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Installing Ducting for New Home Addition

3109 Views 12 Replies 2 Participants Last post by  marshstate
I'm adding 1,000 sq. feet that has a bedroom, den, two full baths, and a laundry. I am trying to save a buck by installing the ducting myself. I think I want to use the coil ducting, rather than the stiff-type. Here are questions that I don't know answers to.

1. What diameter do I use? Is it different for bedroom, den, bathroom, laundry?
2. Can I attach coil ducting to the vent outlets with zip ties and still meet code?
3. Can I lay the ducting along the ceiling rafters, rather than suspending them from the roof rafters and still meet code?
4. Is it ok to run two air vents at opposite ends of the bedroom and the same at in the living room?
5. How do I end the ducting in the cavity where the return air vent will be?
6. What am I not considering?
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Who is doing the addition?

Are you replacing your existing HVAC A/C,Furnace or adding another system?

While just adding a supply line here and there and doing the same with return seems OK it is not.

You really need to be sized correctly taking into account the following

1. Sq ft of addition
2. Doing a Man J for heat gain/loss.
Doing the man j will take into to consideration for windows, insulation, exterior walls, and much more
3. Needed capacity of HVAC equiptment (heating and cooling)

Basically, it may be wise to hire a pro to caculate and design for the equiptment and duct you will need.

When sizing duct you will have to learn sizing for CFM, Sizing for static pressure, Sizing for friction loss.

It would be wise to have a pro help you plan for your addition so that when every thing is done you have a system that conditions the air through out rather than not.

I have seen so many jobs where it is just hacked together and then we have to tear out to fix the problems with the duct.

Every situation is different so planning is always an on going project. Spend a little now for help or spend alot latter when it has to be torn out.
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Google Manual J

Manual J is HVAC Residential Load Calculation most utility companies will come out for free and do this. This will tell you what size system you need for heating and cooling.

Manual D is for duct sizing

It all starts with the "J"

Yes it may cost something to get help with design but it will be worth it.

For example a rooms on the South need more CFM than the North and a room on the SW needs a little more than a room on the NE.

All this is considered in duct sizing plus cfm, length of runs and size, register size.

I know you don't want to hear this but, hire a pro to help.

Who are you using to run gas lines and to set up the equiptment once it is installed?
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If you have a tech to help ask him?her if they can size duct.

Try this link for software, this one is free

There is another site that HO can use and it cost about $50 but I can't find it
CFM = Cubic Feet per Minute


Let's say that you do the calculation and it says you need a 1 ton A/C

For every ton of cooling you will need between 350/450 of CFM.

Then you size the trunk to 400cfm and let's say for 400 CFM you need a main trunk size 10 X 10 at .1 static pressure.

We use a duct-a-lator to size

So the trunk is 10 x 10 and the transisiton from the furnace will be what ever size the opening of the furnace/coil will be to the main trunk.

From there you will size off the main trunk with branch ducts

Just keep in mind that A/C require more air flow than heat so size off A/C

Hope that helps
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2 ton A/C for cooling and I don't see anything for heating but I would go with a 50K BTU furnace. 50K will be fine for 1000sq ft.
2 ton system needs 800 cfm

Main trunk needs to be 10 x 14 or 140 sq in, so if you don't have room for 10 x 14 you can change the size so that the total sq in equals approx 140

Then you will need to size the rooms keep in mind the following

Duct size = CFM

6" = 110 cfm
8" = 220 cfm

When you size for rooms you cannot exceed 800-900 cfm total branch runs

See, seven 6" runs will be 770 cfm you just need to keep it in the range of 350/450 cfm pr/tn or in your case 2 ton equals 700/900 cfm.

This is where your tech will come into play for the room size

Return air size will be the same size as supply air. You could go as large as 20 x 20 in a filter grill and reduce down to the 140-150 sq in range or just a little larger.

Static pressure comes into place when sizing and you need approx .2 in the return and .3 in the supply for a total of .5 total static pressure. The supply will be a positive pressure and the return will be a negative pressure. Even though one is a + and one is a -, you still add them together to get a total. Residential furnaces are set for a .5 static pressure. You use a Manometer to measure this in inches of water.

Clear as Mud?:eek:
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