# Duct Size



## nholmes8669 (Oct 26, 2010)

I have a 3.5" x 12" supply duct running from my basement up to my 2nd floor.  The supply pipe from the furnace to the rectangle duct is a 6" round and also from the end of the rectangle duct to the register.  The heat just trickles out upstairs.  I was wondering if changing the 6" round to an 8" round off of the furnace would make much of a difference or can someone recomend a different size to get addequate airflow to the 2nd floor?  I tried a booster fan and that really didnt make much difference.  Any help is appreciated.


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## paul52446m (Oct 26, 2010)

nholmes8669 said:


> I have a 3.5" x 12" supply duct running from my basement up to my 2nd floor.  The supply pipe from the furnace to the rectangle duct is a 6" round and also from the end of the rectangle duct to the register.  The heat just trickles out upstairs.  I was wondering if changing the 6" round to an 8" round off of the furnace would make much of a difference or can someone recomend a different size to get addequate airflow to the 2nd floor?  I tried a booster fan and that really didnt make much difference.  Any help is appreciated.


  This is a engineering question . I lay out a hot air ducts
 1.0 static pressure and return air to a .o8 static pressure.  Using these numbers
 you can get 150 CFM of air through the 3.5x12. 110 CFM through a 6" round,
 and 220 CFM through a 8"Rd.  The hole thing depends on how many ell , what kind of ells, and how the rest of the ducts are laid out. A lot of guys restrict 
 the air flow by not making the return air large enough. Have you checked your blower speed to see what speed you are on? Later Paul


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## nholmes8669 (Oct 28, 2010)

paul52446m said:


> This is a engineering question . I lay out a hot air ducts
> 1.0 static pressure and return air to a .o8 static pressure.  Using these numbers
> you can get 150 CFM of air through the 3.5x12. 110 CFM through a 6" round,
> and 220 CFM through a 8"Rd.  The hole thing depends on how many ell , what kind of ells, and how the rest of the ducts are laid out. A lot of guys restrict
> the air flow by not making the return air large enough. Have you checked your blower speed to see what speed you are on? Later Paul



The blower is a 3 speed blower and is set at the highest setting.  I am unsure of the cfm's at each speed.  It is a comfortmaker 75,000 btu furnace 80%.  There is no return air duct on the 2nd level but I plan on adding one when I add a bathroom up there in the next couple months.  The rest of the ducts throughout the house are 6" round.  It is an older house and all the return air runs on the first floor used the floor joists as ducts.


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## paul52446m (Oct 28, 2010)

nholmes8669 said:


> The blower is a 3 speed blower and is set at the highest setting.  I am unsure of the cfm's at each speed.  It is a comfortmaker 75,000 btu furnace 80%.  There is no return air duct on the 2nd level but I plan on adding one when I add a bathroom up there in the next couple months.  The rest of the ducts throughout the house are 6" round.  It is an older house and all the return air runs on the first floor used the floor joists as ducts.



You have not give enough info to even begin to do any engineering. Like SQ' of lower story, SQ' of upper story Sizing of all the return air ducts, size of all the 
 supply air ducts. They make 75,000 btu furnaces with 2 ton, 3 ton, and 4 ton blowers A 2 ton blower is max. 800 CFM, a 3ton is 1200 CFM, a4 ton is 1600 CFM. If you give me all the numbers off the furnace than maybe i can tell which
 blower you have. One thing you can test is take off the blower door tape the door switch so it will make contact. Now start the furnace and with the blower running check the air flow up stairs with door off and with door on. If you get a lot more air flow with door off than your return air ducting is not large enough. Of course if you put so much CFM. up stairs you have to take that much return from up stairs. later paul


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## nholmes8669 (Oct 30, 2010)

Ok the model # of the furnace is GNE075B12G1.  The 1st floor square footage is 650, the 2nd floor is 300 (its a 1 1/2 story).  There is a 8"x24"x8' supply trunk on the top of the furnace with 4 - 6" round ducts coming off of it for 1st floor rooms and 1 - 6" round duct for the basement and 1 - 6" round duct for the 2nd floor.  The return air duct at the furnace is a 8" x 24".  I did what you said and ran the furnace with the door removed and the airflow coming out of the 2nd floor vent was the same as with the door on.  Let me know what other info you need.  Thanks.


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## paul52446m (Oct 30, 2010)

Do the runs down stairs blow hard? How many turns does that up stairs run
 have in it? Are you sure that the blower is on hi speed?  later paul


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## nholmes8669 (Oct 31, 2010)

paul52446m said:


> Do the runs down stairs blow hard? How many turns does that up stairs run
> have in it? Are you sure that the blower is on hi speed?  later paul



All the runs downstairs do blow hard.  I actually have 2 of them closed to try to get more airflow upstairs.  The run upstairs has 3 - 90 deg bends before it goes upstairs and 1 - 90 deg bend upstairs to get it to the vent.  The blower is definately on high speed, I looked up the online manual and had to switch the wires on the control panel.  It was on med-high (orange) and I changed it to the black (high).  Here is a link to the online manual.

http://icpindexing.mqgroup.com/documents/086477/44101330700.pdf


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## paul52446m (Oct 31, 2010)

If you only have 6 hot air opening then you should leave them all open so you don't over heat the furnace. A blower can only built up so much static pressure, after that it will just start bypassing air inside itself. Normally a 6" pipe can only move 
about 110 CFM of air, so if you don't have large enough ducts to use hi speed, then you are not moving any more air on hi speed than you was on med speed. 
 Sounds like you need to run another run up there. Is the up stairs run in a room that has a open door so it does not pressurize the room and stop the air flow?
  Later Paul


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## nholmes8669 (Oct 31, 2010)

paul52446m said:


> If you only have 6 hot air opening then you should leave them all open so you don't over heat the furnace. A blower can only built up so much static pressure, after that it will just start bypassing air inside itself. Normally a 6" pipe can only move
> about 110 CFM of air, so if you don't have large enough ducts to use hi speed, then you are not moving any more air on hi speed than you was on med speed.
> Sounds like you need to run another run up there. Is the up stairs run in a room that has a open door so it does not pressurize the room and stop the air flow?
> Later Paul



Yeah I plan on running another duct upstairs when I add a bathroom up there next month.  I am also going to add a return duct as well.  The room upstairs does not have any doors and is open all the way down the stairs to the 1st floor.  
  Do you think that the 6" duct from the furnace to the 3.5"x12" duct running upstairs is choking down the ariflow?  Should I increase it to an 8" to give it more airflow at the vent upstairs?  What size supply and return ducts do you recomend when I add them upstairs?


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## paul52446m (Oct 31, 2010)

nholmes8669 said:


> Yeah I plan on running another duct upstairs when I add a bathroom up there next month.  I am also going to add a return duct as well.  The room upstairs does not have any doors and is open all the way down the stairs to the 1st floor.
> Do you think that the 6" duct from the furnace to the 3.5"x12" duct running upstairs is choking down the ariflow?  Should I increase it to an 8" to give it more airflow at the vent upstairs?  What size supply and return ducts do you recomend when I add them upstairs?



It would help if you run 8" to the up stairs run. I would run the new hot air run in 8" and split it into too 6" up stairs, one to bath and one to main part. Run a 8" 
 return air and try to get the return hi if you can. Later paul


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## nholmes8669 (Oct 31, 2010)

paul52446m said:


> It would help if you run 8" to the up stairs run. I would run the new hot air run in 8" and split it into too 6" up stairs, one to bath and one to main part. Run a 8"
> return air and try to get the return hi if you can. Later paul



The only way to get any ducts upstairs is through a wall cavity so I think my choices are limited to an oval or rectangle duct for both the supply and return.  I will do like you said and swap the 6" for the 8" for the existing duct to the second floor.  Which is better, oval or rectangle?  What is the biggest size that will fit in a wall cavity for each?


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## paul52446m (Oct 31, 2010)

A 6' oval pipe is only good for about 90 to 70 CFM of air. 6 rd is 110 CFM
 8" rd is 225 CFM  paul


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## nholmes8669 (Nov 3, 2010)

paul52446m said:


> A 6' oval pipe is only good for about 90 to 70 CFM of air. 6 rd is 110 CFM
> 8" rd is 225 CFM  paul



So would it be better to go with a rectangle duct then?  Like a 10x3.5 or 12x3.5 to get it upstairs?


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## paul52446m (Nov 3, 2010)

Thats a lot better than oval. 10x3.5 is good for 120CFM
12x3.5 would be about 150CFM
 If heat can not go back up the studded space, just use  that space for return air. Return air has to be larger that hot air to move the same amount of air.
 Later Paul


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