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Old 09-21-2008, 02:51 PM   #1
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Increase duct size to cool room?


Our spare bedroom stays too hot/cold year round. Thermostat is in another room. The spare room has 2 very large windows facing south with a 6 inch duct and register and a return. It is always 4-5 degrees different from the room with the thermostat. I have purchased everything to increase to a 10 inch duct and register. Wanted to see if there is any reason to not make this change. I figure that we can control the temp by opening/closing the register in this room and that a 10 inch duct would give us more control than only upgrading to an 8 inch duct. Already moved the 6 inch duct from the side to the top of the air handler with no noticeable effect. Opening/closing registers in room with thermostat is not practical or efficient for us. Thanks.

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Old 09-21-2008, 02:56 PM   #2
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Increase duct size to cool room?


Your system was not designed properly or is not balanced.

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Old 09-21-2008, 07:35 PM   #3
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Increase duct size to cool room?


I would try it or add another 6 inch run to the room. Just make sure to put an inline damper in the duct to allow you to regulate the airflow somewhat. Also you should try and have a register under each window as this is the greatest heat loss/gain in your room.
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Old 09-21-2008, 08:54 PM   #4
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Increase duct size to cool room?


Chris is right. Your system was poorly designed.

I doubt you'll have success with increasing the duct size. If anything, the reduction in static pressure that the larger duct would create might lessen the duct's effectiveness.
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Old 09-21-2008, 09:42 PM   #5
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Increase duct size to cool room?


Some friends had a house that had had an addition added onto it, naturally at he far end of the house from the air handler. It was pretty easy to fix once we thought about it. Yes it had thrown the system out of balance, because it had added to the house.


The first thing that we did was to wrap the long run of pipe with a nice thick coat of insulation. That pipe was loosing a lot of cool running that far in a hot attic.


The house was in Florida and didn't have ridge vents, so we ran on the length of the roof. Attics in Florida get hotter than blazes, so we also threw in a thermostatically controlled fan and blew in insulation to the level it needs to be in Florida if you don't. A little of the track, but if your going to be up in your attic have a look around. I bought a house to revamp and about had a heart attack when the first power bill showed up. I climbed up into the attic that was just high enough to crawl down the middle up and was very surprised to find it getting cool as I neared the far end of the attic. I found that the plumbers that had re plumbed the house for they elderly lady that had owned it had leaned over the first duct leading out of the main to connect the water. They had bent it down and there was a 4 inch wide opening where the first duct and main met. They let that lady pay to air condition her attic for a lot of years.


Then we put in an air activated booster fan into the duct that lead to the addition. When the A/C kicked on the fan detected the air movement and turned itself on. The fan was adjustable for the amount of extra cool air it needed to pull to keep the addition as cool as the rest of the house. I would think that they all are. If you didn't pull enough the room would still be warm and if you pulled way to much that room would be freezing at the expense of the rest of the house.


I'm about 99% certain that I've seen the booster fans that screw over vents that would really make it easy. I wouldn't use one that you couldn't adjust the air flow. You might get lucky or you might throw the who house out of wack. We wrapped the duct in hopes of avoiding that.


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Old 09-22-2008, 09:44 AM   #6
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Increase duct size to cool room?


Use the 10" stuff you got and add a return from that room.
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Old 09-23-2008, 12:47 AM   #7
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Increase duct size to cool room?


You dont say much about the room besides the windows. Is it over a garage? Over a crawlspace? Does the room have any kneewalls? what about your hvac system--do you have a variable speed fan or no. Are you currently getting good airflow compared to the other rooms? do you have dampers and have you adjusted them? Thru what does the duct travel thru to get to the room? An exterior wall? Is it the longest run of your system? I highly doubt that increasing the duct size alone will do anything. A larger duct could very well decrease the velocity of the air. The booster fans;despite the post above, almost never work out right. You could have some insulation issues with the room that when fixed could solve your problem. I fixed a room over my garage here in cold ohio that was always 10 degrees or so off the rest of the house and I fixed it by fixing the insulation and air infiltration issues.
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Old 09-23-2008, 08:18 PM   #8
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Increase duct size to cool room?


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I fixed a room over my garage here in cold ohio that was always 10 degrees or so off the rest of the house and I fixed it by fixing the insulation and air infiltration issues.
This is where I would start.
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Old 09-26-2008, 04:57 PM   #9
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Increase duct size to cool room?


Thank you so much for considering my problem. I have decided to bring in a technician from the company that originally installed the system. I hope to use their evaluation and recommendations, along with your inputs, to decide on a course of action. This is an incredible forum and I really appreciate the time you all took to reply. I really like the idea of a second 6" line thru the attic to be placed over the windows of this second story room. The 10" material can easily be exchanged at home depot. Thanks again.
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Old 09-30-2008, 12:18 PM   #10
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Increase duct size to cool room?


A 6 inch duct will handle 140 cfm, a 7 inch dust will handle 210 cfm and an 8 inch will handle 300 cfm. You didn't state the total cubic feet off the room but the average room requires at least a 7 inch duct. If there is a wall stack then it needs to be a 3 1/2 x 14 to handle a 7 inch pipe.

Don't go with another 6 inch pipe. Go with a 7 or 8 inch pipe and if it is in the attic then make sure it is insulated. Better a big pipe that can be dampered down than a small pipe that won't work.

Make sure a damper that is accessable is installed so that fine tuning of the room can be done.

A return in that room will do nothing to fix your problem.
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Old 09-30-2008, 01:37 PM   #11
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Increase duct size to cool room?


The room is about 12'x12'x9'. Would you recommend a single 8" duct or another 7 or 8" in addition to the existing 6"? Thanks
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Old 09-30-2008, 01:54 PM   #12
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Increase duct size to cool room?


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Originally Posted by tchristian View Post
The room is about 12'x12'x9'. Would you recommend a single 8" duct or another 7 or 8" in addition to the existing 6"? Thanks
That is roughly 14000 cu ft. An 8 inch duct would give you 300 cfm and that is an air change of roughly 4.5 per hour which is good.

I would go with an 8 inch insulated (R8) flex duct in the middle of the room in the ceiling.

Make sure that the supply to that duct is large enough to feed an 8 inch line or it is a waste of time and energy.

I don't know where your furnace is located so I can't tell you how to plumb the room. But if you have a 3 1/2 x 14 wall stack then there is not enough air to justify an 8 inch duct and you would have to go with a 7.

Keep the 6 in there unless you can modify that to an 8 inch. Dependant on location that would be the only duct needed of you could do that.

Two 6's would not do the job.

The bottom line is that there has to be enough ducting to supply whatever you put up there.

Last edited by Marvin Gardens; 09-30-2008 at 02:00 PM. Reason: Added some clarification.
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Old 09-30-2008, 02:04 PM   #13
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Increase duct size to cool room?


Thanks! Easy to change the duct from 6 to 8". Runs from the air handler (which is huge and not over-taxed) thru the attic, 30' to the side of the room nearest the large windows. I will simply change the duct to 8" with an 8" collar, boot and register. Thanks for the input!
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Old 09-30-2008, 02:24 PM   #14
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Increase duct size to cool room?


Is there a return in that room? A return duct could do more to 'normalize' the room than changing the supply run.
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Old 09-30-2008, 02:33 PM   #15
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Increase duct size to cool room?


Quote:
Originally Posted by tchristian View Post
Thanks! Easy to change the duct from 6 to 8". Runs from the air handler (which is huge and not over-taxed) thru the attic, 30' to the side of the room nearest the large windows. I will simply change the duct to 8" with an 8" collar, boot and register. Thanks for the input!
Wow, that was easy.

Make sure it is insulated.

Let us know if that fixed the problem.

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