Can anybody explain why I should not shorten my (2) 12" flex runs?
They are currently 25' and 20' and I can probably get them down to
15' and 10' (yes without kinking them). Assuming registers are all equal distance.
I don't want to get to technical on this please, I just don't see why they routed the 12" flex so far way from the plenum, to me it makes sense to be closer to it, like in configuration A, shortest run to center of Trunk right?
Current configuration B just doesn't make any sense to me at all.
Are the flex ducts located in a conditioned basement? If so run round ducts to the trunk lines without insulation or a minimum bubble wrap which is easy to do labor wise.
For the equivalent loss from a 12" flex run installed correctly (that is pulled tight) you can use a 10" round duct.
How much is the CFM for your 12" flex runs? Installed properly 12" flex is bigger than needed for a 4X12 trunk line. 10" flex would be the correct size for a 4X12 duct sized for 200 CFM max flow down each trunk.
Of course a 8" round duct would also suffice to closely match the same friction loss you would get down a 4X12 duct.
Might want to just run them straight across to reduce the long runs somewhat and then use a Y going into the trunk lines for the best air distribution.
Regardless of rather you use flex or round duct I would connect them in the center of the trunk lines with a Y for superior air flow and less friction loss. Thank you very much
Last edited by glennac; 06-17-2019 at 02:13 PM.
"I could have ended the war in a month. I could have made North Vietnam look like a mud puddle."
"I have little interest in streamlining government or in making it more efficient, for I mean to reduce its size. I do not undertake to promote welfare, for I propose to extend freedom. My aim is not to pass laws, but to repeal them. It is not to inaugurate new programs, but to cancel old ones that do violence to the Constitution." Sen. Barry Goldwater
I don't see why it would be cheaper to install longer runs?
The take-offs are just round holes cut in the rigid ducting, there is nothing else after that except the vents/registers.
Flex was there when I bought the house, (manufactured home is 15 years old) that's what they used, but I agree with what glennac stated to just use round rigid ducting straight to the center of the trunk and connect it with a Y, great idea.
If your AC is over one ton which I suspect it is then you need bigger trunks. If it is a two ton system then you would need a bear minimum of two 6X12 trunk lines to correctly handle the air flow for each trunk. Thank you very much
"I could have ended the war in a month. I could have made North Vietnam look like a mud puddle."
"I have little interest in streamlining government or in making it more efficient, for I mean to reduce its size. I do not undertake to promote welfare, for I propose to extend freedom. My aim is not to pass laws, but to repeal them. It is not to inaugurate new programs, but to cancel old ones that do violence to the Constitution." Sen. Barry Goldwater
I have a 4 ton ac/coil. The trunks are about 4"x 15" and both run about 60'. I'm not worried about flow, as I feel I could use more air. That's another post I have on this site to upgrade my motor for more CFM, but not getting far with that
But again, airflow is not a problem as far as too much. I added 3 registers to this system on 8" flex and the motor is quieter and obviously the flow is a bit less.
My plan is to just cut out whatever wasted 12" flex there is, because yes rigid ducting would be awesome, I just don't want to be under the house that long piecing the system together with mastic and insulation, sheet metal screws etc...
My system works great, I just want to cut off excess 12" flex and update my blower 200-300 CFM and I'll be good to go.
Forgot to mention, I have a 3 ton furnace and they added a 4 ton coil/compressor which is the reason I want to upgrade the motor.
I really wish I was a home owner that knew the answer to your question. I could only guess at the CFM's by maybe looking up my furnace model number...unless it's on the blower itself...I didn't see one on first inspection...
Some of this HVAC stuff can get complicated and "duct pressure" sounds like one of them. Do you have a formula I can try?
I try to educate myself, but everybody I see has some type of gadget used to test duct pressure, so not sure I can answer that?
You should try for 400 CFM per ton but no less than 300 CFM. Also try to keep your velocity in the range of 600 ft./min. in all your low pressure ducts for residential use for recommended design. Figure out what you need from that. Thank you very much
"I could have ended the war in a month. I could have made North Vietnam look like a mud puddle."
"I have little interest in streamlining government or in making it more efficient, for I mean to reduce its size. I do not undertake to promote welfare, for I propose to extend freedom. My aim is not to pass laws, but to repeal them. It is not to inaugurate new programs, but to cancel old ones that do violence to the Constitution." Sen. Barry Goldwater
I really wish I was a home owner that knew the answer to your question. I could only guess at the CFM's by maybe looking up my furnace model number...unless it's on the blower itself...I didn't see one on first inspection...
Some of this HVAC stuff can get complicated and "duct pressure" sounds like one of them. Do you have a formula I can try?
I try to educate myself, but everybody I see has some type of gadget used to test duct pressure, so not sure I can answer that?
Thanks
The readings you need to know, aren’t listed anywhere on the equipment. They are taken real time in the field by instruments.
If you don’t know the current conditions, and don’t know the conditions your after, what do you expect to do here?
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Originally Posted by thracianspec
