Will the size of my ducts have to be changed?

Pawl

If you go the 2.5 ton and have a cfm delivery of 1000 cfm and the manual j says you only need 800, you will have the extra to make up for leaks, turns etc. the folks upstairs will be thankful.[/QUOTE]

From using the duct sizing portion of this program the total amount of cfm I have for my house is 609. You can see the total in the picture I displayed.

1.) Is a 2.5 ton too big for this system?

2.) Is a 2 ton unit cutting it close with 800 cfm if I only need 609?

A 2.5 ton (1000 cfm delivery) is giving me a 391 cfm cushion if I only need 609 cfm. I have no problem with either size I am just trying to keep from having the short cycling issues and humidity.

digitalplumber

Interesting, I would have assumned this program would have told you what individual room size duct to use.


You need a Air Duct Calculator:

If I am reading mine right and using .10 for friction (Its recommendation):

I used 70 cfm for allisons bedroom and I get 5" rigid and 6 with a flex.

Extra Bedroom is 5 (4.5 actual) rigid and 5 on my flex.

Bathroom have to use 15 cfm which is 3 rigid and 3 flex

Landing is 3 and 3

Again take these with a grain of salt, hopefully somone will chime in and same I am close, right or wrong!

__________________
digitalplumber is just a name for use here, I am not a plumber!

digitalplumber

From using the duct sizing portion of this program the total amount of cfm I have for my house is 609. You can see the total in the picture I displayed.

1.) Is a 2.5 ton too big for this system?

2.) Is a 2 ton unit cutting it close with 800 cfm if I only need 609?

A 2.5 ton (1000 cfm delivery) is giving me a 391 cfm cushion if I only need 609 cfm. I have no problem with either size I am just trying to keep from having the short cycling issues and humidity.[/QUOTE]


Again earlier I believe BT recommended that you go the larger unit based on your numbers. I would for the peace of mind, but thats me.

__________________
digitalplumber is just a name for use here, I am not a plumber!

Pawl

Central Air: Figuring out duct size for supply/return trunks
The easiest way to size your duct work is with a ductulator, but most people don't have one so heres another easy way you can do with a calculator.
the formula is V X A = CFM.

V stands for velocity like in feet per minute, the standard residental design for trunk ducts is 900 fpm so V = 900
A is the area of the duct in sq ft and CFM is cubic feet per minute.

so like you said 400 cfm per ton (another industry standard) 2.5 tons = 1000 cfm.
the equation looks like this.....1000/900= A in sq feet
so 1000/900= 1.1 sq feet is the area of your trunk duct at the unit.
1.1 x 144 (144 to change to sq inches) = 160 sq inches so you can pick any multiple of sizes 20x8=160 16x10=160 etc.
start out with 20x8 pick up half your run outs and and do the cals again and pick a new size for the rest of the supply trunk. (500/900=.55 etc)
few tips when you pick the last size make it a little larger the air cools of as you get near the end of the run so you need more of it.
no run outs smaller than 6" dia., all with dampers
insulate everything in the attic
make the return the same size as the supply (the air don't know which way its going)

if you want make the sizes a little bit bigger you can't get hurt making them larger. but 25 x10 seems a little large for 2.5 tons

here's the table we used for years sizing run outs.
6" dia. 150 cfm or less
8" dia. 250 cfm or less
10" dia. 450 cfm or less
12" dia. 650 cfm or less

beenthere

That table will give you a high static pressure in you system.

6" less then 100CFM
7" less then 130CFM
8" less then 180CFM
The above is for room supplies.
The below is trunk lines.
8" up to 250 CFM
9" up to 310 CFM
10" up to 390 CFM
12" up to 580 CFM

Pawl

beenthere,

1.) What about a bathroom that calls for 13 cfm (38 sq ft room) from the load calculation? Can you give me some smaller diameter sizes to get closer to 13 CFM? The old duct leading to that bathroom is 5".

2.) I have a total of 154 cooling CFM/141 heating CFM needed for 2nd floor that will be running from 1st floor AC unit up to 2nd floor. An 8" trunk line running through 1st floor attic is going to transition to a 9' rectangular duct going vertically up wall into 2nd story attic and then back to 8" round across ceiling joists branching across over bedrooms to each register. The original vertical rectangular duct is 14"x 3" and there was no room for insulation because it would only fit 14" wide between roof rafters/wall stud. I was thinking maybe 12" x 4" duct (48 sq inches) giving me 1" on each side for insulation and then just bring the wall out an extra inch to make this fit. And 8" duct measures 50 sq inches.

3.) Are those numbers shown here for supply/trunk lines above also used for the return ducts back to ac unit?

4.) Is there any advantage to trying to run one larger return duct versus two separate return ducts (1st flr/2nd flr) back to ac unit? The obvios is less material to install or is it not a good idea?

Attached Thumbnails

 

digitalplumber

Having returns in at least all bedrooms allows the door to be closed. Having one up and one down is good but more makes for more comfort.

__________________
digitalplumber is just a name for use here, I am not a plumber!

Pawl

I just realized I didnt say that right. I plan on putting returns in both upstairs bedrooms and at the top of the stairs. I will have to bring all the upstairs return ducts/vents into one 8" return duct in the 2nd story attic and run that duct back to the ac unit.
But I was asking if when I bring that 2nd story 8" return duct down into that open wall in the closet and send it back into the 1st story attic could I tie that duct at that point into the 1st story 12" return duct and just send one duct back to the ac unit for both floors.Might save some work,some material,some space in the attic.
I would probably have to run an 8" return duct in the 2nd floor and transition on the 1st floor into a 12' return duct to handle 610 cfm for the whole house.
If I have to run separate return ducts back to the ac unit for each floor than I need one 12' round duct for 1st floor and one 8" duct for 2nd floor. Maybe separate ducts is better because "beenthere" said the 12" duct is rated up to 580 cfm and my whole house adds up to 609cfm.

beenthere

I use a 5", with a damper in it. So it can be throttled down. You can use a 4" if you want for that low of a CFM.

I believe the CFMs the software is saying you need, is based on a 1.5 ton unit still. Correct it to the size you are going to install.

Pawl

The 13 CFMs for my upstairs bathroom is based on a total of 600 total cfm which I believe is equivalent to a 1.5 ton unit. A total of 800 cfm (2 ton?) raises the upstairs bathroom cfm requirements to 18 cfm and a total of 900 cfm raises it to 20 cfm. The program only allows me to enter between 300 cfm and topping out at 900 cfm with 50 cfm increments.

digitalplumber

I'm confused, after you enter all of your data are you telling me that you are entering the data for the Total AC CFM manually?

I thought that the program would tell you this after you entered all of the individual room sizes?

Someone correct me if I am wrong, but there should be no manual entry of the total cfms correct? the calc program should tell us this?

The minor increase in CFM would seem to be no big deal to me.

__________________
digitalplumber is just a name for use here, I am not a plumber!

beenthere

The program allows manual input so as to obtain the sensible/latent ratio you need. I believe it limits air flow to 450 CFM per ton though.

beenthere

5" will cover your bathroom. Its good for about 55 CFM quietly.

Pawl

1.) Under tools for the program I clicked on "CFM per room calculation"

2.) Next I have to choose either one air handler for the whole house or one air handler for each zone.

3.) Next I have to choose the cfm for the equiptment chosen for this house. I assume 1.5 ton equates to 600 cfm for the whole house, 2 ton is 800 cfm and 2.5 ton is 1000 cfm but it tops off at 900 cfm. The choice starts at 300 cfm and goes up in 50 cfm increments 350,400,450 to 900 cfm.

4.) So whatever amount of cfm I chose between 300 and 900 it gives me a breakdown for the whole house room by room with one column for cooling cfm for each room and one column for heating cfm for each room.

5.) So I chose 600 cfm and got 13 cfm for upstairs bathroom, then I chose 800 cfm and got 18 cfm for upstairs bathroom, then I chose 900 cfm and got 20 cfm.

6.) I did not do what beenthere is talking about with entering manually because I dont know how to do that nor even understand what that would tell me. But I am sure it can be done.

digitalplumber

Thats interesting, I always assumnned that the purpose of a manual j was for it to ultimately tell YOU the total cfm necessary based on the numbers you entered?

So do you then look at the final Sensible and Latent cooling BTU's and compare these 2 to determine if your original unit size is correct??

Do you have to enter a number there?

__________________
digitalplumber is just a name for use here, I am not a plumber!