# Thread: How Much Air can 12 by 8 ductwork handle

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## How Much Air can 12 by 8 ductwork handle

I recently posted about adding a second HVAC system to my house. You guys have pointed me towards first getting an energy audit to see where I can tighten up the house and make it more efficient. I'm pursuing that right now. Just waiting to get call backs. However, I currently evaluating the quotes that I already have. One of the guys wants to put a 2 Ton Heatpump in with a 3 ton coil in the attic. I've had one manual J calculation in the past for the upstairs and they said I was between 1.5 tons and 2 tons needed. They would go for 2 tons. I have a horrible insulation job in the attic and I'll be taking care of that after I get a system installed (I should say if). Anyway, the ductwork that currently goes from the basement to the attic on the second floor is 14 by 8 (unlined sheet metal). When it gets to the attic, it's reduced to 12 by 8 (unlined sheet metal with insulation on the outside. 9 flex duct lines run off the 12 by 8. My question is, can that size duct work handle a variable speed 2 ton unit? Will I get 800 cfm through there without it sounding like a jet engine? Thanks for any advice.

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Originally Posted by breenemeister
I recently posted about adding a second HVAC system to my house. You guys have pointed me towards first getting an energy audit to see where I can tighten up the house and make it more efficient. I'm pursuing that right now. Just waiting to get call backs. However, I currently evaluating the quotes that I already have. One of the guys wants to put a 2 Ton Heatpump in with a 3 ton coil in the attic. I've had one manual J calculation in the past for the upstairs and they said I was between 1.5 tons and 2 tons needed. They would go for 2 tons. I have a horrible insulation job in the attic and I'll be taking care of that after I get a system installed (I should say if). Anyway, the ductwork that currently goes from the basement to the attic on the second floor is 14 by 8 (unlined sheet metal). When it gets to the attic, it's reduced to 12 by 8 (unlined sheet metal with insulation on the outside. 9 flex duct lines run off the 12 by 8. My question is, can that size duct work handle a variable speed 2 ton unit? Will I get 800 cfm through there without it sounding like a jet engine? Thanks for any advice.
It would be suitable for 400 cfm The 14 x 8 will carry 490 up to the attic

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Originally Posted by second opinion
It would be suitable for 400 cfm The 14 x 8 will carry 490 up to the attic
Thanks, so in other words, if someone is going to cut into the ductwork in the attic and separate it from the one unit that's currently running the whole house, I'll need larger ductwork in the attic for a unit up there. Right?

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Originally Posted by breenemeister
Thanks, so in other words, if someone is going to cut into the ductwork in the attic and separate it from the one unit that's currently running the whole house, I'll need larger ductwork in the attic for a unit up there. Right?
Yes that would be correct

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Originally Posted by second opinion
Yes that would be correct
If they were to cut into that main trunk in the middle and send half the air in each direction, like with a T formation, would that work? The estimate didn't give me that information. I'm trying to figure out what questions to ask. Thanks.

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If they double the size.

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If I understand you correctly, you have one system (now) for the etire home that is in the basement with a 14 x 8 riser (duct) to the attic to take care of the 2nd floor?

What size is your home? Each level? What tonnage is in the home now? Are all the rooms on the 2nd floor having cooling problems?

Is there a reason for the "riser" to reduce from 14 x 8 to the 12 x 8; i.e., a joist in the way?

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Originally Posted by second opinion
If they double the size.
I actually asked the contractor about this the other day. He said he would cut into the ductwork in the middle and be sending half the air one way and half the air the other so that it would handle it. Given where the flex lines come off the main trunk, that would have it sending half the air to 4 6 inch supply lines on one side and 5 6 inch supply lines on the other. The side with 4 is all the master suite with a vaulted ceiling that peaks at like 12 to 14 feet, maybe 400 square feet. The side with 5 supplies is the rest and has a normal ceiling. It makes sense in my head, but I'm not an HVAC guy. What do you think?

Originally Posted by George2
If I understand you correctly, you have one system (now) for the etire home that is in the basement with a 14 x 8 riser (duct) to the attic to take care of the 2nd floor?

What size is your home? Each level? What tonnage is in the home now? Are all the rooms on the 2nd floor having cooling problems?

Is there a reason for the "riser" to reduce from 14 x 8 to the 12 x 8; i.e., a joist in the way?
You are correct, the single unit is in the basement and the 14 x 8 riser goes to the attic. It reduces down to 12 x 8 after a T junction. One side of the T is very short and currently has 5 supply lines coming off it. 4 of those supplies feed the master bedroom. The 5th one feeds the downstairs bathroom. I would have this blocked off and feed that bathroom from the basement if I do anything. The other longer end after the T has 5 supplies that service a small bathroom and three bedrooms. The largest of the bedrooms has 2 supplies. My guess is that the reduction to 12 x 8 was done to keep the air velocity up, but again, I'm not an HVAC guy. First and second level of the home is around 1900 square feet. First floor is 960, second is 930. The second floor is overall warmer than the first floor as there is markedly less air flow and the 2nd floor heats up first. Some rooms are hotter than others, but all of them are warmer than downstairs. The room on the north end of the house which is at the furthest end of the ductwork run is the warmest, anywhere from 2 to 5 degrees warmer than the thermostat downstairs, depending on the weather. I also have a computer in there that doesn't help things.

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Originally Posted by breenemeister
I recently posted about adding a second HVAC system to my house. You guys have pointed me towards first getting an energy audit to see where I can tighten up the house and make it more efficient. I'm pursuing that right now. Just waiting to get call backs. However, I currently evaluating the quotes that I already have. One of the guys wants to put a 2 Ton Heatpump in with a 3 ton coil in the attic. I've had one manual J calculation in the past for the upstairs and they said I was between 1.5 tons and 2 tons needed. They would go for 2 tons. I have a horrible insulation job in the attic and I'll be taking care of that after I get a system installed (I should say if). Anyway, the ductwork that currently goes from the basement to the attic on the second floor is 14 by 8 (unlined sheet metal). When it gets to the attic, it's reduced to 12 by 8 (unlined sheet metal with insulation on the outside. 9 flex duct lines run off the 12 by 8. My question is, can that size duct work handle a variable speed 2 ton unit? Will I get 800 cfm through there without it sounding like a jet engine? Thanks for any advice.
You say a three ton coil, does that mean he will be using a three ton air handler (and thus you are looking at moving three tons of air rather than two)?

Is it Variable Speed that can have the airflow set to 2 tons?

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Originally Posted by S.T.Ranger
You say a three ton coil, does that mean he will be using a three ton air handler (and thus you are looking at moving three tons of air rather than two)?

Is it Variable Speed that can have the airflow set to 2 tons?
Yes, it would be a variable speed unit that could be set. Here are the parts.

Bryant 225BNA024****A Heatpump
FV4CNF002 Air handler
10 KW heatstrips

Thanks for the input.

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Reviving this old post as I'm getting closer to deciding on having work done. I know that 12 by 8 duct can only handle 400 CFM. The main equipment I'm looking at now for the attic is Goodman MBVC1200 air handler and SSZ140241 heatpump. Variable speed air handler and 2 ton heatpump. If the contractor cuts into the duct in the middle and makes a T there and sets the unit to max of 800 CFM, will I be OK? This would result in four 6 inch flex duct supplies on one side and 5 on the other I think. In my head, I theoretically see 400 CFM going in each direction. The duct in the basement that services the first floor is set up in a similar fashion, but upflow and the duct there is 14 by 8, so it can handle 490 CFM to each side. For a total of 980 CFM I suppose.

12. If all the ducts are accessible and visible in the attic, the comtractor should design the entire duct system. Hire a contractor you trust and rely on their experience. They already did the man j, use that information to complete a system design.

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## ONE NEEDS TO

ANALYZE THE WHOLE DUCT SYSTEM TO DETERMINE AIR FLOW GIVEN THE FAN CURVE.

1000 CFM in 30 Feet of 12" x 8" Straight Galvanized Steel Duct creates 0.11" pressure Loss.

dP total is all about knowing ALL the Fittings/ Transitions and the dP for each and every component.
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