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Old 01-01-2009, 02:32 PM   #16
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Return Air Vents


Biggles-

Really great idea. Will definitely try. I am concerned however because it seems like I just don't have enough return air at all for the system?

What do you think?
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Old 01-01-2009, 08:02 PM   #17
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A 5 ton is way over kill for you house.
So according to your manual for a 5 ton unit depending on your DIP settings you should be pushing around 1600 cfm. Now with your return sizing your static pressure is going to be really high and your not going to be pushing that now. But you probleyt should have around a 18-20" or equilant size of return duct. Ofcourse you can split this up into several smaller ones. Make sure you have enough grill square feet to cover 1600cfm.
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Old 01-01-2009, 08:12 PM   #18
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heres a calculator you can use to get some idea of what size ducts you need. http://efficientcomfort.net/jsp/ResDuct_Web.jsp

For a return to flow freely and quietly you should have the air flowing threw the ducts around 600-700 feet per min. 600 is better tho. The slower the air is going the quieter it is. So if you figure 650fpm. and your blower puts out around 1600 cfm. Then you need around 21" round duct. (that puts you at 665fpm and 22" puts you at 606fpm)
Now for a retangle duct. 18x20" puts you at 640fpm at 1600 cfm.

Now longer lenghts 90 degress and flex duct vs. metal duct will make you flow less air so you would even need to go bigger then that.
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Old 01-01-2009, 08:37 PM   #19
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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.
The upstairs vent sounds fine with losely holding it. It shouldnt be sucking so hard, as that will make to much noise. You can figure out from the calculator how much cfm should be going threw that vent at that size with the air traveling at 650fpm.

You probley need to try and block off the lower vent. Cover it patch the hole and such so that truck is only going for upstairs, then try and add another duct for a new down stairs vent. might want to increase the upstairs grill and flex duct size to fit the maximum that the square trunk going to upstairs can hold.
8x12" is equal to an 11" round metal duct. both carry 400cfm at 600fpm. So probley a 12" flex duct would do nicely (since 11" is an odd ball size round up)
Increase your grill size. to maybe a 20x20" or a bit smaller for your 12" duct size. Check for probley grill size for 400cfm and 12" duct.
This will take car of the grill and ducts for upstairs. Then find a good place to run a new duct to downstairs and a large grill. You probley need a 18" duct downstairs to cover the 1200cfm you still need. a 20x30 or 24x30" grill size for downstairs is needed to cover an 18" or 20" duct and 1200cfm.

I currently have a
1) 20x30" grill with a 16" metal duct,
1) 6" fresh air duct from outside
3) 12x6" grills with 7" ducts from each of the 3 bedrooms
all connected to my return plenum fro my 3 ton 1200cfm unit.

This is in a 1600 Sf. house near los angeles california.
Hopse this helps shead some light and give you some ideas.
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Old 01-01-2009, 09:03 PM   #20
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This is fantastic! Thanks so much, way above and beyond any call of duty.

A few questions, clarifications, etc...

1) Definitely oversized, I realized that when I was pulling the info this morning. I guess it is slightly better oversized than undersized, right? And we are planning 150 sq. ft addition on the house, so at least it will be able to accommodate that.

2) I completely agree with the idea of closing off the vent to the main floor and using the current trunk to pull from the upstairs. Just a thought, but instead of having one large return grille on the upper level, could I perhaps have 2 or 3 return air vents on the upper level? It might make sense to get air moving, right? We have one room towards the back of the house that just gets hit big time by the sun in the summer, and it might help to suck some air from there, no? Or do you think it is better to just keep things simple?

3)I understand the need for a large trunk for the main level of the home. I guess I am confused because my floor joists run 16" on center, I believe, so how would, or do people, get 18" ducts through that type of space?

4) MOST IMPORTANT!! Ok, how on earth do I go about "adding" the additional trunk to the return air plenum. I know this is the million dollar question and might not be so easy to explain, but if you could even so much as point me in the right direction, it would be great!

5) Do you see any reason to add return air vents to the basement? It is heated/air conditioned and my mother-in-law lives down there (now you can see why it took me so long to get down there and pull the model numbers).

6) So, at present I am really undersized in my return air, right? Is it possible I've done any damage to the various equipment? We ran thr AC for a full season and now the heat. How urgent is it that I make these repairs? Also, will correcting this issue result in noticeable increase in performance, ie, will the house heat/cool faster, more evenly, better, or will it reduce costs at all? I guess, what exactly are the detriments to not having enough return air?
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Old 01-01-2009, 09:41 PM   #21
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1) Definitely oversized, I realized that when I was pulling the info this morning. I guess it is slightly better oversized than undersized, right? And we are planning 150 sq. ft addition on the house, so at least it will be able to accommodate that.
Yes to some extent. There is problems with it being to big too. Short runs times. The unit kicks on and off to quickly, due to it meeting the demands fast. Now while that sounds good, if the unit doesnt run long enought its harder to circulate air threw the house properly, doesnt filter very long, doesnt have enough time to dehumidify the house, which leads to mositure issues. Also it just plans sucks up to much energy. It's better to have a small unit that doesnt use much electricy and can run for loneger periods of time to circulate and filter the air. The more it circulates the air the more even the house temps are going to be threwout the house, not just in one or 2 rooms.

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2) I completely agree with the idea of closing off the vent to the main floor and using the current trunk to pull from the upstairs. Just a thought, but instead of having one large return grille on the upper level, could I perhaps have 2 or 3 return air vents on the upper level? It might make sense to get air moving, right? We have one room towards the back of the house that just gets hit big time by the sun in the summer, and it might help to suck some air from there, no? Or do you think it is better to just keep things simple?
Sure put a return in every room if you choose. That would help move the air around, and it really helps when room doors are closed. If a room door is closed the air can go into the room but has no where to come out, so it gets lost threw the windows, loseing conditioned air. Also the vent in the bed room just might not blow out much air if the door is closed and the windows are sealed good. It will build pressure in the room and cause the air from the vent to go into another room. With a return in the rooms this helps this if the doors are closed. Plus it help circulate better. Just split the 12" into a few 7" or 8" ducts going to different rooms and such. What ever fits for 400 cfm that the main truck going up there can handle.

Quote:
Originally Posted by stubits View Post
3)I understand the need for a large trunk for the main level of the home. I guess I am confused because my floor joists run 16" on center, I believe, so how would, or do people, get 18" ducts through that type of space?
Well it different for different people. I'm in a single story and my unit is in my attic. So I can fit the duct, but I have to cut and frame out some joists to fit my grill or box which is 20x30 and didnt fit between joists. You can also split this up if need be into smaller ducts. For example. My plentum wasnt tall enough to fit a 16" duct in the side or end of it. So I pulled 2 12" ducts one from the side and one from the end of it and Wyed them together (with a 12"x12"x16" wye) right next to the plentum then ran 16" the rest of the way into the return grill. So you have to get creative. If you can fit one big figure out the CFM split them up. Or open the wall and frame out a big opening.

Quote:
Originally Posted by stubits View Post
4) MOST IMPORTANT!! Ok, how on earth do I go about "adding" the additional trunk to the return air plenum. I know this is the million dollar question and might not be so easy to explain, but if you could even so much as point me in the right direction, it would be great!
Need to seem more pics. You can cut holes in any side of the plentum you can see and go from there. Use the example from above. If you have alot of room you can extend the plenum to a bigger size then add ducts. More pics and me might have some ideas.

Quote:
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5) Do you see any reason to add return air vents to the basement? It is heated/air conditioned and my mother-in-law lives down there (now you can see why it took me so long to get down there and pull the model numbers).
If you have supplies down there then the air needs to make its way from there back to the unit some how. If she keeps the door closed then this falls into the anwser from above. Treat it like a extra bedroom. All this is about is getting the air to flow easy to everyplace in the house and BACK too. So stand next to your return and figure how easy is the supply air in basement going to make it back to the return vent. If its a condition room and has a supply then it would probley benfit from haveing a return around the same size as the supply.

Quote:
Originally Posted by stubits View Post
6) So, at present I am really undersized in my return air, right? Is it possible I've done any damage to the various equipment? We ran thr AC for a full season and now the heat. How urgent is it that I make these repairs? Also, will correcting this issue result in noticeable increase in performance, ie, will the house heat/cool faster, more evenly, better, or will it reduce costs at all? I guess, what exactly are the detriments to not having enough return air?
Yes your undersized by alot. No probley no damage. Not life or death, more about when do you want the unit to work properly and save money. As far undersize as you are you will see a big diff. in the performance and it will reduce the cost of your electricy bill.
The fan is haveing a hard time spinning when theres no air flowing threw it. Also its not flowing much air threw it so its going to take more time to heat or cool and its going to use alot more enrgy to do it in. Think of driving you truck with a trailer behind you. The fans trying to spin, but its just not going fast enough cuz its being held back. This results in you useing more gas to try and move, and even after useing more gas, you still are not going as fast as you normally would be.
So mostly you mwould notice lower electric bills, faster cooling and heating, and less noise from your unit.

Some one else can probley chip in on how bad it is for the unit, but altho Im sure its not good I dont think its going to fall apart.
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Old 01-07-2009, 07:45 AM   #22
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This is fantastic. Thank you so much for all the information and help.

I will upload some pictures of my current setup tonight, running the duct to the main level shouldn't be difficult, but I am concerned about how to tie it all in to the plenum.

Thanks!
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Old 01-07-2009, 09:46 AM   #23
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Are you POSITIVE there isn't an opening for the return duct in the basement somewhere? Maybe in an area you can't quite see? Because all the information so far indicates the return is so undersized I can't see how any HVAC tech/installer would do it that way.

It's not too hard at all to add duct in. As long as you have working room. If I were you, I would extend the 16x22 return box all the way back to the main trunk (on the second level). That 8" round is killin you, and never mind how bad of an idea it was to use flex duct for return in the first place.

Get yourself a pair of tin snips (there are different "directions" to tin snips, I hate to say this but research them a bit before you buy them). Mark your 16x22 cutout on the return main. Drill holes at the corners. Connect the dots using the tin snips. Now you have a cutout.

Obtain the 16x22 duct - this may mean having a sheetmetal place make it for you, or building it yourself, or maybe you get lucky and find it at Home Depot (don't bet on it). The pieces that connect into the return main and the plenum will have tabs on it - you insert those into the cutout on the return main (and I suppose the same cutout on the plenum box at the grill), bend em over, apply tape or duct sealant paste (mastic), connect all duct in between, and away you go.

I suppose you could use any duct size smaller then 16x22 - so you could find some stuff at the local borg (HD or Lowes) that would be relatively inexpensive. But anything bigger then 50 sq inches (area of an 8" roudn duct) will help. Even using a round duct that's as big as you can find will help.

Actually if you can find like a 15" round duct, including takeoffs, that would be a real easy solution.

I'm feeling rather helpful today - here's a place that sells the takeoffs - you'd need 2

http://www.alpinehomeair.com/viewcat...ke_Off_Collars

Then add a few lengths of 14" duct (typically sold in 5' or 6' lengths, you just slide em together).

Hopefully you can do a straight run between the 2. Get a 8" cap to seal off the existing flex duct takeoff.

Going from 8" to 14" will triple the area of the duct. Having a straight run of smooth walled duct will help out too.

It gets hot in the summer because heat rises. And your AC isn't able to effectively pull the hot air out of the 2nd story too cool it back down. So the heat just builds up.

For the same reason, in the winter your system appears to work great. But there probably is a big temperature diff between the 2nd floor and the basement.
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Old 01-07-2009, 10:29 AM   #24
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Glad you're in a helping mood.

I am absolutely certain that the basement has no return air vents. The folks who "flipped" the house ended up being really shady, bad plumbing, bad electrical, and of course bad HVAC, but we are working our way through the issues, fixing stuff when and where we can. Thank you and everyone else for your help on this project!

So, a couple of follow-up questions.

Do you agree with integlikewhoa that the whole trunk is undersized? The trunk itself is only 8x12. Would it even be possible to connect a 16x22 duct to that? I definitely could not connect a 15" round to it.

What I am thinking about doing is the following...

1) Connect 2 9" round ducts to return air plenum to take care of the main floor of the home, one in the dining room, other in the living room.

2) Close off the current main floor return so that the 12x8" rectangular return air duct just takes care of the second level. In the attic I'll attach 3 7" round ducts to the trunk with registers in the hall, master bedroom and the guest bedroom.

3) I'd like to add one or two small return vents in the basement as well.

I have plenty of room to work in the attic, not a problem at all. I am pretty sure I can find or make room in the basement utility closet for all of the duct work, but I am not sure where to begin at all. Given that presently there is only one return are trunk, the 12x8 basically just attaches directly to the plenum and runs straight up the whole house, from basement to attic. I'll post some photos tonight to see if you can help me figure out how to tie all this into the plenum.

Do plenums come in different sizes? Is it possible I will need to replace the plenum to pull this all off? Is there any limit to how many times you can hack into a plenum? Any thoughts or suggestions would be appreciated!
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Old 01-07-2009, 11:10 AM   #25
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Do plenums come in different sizes? Is it possible I will need to replace the plenum to pull this all off? Is there any limit to how many times you can hack into a plenum? Any thoughts or suggestions would be appreciated!
You can buy plenum's in all different sizes, but they uselly don't fit right to the unit or atleast in my case. I had a local sheet metal guy make one up for me to fit what I wanted to do. They can make then wider at one end, longer or what ever you need to fit the size of ducts you need. There is not really a limit to what you can hook up to it. I have all (5) sides of mine used up. Between a fresh air intake a pressure bypass, 2 12" (that wye into one big 16") and a 10" (that wyes off to smaller returns in the bedrooms). My plenum is 24" long of the unit and around like 15x20 or so what ever the unit size on the filter end.

I think the 2 9" your talking about are going to be way under sized for the main floor return. You need around a 20" down stairs. Installing two 9" is about a 1/4 of the flow you need. each 9" will flow around 275cfm or so. So thats only around 550cfm. We said before that your current 8x12 trunk for upstairs flows around 400cfm. So with 2 9" and your exsiting you will only flow around 950cfm. You should be atleast 1600cfm and I always like to keep it on the high side for bends, flex and other things that will make acutal even less then what I'm showing.

A 20" duct will flow around 1,327 cfm depending on lenght and such. An 18" (the next size down) flows around 1100cfm. With your existing trunk handling around 400cfm of your 1600cfm total I would go with a 20" and youll be about right where you should be.
A 12" duct will flow around 475cfm. So even 2 12" for down stairs wont be big enough. Now 2 12" and one 9" sounds about right.

Last edited by integlikewhoa; 01-07-2009 at 11:20 AM.
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Old 01-07-2009, 11:14 AM   #26
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Do you agree with integlikewhoa that the whole trunk is undersized? The trunk itself is only 8x12. Would it even be possible to connect a 16x22 duct to that? I definitely could not connect a 15" round to it.
I got the impression somewhere that the trunk was bigger.

8x12 is ridiculously undersized. Like I said, it makes me suspect there's an outlet right near the furnace/ac somewhere that you mighta missed.

If you can, replace the whole trunk. Any renovations you make at this point look pretty major. And it means some extended downtime on your furnace. So you may want to considering hiring someone - whom you can give a drawing to, then then come out one day, measure everything, then come out a second day and complete all the work. It'll cost $$ but it'll get done quickly and save yourself a lot of frustrating doing the work yourself. Or at least wait till spring.

Quote:
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1) Connect 2 9" round ducts to return air plenum to take care of the main floor of the home, one in the dining room, other in the living room.
Beware of adding sound to areas you don't already have it. You may find it quite annoying.

Quote:
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2) Close off the current main floor return so that the 12x8" rectangular return air duct just takes care of the second level. In the attic I'll attach 3 7" round ducts to the trunk with registers in the hall, master bedroom and the guest bedroom.
Not a bad idea. But if your trunk is 96 sq inches, there's not a lot of point to adding more then 96 sq inches of run between the trunk and the registers. Doing the math, I see that the 7" ducts combine to be about the same area as the trunk. So that makes sense.

Plenums come in different sizes and you may be able to buy a premade one relatively cheaply that will help. The bigger the better. THere's no real limit to how many times to add a takeoff. But when you do, arrange your takeoffs logically, trying to keep in mind that the air has to flow thru them. Try to make the airpath as smooth as possible.

I would also consider adding balancing dampers to each run while your at it. So you can try to redirect the airflow at a later date, or even turn off runs you may decide you dont want to use. Like when your wonderful mother in law moves out.
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Old 01-07-2009, 11:22 AM   #27
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Awesome.

Thanks!

So, it is possible I can get by with my current plenum... just depends on how much room I need to get this all hooked up correctly, right?

Just out of curiosity, what did it cost to have your plenum done custom?

I am not sure how I came up with the 2 9"s, must have misinterpreted something. Would 2 14" round work (or maybe 2 12? Could I tie that into the plenum with a 14x14x14 wye?

Thanks!
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Old 01-07-2009, 11:26 AM   #28
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I'll definitely wait till the spring to do any of this. Hopefully can get it done before the summer hits, DC summers are hard.

I've been all over the basement, it isn't that large really, and alas, no return air vent, as crazy as it seems.

I'd rather do the work myself, not just to save $$ but because it is interesting. Replacing the whole trunk would be a lot of work, whereas I can run a couple of ducts to the first level pretty easily.

I know it won't be perfect, but trying to fit a modern AC/Furnace into a 75 year old brick rowhouse probably never is.
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Old 01-07-2009, 11:36 AM   #29
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Also something to remember another reason you have such large or so many returns is cuz you have a unit that is way oversized for your size house. A 5 ton unit is used alot in 2500sf homes. I'm slightly oversized according to a MAN J for my 1600sf house with a 3 ton unit. My unit flow less cfm since its a smaller unit. around 1200cfm. The general rule of thumb is 400cfm per ton. Your unit for a 5 ton actually flows less then normal I would say at only 1600cfm. That actually helps you out. If it was a new high efficent unit you would probley be flowing 2000cfm out of a 5 ton unit. That wound mean bigger ducts and more of them. So this might help you as your trying to understand why do these ducts seem so big. Your unit is way oversized which you already know. But Everything comes in proportion. An oversized unit for you house, means oversized ducting, oversized electric and gas bill, oversized wiring (for the a/c compressor) etc...... Bigger is not always better. And the 150sf your planning on adding is not even dent in what this sized unit is normally installed in. 2500sf houses normally. Hope this Helps, JIM
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Old 01-07-2009, 01:25 PM   #30
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I think I am understanding more and more. It sucks because the furnace and AC are both brand new (6 months or so). What a shame, and waste of money, that flippers put in such oversized equipment. Well, what can I do at this point, right? Tearing it all out and starting over is just prohibitively expensive!

So, a couple more quick questions...

1) If I do all this work now and say in 10 years I end up replacing all the equipment with correctly sized equipment, am I going to be stuck with oversized return ducts? Or does it not really matter?

2) So, it is possible I can get by with my current plenum... just depends on how much room I need to get this all hooked up correctly, right?

3) Just out of curiosity, what did it cost to have your plenum done custom?

4) Would 2 14" round work (or maybe 2 12"? Could I tie that into the plenum with a 14x14x14 wye or does one arm of the wye need to be larger?

5) If I tie in a vent or two for the basement, are they going to be super load because of their proximity to the furnace? Or, given that I'll have increased overall air flow, will it all sort of balance out?
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