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Duct booster fan vs axial

871 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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Is this run through a hot attic space?
Flex at best is usually an r-8 and I have little confidence that it even performs that well. R-8 from fiber insulation would be 2" thick. Ducts also have a lot of surface area.

I'm a retired energy auditor and have tested heat loss/gain through ducts and the solution is just MORE insulation.

If they used a foil exterior although it can help it is not as magic as often claimed.

You mentioned a difficult installation, thus I assume adding more insulation would also be difficult, but any areas you can improve will help.

Where are you measuring the heat rise and what are the numbers?

The duct is gaining heat in two ways, convection from the hot air in the attic and radiant energy from the bottom of the roof above.

What measures do you have in place for attic ventilation, every little bit helps.

If you can slide in or wrap a foil faced product to shield that duct from the radiant source it could help. If exposed fiberglass then the radiant energy is just shinning right through. Although I don't believe the claims for foil/bubble/foil wrap it is a workable foil surface. Tough to pull in a roll or aluminum foil.

If new shingles are in your future they offer ones that reflect the infrared energy thus blocking it before it gets inside.

There are also paints that can be applied to the bottom of the roof to reduce its admittance.

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I've been contemplating how velocity would affect the heat gain. As the inside of the duct cools from the air flow the heat source drops and has to be replenished from wherever in the attic. Ultimately it is the replenishment rate that contributes to the heat gain. 2° maybe, but not sure that would make a big difference.

My preference would be to drape where possible a layer of some reflective material. Think of the hamburgers you buy wrapped in foil, they stay hot, in your case cool.

No need to remove the r-19. That is why I said draped and underlined it as opposed to wrap. As long as the foil is between the duct and the bottom of the roof it will be blocking much of the radiant heat. Some construction in the deep south uses roof sheathing that has a foil bottom surface.

If you were to start over you might omit the foil layer on the duct, but the duct itself is also a vapor barrier. Ideally a good air barrier would keep the humidity away from the cold surface but we are not in an ideal world.

Drape an easy section to access and compare the temperature under that area to another area without the foil drape obviously with the ac running.

All you need is the foil, but with a little insulation it adds some bulk making it easier to handle. I have used the foil/bubble/foil in a few places ant as a reflector it is fine.

Your choice.

As long as it is draped over the top and not wrapper around the duct I don't see any added risk

Overlap the edges along the length but do not wrap.
This is just a guess but the r-19 was not performing at its potential being exposed to the very hot bottom of the roof. Radiant heat transfer is not only significant it is fast and shines right through the fiberglass. Your total r-value should be doing better and the foil over the top is best I can suggest without starting over, and I know that is not an option.

"doesn't fiberglass work when it lays on the attic floor to shield heat from the attic into the house?"

Insulation likes to be enclosed on all 6 sides like inside wall cavities. To compensate for the poorer performance codes require a lot more insulation in a ceiling. When calculating we still use the r-19 when that is what is installed, but with a wink. There are many places in a home where an auditor has to fudge the numbers to get the actual heat gain/loss to match the calculated.

Before the extra insulation was added there should have been an air sealing effort. If they took the insulation level "to r-30" as opposed to adding r-30 to what was there then you have a minimum level of insulation, but there was probably a difference. To see the difference it would be in the cost of cooling and it gets a bit complicated adjusting one year to another based upon the weather. If the added insulation was a neat job adding more would make little difference. Only time I find it cost effective is to add more while they are there, very little added cost.

If you post a picture of that attic it might help.

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