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Discussion Starter · #1 ·
Would like to re-work flex ducts for better air flow and static pressure.

Left side of drawing shows existing configuration. Right side is my proposed.

Could use your input on the duct board size off the trunk.

Am I helping or hindering the system by this proposed change?


Rectangle Slope Font Parallel Schematic
 

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Would like to re-work flex ducts for better air flow and static pressure.

Left side of drawing shows existing configuration. Right side is my proposed.

Could use your input on the duct board size off the trunk.

Am I helping or hindering the system by this proposed change?


View attachment 707328
Have you checked your static pressure to see if it's within the air handler specs?

I am having a similar problem, but I am waiting for fall to go up in the attic. My units specs are .2 W.C. and my static pressure is .7 W.C.. I know .2 is not a set number and the unit can run at a .4, but not a .7. if your unit static pressure specs is .5 you are good if it's at .7.

The whole duct system requires many factors and calculations for a proper design. I been reading about it and watched many HVAC YouTube videos and now I know why many HVAC techs don't even bother with it. Factors are size of unit, amount and size of vents, air flow, air velocity, room size and location, duct sizing, the size of the return and room temperature. Some rooms may be getting more sunlight and require more air flow, because they get warmer.

With flex duct proper installation is key. The flex cannot be laid on the floor, needs to be hung with straps every 3ft, should not have tight turns and a tight seal at the connections. I think is like 70% of homes have issues with the ducts system.
 

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Your current layout doesn't look bad in terms of design. Mine is a huge octopus going all over the attic.... I don't have a trunk, but distribution boxes connecting the ducts..... Horrible design. My supply plenum is small and the whole system is filled with restrictions.
 

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Just looking at part of the installation and making a decision is foolish. You need the whole system to make decisions.

I hate flex duct and when it is possible I remove it. Metal duct is superior and easier to maintain air volumes. If you have to use flex then make it tight, firmly, and use metal large radius 90 ells.

The second way IMO will lower air volumes to the rooms your changing.

I aint no duct guru.
 

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Discussion Starter · #7 ·
The flex is fine but without knowing how much air the rooms need we can't say how big the ductwork needs to be.
I'm asking, with the information provided, what dimension duct board would be needed to supply correct amount of air for the two 7" flex ducts going to the two 6" ceiling supplies? 10x10, 8x8, 8x10, .... ?"

The existing configuration is what was installed by the HVAC company that replaced the HVAC system. The original flex ducts were 6". They increased the flex to 7" and added a reducer on the 6x10x6 ceiling vent.

They would not do a room by room heat gain/heat loss. Just connected to existing ceiling registers So, do you size cfm air flow for the 7" flex duct or the 6" celing register?
 

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Discussion Starter · #8 ·
Have you checked your static pressure to see if it's within the air handler specs?

I am having a similar problem, but I am waiting for fall to go up in the attic. My units specs are .2 W.C. and my static pressure is .7 W.C.. I know .2 is not a set number and the unit can run at a .4, but not a .7. if your unit static pressure specs is .5 you are good if it's at .7.

The whole duct system requires many factors and calculations for a proper design. I been reading about it and watched many HVAC YouTube videos and now I know why many HVAC techs don't even bother with it. Factors are size of unit, amount and size of vents, air flow, air velocity, room size and location, duct sizing, the size of the return and room temperature. Some rooms may be getting more sunlight and require more air flow, because they get warmer.

With flex duct proper installation is key. The flex cannot be laid on the floor, needs to be hung with straps every 3ft, should not have tight turns and a tight seal at the connections. I think is like 70% of homes have issues with the ducts system.

In my opinion and experience, hire a mechanical engineer with experience in hvac to do the system design. Give the design to the HVAC company to install per the engineer's design.

It's probably more than 70% not installed correctly. Flex duct was not developed for residential HVAC. It was for commercial suspended ceilings with acoustical 2'x2' or 2'x4' ceiling tiles. You can't walk on the ceiling gridwork held up with wire. Only way to connect the air vents to the supply was flexible ducting. Plus, the duct work is still within the conditioned space of the office building. Not in a 130 degree or 20 degree residential attic.

You can guess as to why it's used in residential HVAC.
 

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Discussion Starter · #9 ·
Just looking at part of the installation and making a decision is foolish. You need the whole system to make decisions.

I hate flex duct and when it is possible I remove it. Metal duct is superior and easier to maintain air volumes. If you have to use flex then make it tight, firmly, and use metal large radius 90 ells.

The second way IMO will lower air volumes to the rooms your changing.

I aint no duct guru.
It's not foolish. As you stated, metal is better than duct board, and duct board is better than flex.

All I'm doing is providing the existing ceiling registers with better, less turbulant, air flow. It's still the same cfm; one straight duct board run with two short flex runs.
 

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Air flow is done (correctly) by the type of room your pushing air into. You do not provide that information.
When I changed the duct in my home, I was changing the equipment at the time. I did the calculations. I discovered there was no benefit to go to larger duct, there was a benefit to removing the flex. This surprised me.

Would you change the rear tires on your car to a bigger size for better gas mileage?
Kind of the same thing. Thinking that you can modify one part of a duct system with out considering the whole system is indeed foolish. Do what you will it is your house.
I have not seen a new installation of duct board in over a decade. I have seen LOTS
of it removed. "Ductboard & metallic Flex, under ideal conditions, are up to 10 times & 23.3 times rougher, respectively, than galvanized steel – this is supported & verified by ASHRAE research and can be referenced in ASHRAE Fundamental Ch 35 & ASHRAE Principles of HVAC Ch 6. "
 

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Discussion Starter · #12 ·
Air flow is done (correctly) by the type of room your pushing air into. You do not provide that information.
When I changed the duct in my home, I was changing the equipment at the time. I did the calculations. I discovered there was no benefit to go to larger duct, there was a benefit to removing the flex. This surprised me.

Would you change the rear tires on your car to a bigger size for better gas mileage?
Kind of the same thing. Thinking that you can modify one part of a duct system with out considering the whole system is indeed foolish. Do what you will it is your house.
I have not seen a new installation of duct board in over a decade. I have seen LOTS
of it removed. "Ductboard & metallic Flex, under ideal conditions, are up to 10 times & 23.3 times rougher, respectively, than galvanized steel – this is supported & verified by ASHRAE research and can be referenced in ASHRAE Fundamental Ch 35 & ASHRAE Principles of HVAC Ch 6. "
Honestly and respectfully, I have no idea what you are trying to say.

Why would it surprise you that removing flex duct would be a benefit? It is almost never installed correctly, stretched completely and supported properly. Fiberglass duct board and metal duct are much better with air flow, less restriction.

I am not increasing the size of any ducting (tires). I'm attempting to replace flex duct with duct board. Given the existing 7" flex ducts attached to 6" supply registers, what size duct board would be needed?

If you are not seeing any fiberglass duct board installed and flex duct is "23.3 times rougher", what are you seeing installed, all metal ducting?

The math is already done. The room needs two 6" supply registers which apparently meets the needed cfm.
 

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I'm asking, with the information provided, what dimension duct board would be needed to supply correct amount of air for the two 7" flex ducts going to the two 6" ceiling supplies? 10x10, 8x8, 8x10, .... ?"

The existing configuration is what was installed by the HVAC company that replaced the HVAC system. The original flex ducts were 6". They increased the flex to 7" and added a reducer on the 6x10x6 ceiling vent.

They would not do a room by room heat gain/heat loss. Just connected to existing ceiling registers So, do you size cfm air flow for the 7" flex duct or the 6" celing register?
Www.loadcalc.net

Do a whole house and room by room yourself then.

Is your ductboard internally lined with fibreglass mat or aluminum foil? Assuming fiberglass mat, an 8×8 would be more than enough to be equivalent to 2, 7" flex extended to 80%.
 

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Discussion Starter · #14 ·
Www.loadcalc.net

Do a whole house and room by room yourself then.

Is your ductboard internally lined with fibreglass mat or aluminum foil? Assuming fiberglass mat, an 8×8 would be more than enough to be equivalent to 2, 7" flex extended to 80%.
Thanks. I have used LoadCalc.

Fiberglass lined, outside foil faced. Understand on the 8x8.
 
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