the second floor. The AC/furnace are in the basement.

The problem we have, perhaps not a problem at all, that's why I am asking here, is that the return air vent on the main level of the house is very loud and sucks down a LOT of air while the vent on the second level of the house is barely able to hold a piece of paper up with the suction.

Any thoughts as to what might be causing the trouble? Is this a problem? Any suggestions for how to improve it?

Thanks!

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Old 12-30-2008, 09:24 AM   #2
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I've learned that air likes to follow the path of least resistance. Apparently with your current set-up it is easier for the the bulk of the return air to travel through the down stairs return grille.

It could be caused by many reasons including larger duct work (or return air passage) to the downstairs grill, proximity to the furnace (it could be much closer), less restrictions (less sharp turns to get to the grill), closed dampers to the upstairs grille, etc...

A simple solution which is usually not so simple is to make it easier for the air to get to the upstairs grille. Some may say that making it harder for the air to get to the down stairs grille may accomplish the same thing but I would not reccomend this as it would likely bump your static pressure.

Also if you have even temperatures throughout the home, a solution to the noise may be as simple as reducing the blower speed (less air flow = less noise).

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Old 12-30-2008, 10:26 AM   #3
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Thanks!

We live in a 2 story (plus basement) row house in Washington, DC. The AC/furnace were of course retrofitted, as is necessary around here. With all brick construction there is not a lot of room for duct work.

I think you're right, the downstairs duct is MUCH closer to the furnace, in fact it is immediately above the furnace. That being said, it isn't a very large vent, maybe just 3"x 6" or so. The upstairs return is larger, I think a 6" round duct.

I am actually less concerned about the noise and more concerned about how well the unit is functioning. I know that return air is important and I want to make sure my system is not deprived (I think it must be!). The house isn't very large, maybe 1,200-1,4000 sq. ft. of living space.

The heat throughout the house is pretty even, but the AC was not so great. Although I hadn't attempted to balance the system (using the louvers on the grilles) and we just insulated the attic. Airflow in some rooms is definitely much stronger than in others.

Any thoughts?
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Old 12-30-2008, 03:36 PM   #4
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Quote:
Originally Posted by stubits View Post
Thanks!

We live in a 2 story (plus basement) row house in Washington, DC. The AC/furnace were of course retrofitted, as is necessary around here. With all brick construction there is not a lot of room for duct work.

I think you're right, the downstairs duct is MUCH closer to the furnace, in fact it is immediately above the furnace. That being said, it isn't a very large vent, maybe just 3"x 6" or so. The upstairs return is larger, I think a 6" round duct.

I am actually less concerned about the noise and more concerned about how well the unit is functioning. I know that return air is important and I want to make sure my system is not deprived (I think it must be!). The house isn't very large, maybe 1,200-1,4000 sq. ft. of living space.

The heat throughout the house is pretty even, but the AC was not so great. Although I hadn't attempted to balance the system (using the louvers on the grilles) and we just insulated the attic. Airflow in some rooms is definitely much stronger than in others.

Any thoughts?
What are the model numbers of your furnace, coil, and outdoor condenser? How old are the units?
Posting Pics of how the duct work connects with the furnace may shed some light.
Once you post you model numbers also post your exact return grill sizes. Someone will likely be able to tell if you would benefit from making them larger.
Ultimately, I think you will need to check the static pressure (or have it checked) to gain more insight to whether or not it is functioning properly.

Lot's of other things can be checked also......

How far are you willing to jump into this is a question you will have to ask yourself.

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Old 12-30-2008, 04:07 PM   #5
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Quote:
Originally Posted by stubits View Post
That being said, it isn't a very large vent, maybe just 3"x 6" or so. The upstairs return is larger, I think a 6" round duct.
Yea I agree with Key, we need more info on the unit and the ducts. We need info on the unit to see what the CFM or air flow rating is. The return ducts seem to small for what I'm used to.

Your saying you only have a 3"x6" grill and a 6" round grill or duct for your only returns. The CFM rating on that is very low. You should have more Square inches of return vents then you do of supply vents. Which will slow the air flow and keep it quiet.

In your case you probley should do something to increase the return grills, but ideally you want to get the upstairs return to bring in more air then what it is. If you only inlarge the lower vent your only going to throw off the balance of upstairs to downstairs, but will help increase the air flow threw your system with will reduce static pressures, lower energy use of the blower and increase airflow threw the house.

Give us some more info and someone should be able to help you.
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Old 12-30-2008, 05:44 PM   #6
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the sheet of paper trick was a good test how about cutting a piece of card board the size of the 1st floor return and cover 50% of it then check the upstairs for the audible air noise,and it will at least prove the returns are common.cooling side of the system is more effected by a minimal return so keep that cardboard handy for the summer....the spaces basically balloon if there is no returns....try it with cold night and check upstairs during a long heating cycle.
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Old 12-31-2008, 11:10 AM   #7
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Thanks everybody!

I will check out the specific details of my system tonight and will attempt to take some photos.

I can however offer some additional information.

I don't think the system was designed particularly well, although I think much of it was limited by the house itself. There aren't a lot of spaces to run ductwork. As far as I can ascertain, the return ductwork runs straight up from the basement to the attic. The ductwork is rectangular, not round. On the main floor of the house(where the majority of the return air is being pulled from), the return air grill is connected to the smaller side of the rectangular duct. The ductwork continues up to the attic where is connects to a 6" flexible round duct, maybe 10' in length which then connects to the upstairs grill.

We have significantly more square inches of supply vents than we do return vents, for sure.

Given the VERY simple nature of the design, and how little air is being returned from the upstairs, I am not sure how to add more return air vents. I could do so easily in the upper level, given access to the attic, but on the lower level, all I can imagine doing is expanding the current vent and making is taller, but not wider, as the supply duct that goes from the basement to the attic is immediately in front of the return duct in the heating/cooling chase.

The heat has definitely performed MUCH better than the cooling did in the summer.

I will take some photos and get some additional info tonight, but would appreciate any help at all!

Thanks!
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Old 12-31-2008, 01:07 PM   #8
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Quote:
Originally Posted by stubits View Post
We have significantly more square inches of supply vents than we do return vents, for sure.
That's going to be one big problem right there. Need to see what you can to try and even things out a bit. If you find out the CFM rating on the blower in the heater unit, you can find out how big of ducts and the square inches the intake grill should be to some extent.
After you get them up to proper size you'll lower static pressure, increase flow and use less electricity. The fan is going to waste alot of electricity trying to suck threw air threw a pin hole. You will also end up with cooling problems as the coil will freeze up, due to the lack of airflow over the coil.
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Old 12-31-2008, 03:00 PM   #9
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Ok, I need to get down to the basement utility closet to figure out what equipment I have, but I thought I'd share some photos and measurements from the return ducts. It is definitely funky!

As I indicated earlier, we have only one return air "trunk" in the house, it runs straight from the basement to the attic where it branches off via flexible duct to the second floor return air vent. As you can see in the photos, the main level return is effectively a large hole in the side of the trunk (no wonder there isn't much umph left by the time it gets to the attic!

Here are the measurements:

Main Level Return(Bottom Photos):
Ductwork (trunk): 8x12
Opening: 7x10 (area beneath the metal flap) or 7x25(total opening)
Grille: 10x26

Second Level Return(Top Photo):
Round duct: 8" diameter
Grille: 16x22

I'll post photos and more info on the equipment later. Again, the house is perhaps 1,400 square feet of air conditioned/heated space. Also, the basement, where all the equipment is housed, is finished and is air conditioned and heated, but there are no return vents there.

Any thoughts?
Attached Thumbnails
Return Air Vents-pc310164.jpg   Return Air Vents-pc310163.jpg   Return Air Vents-pc310162.jpg  
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Old 12-31-2008, 03:07 PM   #10
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Also, I just tried the suggestion of covering half the the main level return air vent with cardboard. Indeed, it increased the amount of return air being sucked in by the upstairs return air vent.
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Old 12-31-2008, 04:19 PM   #11
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An 8x12 return line flows somewhere around 400 CFM at ideal conditions. And that's on the high side. I still don't know what the CFM is of your blower. With that you can figure out the size of trunk line you should have under ideal conditions.
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Old 01-01-2009, 12:05 PM   #12
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OK, I think I have the necessary details...

The furnace/air handler is model number 80PS10EDR01 which seems to be a Rheem 80% Upflow/Horizontal Gas Furnace. From what I can ascertain it seems like it is 100,000 BTU and 5 ton. I think the manual and other info about the unit can be found here: http://www.allreds.net/WS2/item_detail.html?item_num=RHE+80PS10EDR01+++++

The indoor component of our air conditioner is model number RCFA-HM4821AC which seems to be Rheem 4 TON 21", 13 SEER cased coil. More info about this component can be found here: http://www.allreds.net/WS2/item_deta...FA-HM4821AC+++

Finally, the outdoor component of our air conditioner is model number 13AJA36A01 which seems to be a Rheem Corsaire 13 SEER, 3 ton condenser. More info on this unit can be found at http://www.allreds.net/WS2/item_deta...AJA36A01++++++

So, I am certain of all the model numbers, but the rest of the information is based on a simple google search, so I can easily be wrong.

I am guessing, based on the little bit I've learned here that a single 8x12 return trunk is very much insuficient. At the unit itself, the return starts out as a 8x20.

So, with all this information, any suggestions? Can I add an additional return trunk? There seems to be 2 or three supply trunks, which makes a lot more sense. One thought, if multiple return trunks are possible would be to use the current trunk the return air from the second floor of the home and add a trunk to handle the first floor of the home and perhaps a third to return air from the finished and lived in basement?

Thanks!
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Old 01-01-2009, 02:05 PM   #13
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that 2nd floor return is mounted in the ceiling with the furnace in the basement?you say it is 8" duct but if it isn't connected to the main level return it is useless.the 2nd floor needs to be mounted directly above the main level return on the wall,and the 8" dropped down onto the top of the main level return opening.does the 8" duct loop up into the attic and then drop down into the house someplace?that main level rectangular return does that go down into the basement and connects to the side of the furnace..if it does that is the main return the 2nd level is doing nothing doubt it is connected with the original failed that sheet of paper test.if you could open the wall just above that main level return and drop a flex duct down onto the top of the duct in there you would pull a true 2nd floor return with a little restriction on the main level return.

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Old 01-01-2009, 02:09 PM   #14
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It is connected to the main level return.

Here is how it is setup.

The return air trunk goes from the furnace in the basement straight up to the attic where is terminates. As you can see from the photo on the main level of the house, they've cut a large hole, 7"x25" into the trunk itself and attached a grill. In the attic, where the trunk terminates, they've connected a piece of 8" round insulated flexible duct which then connects to a 16"x22" sheetmetal box with grille.

The upstairs return air definitely works, just not well. It will loosely hold a piece of paper. When I covered over half of the main level return air vent with cardboard as suggested earlier, the pull from the upstairs vent was stronger.
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Old 01-01-2009, 02:30 PM   #15
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your real close then with the trunk up into the attic,and restricting the main level return.see the size of that attic return......cut a sheet of card board the size of the main level opening that the grill goes into.then on the sheet of cardboard cut the same size that the attic return is into that sheet going into the main level opening,so your main and 2nd floor returns are the same sizes....see how the air noise is with that.go for the SQUARE INCHES(width X lenght) when doing the main level cutting since it is more retangular and the attic is closer to a square.

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