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Duct booster fan vs axial

864 Views 21 Replies 3 Participants Last post by  dorlando
I am having a heat gain issue in my flex duct. Airflow seems good but I am picking up a lot of heat in the 45 foot run. It was suggested to try a duct fan to move the air faster through the duct so it won't pick up as much heat.

It was suggested I use an axial fan vs booster fan. I can't find much on the axial fans. Any suggestions as to which one I should use? Is it best to put it closer to the register or closer to the supply? It will be a 7 or 8" depending on where I place it because I have a reducer in the line
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I don't disagree but I don't know why I have the heat gain. I have talked to two hvac guys and they are not sure either. I have stretched it tight, I don't have any 90's or deep sags, etc. One guy thinks that I needed a bigger duct because he thinks the air is just swirling around giving it more time to heat up, but the airflow is pretty strong out of the register.

I have been told I should not be picking up that much heat gain even in a 45 foot run. It was very difficult to install so I am not looking to do it over or replace it with a larger duct.

Any ideas on why I can be picking up so much heat or is that normal for flex?
Hi Bud, thanks for your help. I actually did wrap R 19 around it. It isn't air tight as I just took strips of batts and wrapped it. The entire length is not wrapped but a lot of it is, small sections open here and there. I think the flex was R 6. The R19 helped a little but still getting 6-7 degree heat gain. Yes, it is a hot attic in Florida.

The rest of the house has the R4 ductboard. The run to our bedroom using the duct board is about the same length and I don't pick up as much heat.

I don't mind trying a duct fan if that will speed up the air through it.
Hi Bud,

I am measuring at the plenum and then the register into the room. I also went into the attic and measured the temp where the reducer is in the duct, I had 3-4 degree heat gain in the first 25 feet of the run. The attic does have soffits and a ridge vent.

The flex duct has the original foil insulation plus the R 19 I wrapped around it.

Do you think a duct booster fan would work to push or pull the air through the duct faster so it doesn't have as much time to heat up? If I can shave two degrees that would probably make a big difference.
Even though the flex has the foil around it, add more foil?
Bud, I was doing some reading on what you said. Are you suggesting just a foil radiant guard or a foil with insulation like Reflectix? Also, since I wrapped R 19 around the ducts should that come off first? Lastly, do I need to worry about condensation issues by draping this around the flex duct that already has a vapor barrier?
What type of foil do you recommend? I saw one with just a foil and one with a foil plus some insulation?
I found something called Radiantguard, which is just the foil. Can this cause any condensation issues since I already have insulation and a vapor barrier?
I appreciate your help. I am going to ask this just to be safe and not to be redundant or as an insult. I am not as educated in this as you are. I've already spent a lot of time and some dollars on the R 19 and saw little benefit. I understand you are suggesting the foil to block the radiant heat but in your opinion how much of an impact do you think it will make? I assume I should overalp the edges by an inch. Thanks again.
Thanks. For my understanding doesn't fiberglass work when it lays on the attic floor to shield heat from the attic into the house?
Gotcha. I will see what I can find in the foil, try to find something cost effective. I may just go with the foil only since I already have insulation underneath.

Separate question. My house is from the 70's. I had just a few inches of celluose in the attic and I had some more blown in. It was by Certainteed. It wasn't cellouse, maybe some form of fiberglass. They supposedly blew in R 30 but I don't think we ever felt much of a difference. Should I blow in some more and what is the best type to blow in?
Bud, attached area couple of pictures. You can also kind of see the duct that I wrapped with some reflectix. I also used a couple of windshield visors to try and wrap most of the duct.


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