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I need some advice. I have an addition that was not cooled or heated as well as the rest of the house. I found out that two of the three ducts were disconnected and when I reconnected them the addition cools to within one degree of the rest of the house on a 90 degree day. A contractor wishes to upsize two of the runs to the addition from 6 to 7 inches and enlarge the registers. One of the runs he wishes to upsize comes off near the end of the 10 foot long trunk line that is 10x8 inches and has two other 6 inch runs coming off it. I know this is simplistic but a 6 inch circle has an area of 28.27 inches and a 7 inch has an area of 38.4 inches. 2 six inch plus one 7 inch run is a total of 94.94 sq inches. Three 6 inch runs have a total area of 84.81 sq inches which is close to the 80 square inches the 8x10 trunk. I am concerned that the 7 inch duct will steal air from the other 2 ducts coming off this trunk?
The trunk feeding the 8x10 trunk is a 28 foot long 8x18 and has 3 six inch branch ducts coming of it as well a 2x17 register and a 10x 3.5 inch register that come off nearly directly from the trunk. The contractor wants to upsize one of the 6 in branchs to the addition to seven inches.
Using my simplistic method the 8x18 currently supplies a 8x10 trunk and 3 six inch ducts and two registers. This is an area of 84.81 for the tree 6 inch runs and 25 and 25 inch for the registers.[ I decreased the area of the registers to approximate the open space between the grills]
This is a total of 214 sq inches. The 8x18 only has an area of 144 sq inches.
My questions are:
Are the demands on the system greater in the winter than in the summer since the temp differential is greater in the winter. It freq gets into the teens in the winter and occasionally to below zero. So my addition with 3 sides to the weather may have more of a problem in the winter than in the summer.
Short of a manual D which I do not know how to do and it does not seem that the contractors around here do them either, does it seem that upsizing two of the runs will increase air flow without stealing significant air from the other runs? The contractor say he will keep dampers on the 7 inch runs in case there is a problem. If he enlarges the registers and we don't get more air flow I worry that this will decrease the "throw" of the a/c air.

2. Originally posted by heetseeker
I found out that two of the three ducts were disconnected and when I reconnected them the addition cools to within one degree of the rest of the house on a 90 degree day.
Sounds like you've solved the problem. Why change anything?

3. Heat, why improve now what is working well.

See how it works in heat, before you fall into attempting to improve what can't.

But if you really want to spend money, e-mail me and I'll give you my address to send me a check for no reason at all.

LOL...
Sorry, just couldn't resist.

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I guess I forgot to say that I am having the current crimped flex duct replaced with insulated metal duct so I am going to do something anyway. The choice is between 6 and 7 inch round ductwork.

5. Why? Need some additional noise in that area? That's what you'll get with the metal. Like trading a ziplock bag for one of those flap ones. Still sounds redundant to me.

6. Originally posted by heetseeker
I guess I forgot to say that I am having the current crimped flex duct replaced with insulated metal duct so I am going to do something anyway. The choice is between 6 and 7 inch round ductwork.
Then make sure the take offs have dampers in them, or you might regret the noise.

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I was surprised about the noise comments. My current metal duct outside the addition don't make noticeable noise. Are you saying that 6 inch or 7 inch or both will be noisy. Is it because too much flow is going to the registers or just because you think metal is noisier than flex?

8. Originally posted by heetseeker
or just because you think metal is noisier than flex?
Sound attenuation. Correct velocity and volume MAY over come this.

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size isnt ...

...everything.How far away is the return? You may get by with adding a return rather than throwing off the supply line. You've done your homework on the hole sizes but I can tell you that you will lose static pressure when you add a supply off of that trunk which will affect the air throw, FPM and changes per hour. Solution? Do a heat load for the whole house and size the ducts accordingly using whats already there and adding whats needed

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The runs the contractor wishes to upsize are each 21 ft long.

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The runs the contractor wishes to upsize are each 21 ft long. Will the air velocity at the register change if the 7 inch is too much and we damper it down as compared to a 6 inch run?

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You have already said the contractor hasn't or doesn't know how to run the calcs to determine what you need.

So he is guessing.

You don't have the ability to run the numbers.

So you are guessing.

We don't have all the info.

So we are guessing.

here is something that is not a guess.
The system works now that you have all the ducts hooked up.
It takes more to move cool air than hot air.
You are concerned if the system will heat well.

If this were my house, I would leave it as it is, until I had a real problem that needed to be fixed. Why guess at something when you might not have a problem at all? What if your solution creates more problems, how will you know what to do? You won't know if the system is going to work in the winter until winter comes. You already have it working in the summer. Leave it alone until you know what the actual problem you need to fix is.

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Thanks for the replies.
Daveslo, What you said is what I have been thinking. Your comment about it being more difficult to move cool air is kind of what I waas looking for. Although the temp differential is greater in the winter, the lack of heating or cooling was about the same in the addition in the summer as in the winter. So it sounds like you also think that a room that is adequately cooled in the summer will most likely be adequately heated in the winter even in a cold climate like northern Illinois.

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