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Old 02-22-2013, 05:03 PM   #76
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Will the size of my ducts have to be changed?


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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Old 02-22-2013, 05:11 PM   #77
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Will the size of my ducts have to be changed?


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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.

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..
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Old 02-22-2013, 06:24 PM   #78
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Will the size of my ducts have to be changed?


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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.

1.) 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..
I plan on using rigid duct. My goal is to get all the information and help I can here and then I am going to meet with a HVAC business that came highly recommended from a friend of mine who has a fabricating sheet metal business. I want him to let me do some of the work meaning the installation of the ducts in my house. I am retired and have worked on many many aspects of my home. I have more time than a company on the clock.
The HVAC people can do the install of the AC/Heating unit/balancing /thermostat/electrical because thats a specialty. I want the supply/returns and ducts put in correctly. I found the original ducts in my walls not sealed and heat has been blowing in the walls since 1965. They probably will be glad to have someone else crawl around my attic. I will have to have him calculate everything before we can figure out duct sizes etc,... My worry is as of now I am somewhere in the area of needing a 2.5 ton unit. If they start trying to talk me out of it by suggesting a larger unit I will be finding someone else. I will have my load calculations with me, weather data for my area so any good HVAC company should be able to work their magic. I have done a lot of the homework on this website.



(As I recall, you base a rooms cfm on the higher ac number.)<<<<<< -

2.) Could you explain this statement some more. What is the higher ac number and where are you finding it? Are you saying the results of a Manual J load Calculations should be printing out the AC number?
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Old 02-22-2013, 06:29 PM   #79
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Will the size of my ducts have to be changed?


In your opinion will an area at the top of these stairs need a supply or return vent? The door you are looking at is the bathroom door and there are two more bedrooms up there to the right.
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Old 02-22-2013, 06:41 PM   #80
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Will the size of my ducts have to be changed?


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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Old 02-22-2013, 07:18 PM   #81
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Will the size of my ducts have to be changed?


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In your opinion will an area at the top of these stairs need a supply or return vent? The door you are looking at is the bathroom door and there are two more bedrooms up there to the right.

If it were me, I would have a return at the top of the stairs and one in each bedroom.
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Old 02-22-2013, 07:25 PM   #82
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Will the size of my ducts have to be changed?


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Originally Posted by Pawl View Post
I plan on using rigid duct. My goal is to get all the information and help I can here and then I am going to meet with a HVAC business that came highly recommended from a friend of mine who has a fabricating sheet metal business. I want him to let me do some of the work meaning the installation of the ducts in my house. I am retired and have worked on many many aspects of my home. I have more time than a company on the clock.
The HVAC people can do the install of the AC/Heating unit/balancing /thermostat/electrical because thats a specialty. I want the supply/returns and ducts put in correctly. I found the original ducts in my walls not sealed and heat has been blowing in the walls since 1965. They probably will be glad to have someone else crawl around my attic. I will have to have him calculate everything before we can figure out duct sizes etc,... My worry is as of now I am somewhere in the area of needing a 2.5 ton unit. If they start trying to talk me out of it by suggesting a larger unit I will be finding someone else. I will have my load calculations with me, weather data for my area so any good HVAC company should be able to work their magic. I have done a lot of the homework on this website.

I agree knowledge is power and if I were you, I would rather err on the too large side than too small and regret it. BT has been giving you good sound free advice!



(As I recall, you base a rooms cfm on the higher ac number.)<<<<<< -

2.) Could you explain this statement some more. What is the higher ac number and where are you finding it? Are you saying the results of a Manual J load Calculations should be printing out the AC number?
Cooling and heat cfm requirements are different, at least when you have a gas furnace for heat. Not sure how it works for full electric and heat pump, but I think they (cfms) are the same.

For instance in my home a room calls for 244 cfm cooling and 172 heating cfm. I think the theory is its easier to heat than cool.

The manual j should tell you all of this, unless I am mistaken.

The one I have for my home given to me by the builders ac company, has all of this info.
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Old 02-22-2013, 08:23 PM   #83
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Will the size of my ducts have to be changed?


another website to fuel your knowledge thirst:

http://www.furnacecompare.com/
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Old 02-22-2013, 08:38 PM   #84
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Will the size of my ducts have to be changed?


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?
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Old 02-22-2013, 08:43 PM   #85
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Will the size of my ducts have to be changed?


Quote:
Originally Posted by Pawl View Post
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?

Are you talking about the CFM, yes its based on your entries. Does it say these are cooling or heating cfm?
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Old 02-22-2013, 11:42 PM   #86
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Will the size of my ducts have to be changed?


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.

Last edited by Pawl; 02-23-2013 at 02:05 AM.
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Old 02-23-2013, 01:18 AM   #87
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Will the size of my ducts have to be changed?


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?
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Old 02-23-2013, 01:50 AM   #88
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Will the size of my ducts have to be changed?


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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Old 02-23-2013, 04:04 AM   #89
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Will the size of my ducts have to be changed?


Quote:
Originally Posted by Pawl
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.
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Old 02-23-2013, 09:17 AM   #90
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Will the size of my ducts have to be changed?


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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