Undersized Return

[Armis]

My return is 8x16, 8x20 supply.
Now return runs about 28 feet away from furnace. First kitchen and living room has one big register, then two bedrooms and Halway each have smaller return registers.
To fix I would like to start with Bigger duct for return, say 10x20 for kitchen, living room (bigger register) then can I downsize to approprated size for rest of the rooms and Halway Or Even USE flex pipe For those returns ducks?
Hope its clear that i am trying to do.

I need roughly 300 cfm for living room, 380 kitchen, bedroom Each 120 cfm 50 Halway.

[HVACTechNC]

Returns in kitchens and bathrooms is a big no-no. Consider disconnecting kitchen return and running it to an adjoining room while making the ducting upgrades. You do not want kitchen odors and grease getting pulled through your HVAC system and distributed through your house. (and coating your coils in the case of grease)

[Armis]

Sorry I made a mistake . Register is in living room, separated with wall.
But living space and kitchen is partially open (50%)

[snupytcb]

what is your reason for wanting to down size the return ducting? is it a space issue? just curiose.

[RANGER-RICK]

For 970 cfm your ductwork seems under sized.

[Armis]

what is your reason for wanting to down size the return ducting? is it a space issue? just curiose.

I want to upsize 8x16 to 10x20 , my question was
Do i need continuous 10x20 return all 28' long , or can it be reduced
After kitchen/living space register, since biggest register is in living room.
Supply to kitchen/living is roughly 680 cfm , rest rooms 300 cfm

[Armis]

For 970 cfm your ductwork seems under sized.

I know

[snupytcb]

I want to upsize 8x16 to 10x20 , my question was
Do i need continuous 10x20 return all 28' long , or can it be reduced
After kitchen/living space register, since biggest register is in living room.
Supply to kitchen/living is roughly 680 cfm , rest rooms 300 cfm

i'm a wet head. i do mostly boilers so i am not big on air flow. as far as i know it is not necesary to reduce a return. you still need to bring back all the air from the start of the run. if you do reduce size you could have a chance of restriction.

[Armis]

i'm a wet head. i do mostly boilers so i am not big on air flow. as far as i know it is not necesary to reduce a return. you still need to bring back all the air from the start of the run. if you do reduce size you could have a chance of restriction.

The reason for downsize is $. 28' of 10x20 duct versus 10' of 10x20 duct and 18' of 10x10 (or flex)

[MountainC10]

You must do a "Heat Load Engineering Study" to determine the proper size system. No rule of thumb ... I use a computer program that determines the proper size for equipment and ducting. Buy one !

Mountain

[snupytcb]

it may be cheaper to hire someone to do a heat load. or you can pay for a "per use" program online. if you hire some one they could tell you other ways to improve also. this is why alot of us pay so much time and money to be a part of this industry, so we can help.

[HVACTechNC]

It is hard to go too big on return air...

If it were my house I would not want to allow cost concerns to allow me to restrict the return air (or even cut it back to the minimum).

By my math, a run of 16" flex has a total of 1.395 sq ft of surface area inside the duct while a 10X20 rectangle has 1.38 square feet...

10X20 square to 16" round (X2)
1 - 25' run of 16" flex
some ul-181 duct tape and straps...
a bucket of mastic

That ought to do the trick and the cost of said materials really isn't that high. If that is what you are looking at doing I wouldn't worry about trying to save too much as there isn't enough materials cost in that to justify much effort in trying to save. You are going to waste hours researching and shopping around to save $20... Not even worth your time.

Or am I not understanding you correctly?

[Armis]

It is hard to go too big on return air...

If it were my house I would not want to allow cost concerns to allow me to restrict the return air (or even cut it back to the minimum).

By my math, a run of 16" flex has a total of 1.395 sq ft of surface area inside the duct while a 10X20 rectangle has 1.38 square feet...

10X20 square to 16" round (X2)
1 - 25' run of 16" flex
some ul-181 duct tape and straps...
a bucket of mastic

That ought to do the trick and the cost of said materials really isn't that high. If that is what you are looking at doing I wouldn't worry about trying to save too much as there isn't enough materials cost in that to justify much effort in trying to save. You are going to waste hours researching and shopping around to save $20... Not even worth your time.

Or am I not understanding you correctly?

Yes you partially anwered my question. I know your calculation is right.
But my question rise from Supply duct. It starts big then gets smaller. Can it be done same with return. Say you need 200cfm supply to A room , 600 cfm supply to B room. Return can be adequate : Big duct and register in B room and get smaller at A room.
Small supply - small return , big supply - big return

[Armis]

You must do a "Heat Load Engineering Study" to determine the proper size system. No rule of thumb ... I use a computer program that determines the proper size for equipment and ducting. Buy one !

Mountain

I have one , question doesn't require calculation

[HVACTechNC]

Again, unless I am misunderstanding, a supply run usually get smaller as runs branch off from the main supply trunk. In this case air is being run down the branches, reducing the airflow in the trunk so smaller is ok as you progress down the run.

If you are looking at running a 16" main return to a return box with smaller runs going to the individual returns, then yes that is ok as long as the smaller runs are properly sized. Say 2 or 3 12" runs feeding a box going to a 16" that runs from the box to the unit, that would be fine. Pair the 12" runs to a 14X20 return and filter grille and bingo-bango you are in business.

If you mean reducing size as it approaches the unit - i.e. going from a 16 to a 14 following the flow of air, not a good idea.

[MountainC10]

Then why are you asking ? ... If you already have a "Program" it should tell you all the required information you need ! ... Email me ... For residential programs it cost just $50.00 for a one time use ...

[Armis]

Then why are you asking ? ... If you already have a "Program" it should tell you all the required information you need ! ... Email me ... For residential programs it cost just $50.00 for a one time use ...

I know all the numbers just not sure how or best way to do it.

[MountainC10]

If you do have the required information ... Then why ask ? ...

[Armis]

If you do have the required information ... Then why ask ? ...


I know all the numbers just not sure how or best way to do it.

[genduct]

My return is 8x16, 8x20 supply.
Now return runs about 28 feet away from furnace. First kitchen and living room has one big register, then two bedrooms and Halway each have smaller return registers.
To fix I would like to start with Bigger duct for return, say 10x20 for kitchen, living room (bigger register) then can I downsize to approprated size for rest of the rooms and Halway Or Even USE flex pipe For those returns ducks?
Hope its clear that i am trying to do.

I need roughly 300 cfm for living room, 380 kitchen, bedroom Each 120 cfm 50 Halway.

What you need is a low resistance path for the air to get back to the unit. So, if that first large return grill in the living area is close enough and sized for the 100 CFMs you need then the rooms with closed doors could have ONE STUD SPACE with a high grill on one side and low grill on the other act like a transfer to allow air back. Even a short run of the so called undersized duct could be OK if short and direct. that first return grill size is more important

[genduct]

Edit

[snupytcb]

can you post a drawing of what you are looking to do? maybe that would help us understand a little better.

[darctangent]

Yes you partially anwered my question. I know your calculation is right.
But my question rise from Supply duct. It starts big then gets smaller. Can it be done same with return. Say you need 200cfm supply to A room , 600 cfm supply to B room. Return can be adequate : Big duct and register in B room and get smaller at A room.
Small supply - small return , big supply - big return

Without a whole host of information that we don't have here, it's like playing a game of darts in a dark room. Do yourself a favor a find a pro who understands airflow, how to calculate it and how to measure it. duct dimensions are only part of the calculation and there will always be intangibles until you can measure the air moving through the system.


by the way, equivalent area of a duct is no way to design a duct system because material and shape factor in as well. Flex duct and rectangular galvanized steel are not the same. In a similar manner, orientation of components can play a huge roll in how the system functions.

If you can't measure, you can't know.

[Armis]

Guys gave me more than enough info.

Thank you very much.