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Will the size of my ducts have to be changed?

15K views 152 replies 9 participants last post by  beenthere 
#1 ·
I am in the early stages of hiring someone to install an a/c - heating unit in my two story house. I currently only have forced heating ducts in my house. My 1965 house was not built with an air conditioner. Only a heating unit. Overtime I have changed about 30% of the ducts using a thicker gauge duct and sealing all ducts with silver backed adhesive tape and topping that with cloth tape with a pasty glue on all joints meaning it will never leak or come apart. I used to be an insulator so I have some experience in doing this. I am thinking the size ducts I will actually need will probably be dependant on the size unit I buy for my house. My neighbor just had a 4 ton unit installed in his attic and he has 1400 sq ft. I have 1950 sq ft and I am thinking 5 ton just because of increased sq footage. I still have a lot to learn. My ducting goes from a 24" plenum attached above the 1st floor heating unit to 8" horizontal ducts reducing to 6" ducts for 1st floor rooms. There are two 8" ducts coming out of plenum and one is used for 1st floor rooms farther away from unit and the other 8" duct is sending air to the vertical reducer headed upstairs which reduces to a 6" duct for the two rooms upstairs. The current heater I have says its input is 100,000 btu/hr and the bonnet capacity is 80,000 btu/hr. I am guessing this would have helped determine size of the duct back then. Is it possible to do the size layout I need by myself,maybe getting it verified by someone in this field and then get a good installer to finish the job? I am retired, remodeled most of my house over the last 25 years so I think this is something I could do. I have been in my attic many times insulating ,running new ducts,etc.... I would appreciate any input. Thanks
 
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#71 ·
So, I also researched the Winter temperatures in my area.The point of measurement was 2.5 miles away from my house. Any thoughts on what I will need in the way of heating.

Days under 38 degrees during each year below:

2007 - ( 6 days under 38 degrees) 32 degrees 2x, 33 degrees 1x, 37 degrees 2x,
2008 - ( 10 days under 38 degrees) 34 degrees 1x, 35 degrees 2x, 36 degrees 2x, 37 degrees 5x,
2009 - ( 7 days under 38 degrees) 35 degrees 2x, 36 degrees 2x, 37 degrees 3x,
2010 - ( 5 days under 38 degrees) 33 degrees 2x, 36 degrees 1x, 37 degrees 2x,
2011 - ( 15 days under 38 degrees) 31 degrees 1x, 32 degrees 1x, 34 degrees 2x, 35 degrees 5x, 36 degrees 4x, 37 degrees 2x.
2012 - ( 11 days under 38 degrees) 33 degrees 1x, 35 degrees 3x, 36 degrees 2x, 37 degrees 5x,
2013 - ( 16 days under 38 degrees) 31 degrees 1x, 32 degrees 2x, 33 degrees 3x, 34 degrees 1x, 35 degrees 2x, 36 degrees 3x, 37 degrees 7x,
* the numbers seem to point out that in the last 3 years it is getting colder here
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
38 degree days during each year below:
2007 zero days
2008 - zero days
2009 - 3 days
2010 - 4 days
2011 - 2 days
2012 - 2 days
2013 - 5 days
39 degree days during each year below:
2007 - 8 days
2008 - 6 days
2009 - 4 day
2010 - 2 days
2011 - 16 days
2012 - 6 days
2013 - 3 days
40 degree days during each year below:
2007 - zero days
2008 - 1
2009 - 7
2010 - 5
2011 - 8
2012 - 3
2013 - 1
41 degree days during each year below:
2007 - 8
2008 - 5
2009 - 8
2010 - 11
2011 - 13
2012 - 12
2013 - 3
42 degree days during each year below:
2007 - 10
2008 - 9
2009 - 12
2010 - 11
2011 - 10
2012 - 5
2013 - 4
43 degree days during each year below:
2007 - zero
2008 - 3
2009 - 18
2010 - 12
2011 - 9
2012 - 15
2013 - 2
 
#74 ·
This picture shows the existing heating supply register. Just for conversation I feel that the new supply register will have to be moved between this ceiling fan and the window you are looking at. I am just extending the supply ducting toward that window. Moving it will also create distance between supply and return.

That is the east side of my house which gets sun on this window from sunrise until about 11am. That is a small round recessed light in the picture. There are four in the ceiling.

The new return will be installed somewhere right above the entrance to this room in the ceiling. With it located in that spot it will put the return closer to the new AC unit area. The entrance to this room is at the end of the hallway. About 8' back down the hallway is the entrance to the original heater in the closet.

Is their an advantage to having the return ducts as close as possible to the AC unit? For example I would think in a bedroom you can mount these ceiling registers/vents anywhere you want but there is obviously a sweet spot for both which will be the most advantageous in performance.
 

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#75 ·
Again, basing on what I have read. Your supply vents need to be over the windows as you have indicated. All rooms.

As far as the return air, across the room from them. A short path back to the return, I assume, is a great idea but maybe getting to worrysome to accomplish or not even a worry.. Remember, you are adding this to a room you did not have before so it has to help.

Hopefully the real experts will chime in soon.
 
#76 ·
This is one of the bedrooms on the west side. It gets sun from about 2pm until sunset so on a hot day this room warms up.It looks to me like this supply duct is probably fine in this spot because it is very close to the window and far enough away to create distance from the return vent. I will just be changing the the old registers for new ones unless someone here has a different opinion or HVAC guy tells me something I dont know.
 

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#77 ·
Again you need to know what the manual j says for the correct amount of ac cfm should be for that room. (As I recall, you base a rooms cfm on the higher ac number.) Once you know that, you can determine if the current discharge vent and supply duct is still the correct size for the room.

Remember it takes a larger flex duct size to move the same air as a solid duct of smaller size rated for the same or close to cfm..
 
#80 ·
Those round supplies are terrible for heating.

Switch to 3 way rectangular ones, that will throw the air toward the outside wall, and the 2 perpendicular walls.

Returns near the air handler sometime give more noise. How close or far they are to the air handler has little to no effect.

If you entered each room individually, then you have a room by room load calc, and the program will print out the CFM each room should have.
 
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#84 ·
I found the duct sizing tool on the program. I checked all the boxes for a total of 653 CFM.

459 cfm on the 1st floor:
~~~~~~~~~~~~~
bath rm 16
master bath 17
living rm 110
kitchen 79
family rm 98
front entry 27
hallway 6
office 42
alexis bed rm 56
dads bed rm 61

148 cfm on 2nd floor:
~~~~~~~~~~~~~~~
allison bed rm 69
extra bed rm 50
bath rm 12
upstairs landing 10

When I click on supply or return it defaults to 16 x 8 duct. There are two other choices for duct: 12" round or 12 x 10.

* my current heating ducts go from a 24" plenum with 8" tapped into plenum reducing to 6" to each room. The two bathrooms downstairs were 5" coming straight off plenum and bathroom upstairs branched off of 6" reducing to 5".

When I click on return the speed of the air selection defaults to 700. Supply defaults to 900.

1.) Are these numbers based solely on load calculation information I entered into the program? Whatever size unit I pick is separate from these numbers?
 
#86 · (Edited)
I played with the duct sizing portion of the program again and here is what the program came up with:

1st floor shows a total need of 459 cfm. For supply they show 4 configurations of ducts to move this amount of cfm. They also show a figure of 512 cfm that is the maximum these 4 configurations will carry. Choices are 10" round, 14x6, 12x7 or 10x8. I also chose rigid duct construction. The program defaulted to 900 on speed of the air.
The return ducts show all the same numbers and same choices of duct sizes as the supply ducts.The speed of the air went to 700 as I was checking the boxes of what rooms to include. Return ducts are same size duct as supply?

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

2nd story shows a total need of 148 cfm. For supply they show 3 configurations of ducts to move this amount of cfm.They also show a figure of 149 cfm that is the maximum these 3 configurations will carry. Choices are 6" round,8x4,or 6x5. Rigid duct construction.Speed of the air defaulted to 900.
The return ducts show all the same numbers and choices of ducts as the supply ducts except the speed of the air went to 700.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If the 6" round ends up being the right size for up stairs I have about 37' I used on the 1st floor that is only about 5 years old and a thicker gauge metal.
 
#87 ·
1.) With the right size ducts figured out now for the total amount of cfm needed will the balancing of each room be dependant on the air speed and the dampers?

2.) If what I just asked is true are the settings for heating and air conditioning based on different speeds for the blower?

3.) Now that I have figured out my duct size for the layout that will not change if I went with a 2 ton unit or 2.5 ton unit?
 
#88 · (Edited)
The duct size for upstairs 149 CFM requires 6" round duct. There is a transition that goes from round to rectangular that passes vertically up the back closet wall from 1st floor attic to 2nd story attic. So the rectangular transitional piece that was in use was 14 x 3 which is 42 sq inches and 6" round is 28 sq inches so the original rectangular transitional duct might have been too big from the start. What do you think?
I would like to do maybe a 10" x 3" = a 30 sq inch rectangular duct and it would leave me room for insulation. I want to be able to have room for insulation around this 9' long two piece duct.The original duct fit so tight there was no room for insulation. Plus the return duct needs to go beside it.I have to figure out how to make that work. I dont think i could find too many companies that would go to this great length to do this right without charging a lot of $$$. And I dont blame them but I can do this.
 

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#89 ·
Pawl said:
1.) With the right size ducts figured out now for the total amount of cfm needed will the balancing of each room be dependant on the air speed and the dampers?

2.) If what I just asked is true are the settings for heating and air conditioning based on different speeds for the blower?

3.) Now that I have figured out my duct size for the layout that will not change if I went with a 2 ton unit or 2.5 ton unit?

Yes fine tuning the balance of each room will be by dampers at the supply plenum. Realistically because of leaks, turns and friction you expect the numbers to be off some at the supply vents, as I understand.

Yes and obviously controlled by the thermostat when set to heat or cool mode. A furnace can be made for multiple ac applications: 3, 3.5 or 4 tons for example. The unit should be verified upon install that the correct ac cfm is set. Based on 400 cfm per ton: 3=1200, 2=800 for instance. Also, the furnace will have heating cfm setting, usually much lower than ac cfm. If you are asking will you have to make manual seasonal adjustments in the dampers, I would say no you will find a happy medium.

Not sure what you are asking here? Once again I believe this is decided for you by your entries in the program, heating btu's and ac tonnage.. You have to look at what the program is saying your cooling needs are and further what it is suggesting for duct size relative to the ac size. keeping in mind though, in my opinion, that if your home is older and poorly insulated and sealed coupled with a borderline cooling suggestion say close to 2.5 tons, I would go with the higher number.

If you go the 2.5 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.
 
#90 ·
This is the duct sizing for upstairs. At the bottom of the boxes to be checked I have checked the 4 boxes for the 2nd story CFM. You can see the 3 choices of duct size to accomplish the task. Rigid duct is chosen and the speed of air is at 900.Like I said earlier the speed of air defaults to 900 with the more boxes I checked.This picture is for supply air duct sizing.

1.) Regardless of whether I choose 2 ton or 2.5 ton this recommended size ducting will not change. Is that correct?

Currently in my house now the heating duct going to the 2nd floor is 8" round and it travels across the attic but the closer bedroom supply register branches off with a 6" fitting and 2ft of 6" round size running to that bedroom supply register. The new register in this room is going to be moved an additional 4 feet.
The farthest bedroom register which is basically at the end of the 8" run has a fitting on it that reduces the 8" round to 2 ft of 6" round with a 5" round branch that travels for 9' to the bathroom supply register.
2.) My question is the duct sizing on my manual J calcs says I can use 8" round to get the right cfm upstairs but it gives no smaller reducing sizes for branching off to each supply register. Any thoughts?
 

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#92 · (Edited)
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!
 
#91 ·
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.
 
#93 ·
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.
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.
 
#94 ·
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
 
#95 ·
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
 
#96 ·
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
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?
 

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#97 ·
Pawl said:
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?
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.
 
#98 ·
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.
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.
 
#116 · (Edited)
I thought I had everything on my load calc but found a couple more things but hopefully this is the grand total but maybe not. I was reading through all the instructions from the program one more time and they mentioned if I get a low cooling load its because I might have done a couple of things they recommend not to do. The only one I did not do was eliminating all outside shading and inside window covers. They said calculate with inside windows not covered up.So I took them out and the total BTUH on Total Heat Gain went from 15,643 (1.5 tons) with shading,blinds,curtains,roll up shades included to 24,313 BTUH (2 tons) with shading,blinds,curtains,roll up shades NOT INCLUDED.
 

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#118 · (Edited)
I increased that outdoor Summer Temperature on the Manual J program to 86 degrees and it still showed I should use a 2 ton unit. When I increased it to 88 degrees it went to a 2.5 ton unit. I think 2 ton is looking like my choice. What do you think guys?

The Total Heat Loss is 37,849 BTUH. The program says to add 20% to that for a total of 45,419 BTUH. Do they round these amounts to 50,000?

My 1975 heating unit is rated at 100,000 BTUH (input) and 80,000 BTUH bonnet capacity. Of course the house had single pane windows,very little insulation from day one. The house was built in 1965 and this heater says 1975 on it. Maybe the original heater was rated lower and they purchased a larger one or maybe the original one took a dump. Maybe when they bought a new one they told them they wanted a larger one because the original one was not getting it done or the installer sold them a larger one.
My heater is over 30 plus years and it works okay but now I am going from a 100,000 BTUH / 80,000 bonnet capacity to a recommendation of 45,419 BTUH. Is that possible?
 

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#122 ·
Again, as one homeowner to another, I go back to the scenario I had. Although they claimned 3 ton in my home was adequate, I found that the high summer temp they used wass not realistic and the lessor 3 ton just ok.

Reminds me of software for a computer, it says on the box a "suggested" minimum computer spec for software use. But if you stay with the minimum, it will work but be slow. If you increase, obviously it works better.

I read back thru this and I believe that on at least 2 times, BT has suggested that 2 tons would just barely be enough.

You have to make the decision: do you go 2 tons knowing that if you go out of your design temps in summer, that your unit may run a long time with minor impact. Or do you go to 2.5 knowing that you will be comfortable all of the time.

Me, I am going with the 2.5!
 
#120 ·
Beenthere - They mention Arizona/Nevada in the program as an example of states located in a dry climate. I am just assuming California is included even though it is not mentioned. We dont have much humidity at all where I live. The program says latent gain is not included in a dry climate so when I am shopping that whatever unit I get is going to be based entirely on sensible gain. Correct?
 
#121 ·
Your Summer grains of moisture is 84, so your not in a dry climate. A 2 ton unit will still just meet your sensible loss. You can use a 50,000 BTU 80% furnace. Or a 40,000 BTU 95% efficient furnace.
 
#123 ·
1.) Is the 50,000 BTU 80% furnace the same type I am using now?

2.) Would you explain the 95% efficient furnace? It sounds like I am getting less loss of heat with this style.

3.) Would you recommend one over the other? Pros/Cons

- is one clearly better than the other?
- price?
- anything else?
 
#127 · (Edited)
BT - will this information you mentioned on an earlier post apply to my 1000 cfm breakdown below? ------> The duct system should be designed for the full 400 CFM per ton (1000CFM for a 2.5), and then the blower set to only move 350 CFM per ton(875CFM). The installers should balance it out to each rooms needs then. With any excess air being equally allotted to each room.
 

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#138 ·
I used the duct sizing system and this is what I got for the "supply" for upstairs. I entered 1000 cfm and for "speed of air" it defaults to 900 with the word "recommended" beside it which is what BT said (875). The current trunk line is 8" for heating so the 7" round for supply would be great because of a couple of tight spots.
For "return" ducts it says the same thing as "supply" ducts the only difference is they reduce the speed of air to 700 with the word "recommended" next to it. Is that sound right? Then are the "return" "ducts" calculated at 75% the speed of air of the supply ducts?
 

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