# NFA of Turbine vents



## RadiantRoofer (Aug 11, 2009)

Hi, I would like to know if anybody knows what the nfa rating is for a turbine vent? And if you, also can only find a declaration saying that two will vent a 1300ft attic?

Thanks

Radiant


----------



## Gary in WA (Mar 11, 2009)

http://books.google.com/books?id=cBa9C_yVna0C&pg=PA75&dq=turbine+vent+NFVA#v=onepage&q=turbine%20vent%20NFVA&f=false

The problems will turbine vents: http://books.google.com/books?id=Z8...=book_result&ct=result&resnum=9#PRA1-PA604,M1 Sorry, page 610.

Be safe, G


----------



## Ed the Roofer (Jan 27, 2007)

I typically see those turbine vents not moving due to rust or other environmental conditions or mechanical problems, such as the ball bearings wearing out, but as far as the "Initial" functionality, they work quite well.

Ed 


http://www.airvent.com/professional/...windTurb.shtml


http://www.airvent.com/professional/...nd-specs.shtml


*Shttp://www.airvent.com/professional/products/staticWind-specs.shtmlpecifications
Airhawk® Wind Turbines*


*Size**Type**Description**Color*12"AluminumInternal brace
with baseMill, Black, Brown, White and Weatherwood12"AluminumExternal brace
with baseMill, Black, Brown and Weatherwood14"AluminumInternal brace
with baseMill, Black, Brown, and Weatherwood
*HOW MANY WIND TURBINES DO I NEED?*
Use the chart below to determine the number of wind turbines you'll need for various attic sizes.
*Attic Square
Footage**12-inch Wind
Turbines Needed**Attic Square
Footage**14-inch Wind
Turbines Needed*Up to: Up to: 130021700218003220032400428004


----------



## RadiantRoofer (Aug 11, 2009)

Ed, I agree with you. In most cases, by the time the roof is being replaced and I am called in the turbine hasn't been moving for awhile. But I am really interested to know what the nfa on the turbines are. Either the 12 or 14 inch I don't care. All other venting has an nfa rating with exception to power vents. I believe there is a reason the nfa isn't published, I would really like to know what it is though.

I would like ventilation standards to be improved upon, so homeowners can compare the options equally. But without the nfa numbers on Turbines it is hard to compare. But I would think that info should be out there somewhere.

Thanks


----------



## Ed the Roofer (Jan 27, 2007)

In one of the www.AirVent.com brochures where they compare turbines versus mushroom vents versus Their brand of Ridge Vent they state that 5 mushroom vents equals 1 turbine vent, but they don't state which size turbine vent.

So loosely, you can figure 250 Square Inches of NFVA for each turbine vent.

Ed


----------



## RadiantRoofer (Aug 11, 2009)

*Looking for the truth behind turbine vents*

I see what you are saying but I believe NFA is calculated by how many inches of sheathing are cut away to allow air to flow naturally. so for your typical pot vent or static vent with an NFA of 50 the hole you cut is 7x7 aproximately for ridgevent with an NFA of 18 per ln foot you cut back 3/4" on both sides to get your 18 NFA rating if you where to cut it back an inch you would think it would increase your NFA to 24 However the gap provided by the ridge vent is 3/4 so that is your bottle neck (assuming convection is the only force in play). 

I suspect that for a 12" turbine the hole you cut has an NFA of 113 and an NFA of 154 for 14".

Let me know what you think. If this is the case why do turbines say they vent much larger areas than the NFA would suggest?

Thanks,

Radiant


----------



## Gary in WA (Mar 11, 2009)

Talk about some hard to find information! I see why they don't want you to see it, depends on how fast the wind is blowing. Otherwise, for the hole size you are correct. Here, after 25 minutes of searching, I found it at the corner store without the helpful hardware man: 
http://www.acehardwareoutlet.com/(cwdyjqrqynv0zi450hkjjdf4)/ProductDetails.aspx?SKU=50585

Be safe, G


----------



## RadiantRoofer (Aug 11, 2009)

Thanks G, finally some useful information.

Radiant,


----------



## Ed the Roofer (Jan 27, 2007)

GBAR in WA said:


> Talk about some hard to find information! I see why they don't want you to see it, depends on how fast the wind is blowing. Otherwise, for the hole size you are correct. Here, after 25 minutes of searching, I found it at the corner store without the helpful hardware man:
> http://www.acehardwareoutlet.com/(cwdyjqrqynv0zi450hkjjdf4)/ProductDetails.aspx?SKU=50585
> 
> Be safe, G


At last, something in print with the ratings, although it doesn't make sense from the formulas used for the NFVA.

*"ATTIC AIRE" TURBINE HEAD*


12" dia. opening
345 CFM at 5 MPH
544 CFM at 8 MPH
969 CFM at 15 MPH
95 sq. in. free area
A standard mushroom vent, with a hole cut out 10" x 10" delivers 50 square inches of NFVA due to the screening restricting the flow by approximately 50%.

So, using that assumption, with a screen for the 12" x 12" hole for the turbine, without a screen, it should be 144 square inches of NFVA or with a screen, 72 square inches of NFVA.

The biggest problem with the static air mushroom vents, besides not enough of them being installed, is that the roofers or carpenters rarely cut out the hole to the correct dimensions.

If they use a 7 1/4" saw blade, they just plunge it 4 times for a 7 1/4" x 7 1/4" sized hole, which only provides about half of what it was intended to deliver for exhaust ventilation.

Ed


----------



## RadiantRoofer (Aug 11, 2009)

> A standard mushroom vent, with a hole cut out 10" x 10" delivers 50 square inches of NFVA due to the screening restricting the flow by approximately 50%.


I think there are to many roofing terms availabe it makes life confusing. So let me see if this is right; mushroom vent=pot vent=static vent= passive vent? Is that right? 

I use the term static vent for what I install, which if I have to use them, is the duroflo weather pro rated at 50 nfa and has a 7 x 7 opening. now 7x7=49 NFA. You are right though, the Airvent Square vents require a 9" x 10" opening and only have a 61 NFA, and I often see the holes cut too small.

Radiant


----------



## Ed the Roofer (Jan 27, 2007)

RadiantRoofer said:


> I think there are to many roofing terms availabe it makes life confusing. So let me see if this is right; mushroom vent=pot vent=static vent= passive vent? Is that right?
> 
> I use the term static vent for what I install, which if I have to use them, is the duroflo weather pro rated at 50 nfa and has a 7 x 7 opening. now 7x7=49 NFA. You are right though, the Airvent Square vents require a 9" x 10" opening and only have a 61 NFA, and I often see the holes cut too small.
> 
> Radiant


It just goes to show you how the Manufacturers NFVA is probably rarely ever achieved.

Especially with the mushroom vents, plus all of the other names you mentioned referring to the same thing and don't forgwet what insurance companies call the, Turtle Vents".

I would venture to say that way more than 90% of all old vent holes are cut out to a maximum of 7" x 7".

The ventilation is automatically cut in half right there.

At least you have read various instructions. Most don't, or at least don't care.

Ed


----------



## RadiantRoofer (Aug 11, 2009)

If your going to do it do it right! In most cases it takes very little extra effort to do something right vs wrong.

Radiant


----------



## zpm (Jan 6, 2009)

Ed the Roofer said:


> At last, something in print with the ratings, although it doesn't make sense from the formulas used for the NFVA.
> 
> *"ATTIC AIRE" TURBINE HEAD*
> 
> ...


A typical mushroom vent (Lowes) has about a 9" diameter opening in the bottom. That's 64 sq inches (PI R squared). Screening and vent design reduces that to 51 sq inches.



> So, using that assumption, with a screen for the 12" x 12" hole for the turbine, without a screen, it should be 144 square inches of NFVA or with a screen, 72 square inches of NFVA.


Again, you're putting a round vent opening over a square hole. A 12" diameter hole is 113 sq inches. That may be further reduced (as in the specs above) if the area of the vane openings in the turbine are less than 113 sq inches. 

I'd be a little suspicious of those CFM figures when spinning.


----------



## PerpetuallyRepairing (Apr 7, 2008)

The issue is that NFA (net free area) doesn't account the effect of wind velocity. If you ask me it is dicey to base your venting on assuming you will have enough wind to make it work.

I just bought $1000 worth of custom gable vents. Got them in and found the NFA was half of what the manufacturer stated. They took them back & said I was the first one to ever measure it. Then I checked my continuous soffit vents. They all advertise 9 in2/foot. A local manufacturer measured 6in2/foot. I returned those & the ones i got were 12 in2/foot. Same specs, a factor of two difference. 

So buyer beware.


----------



## EBaller55 (Sep 22, 2009)

*which manufacturers*

@PerpetuallyRepairing. which product did you use for the continuous soffit vent that has 12in2/foot? I am about to install the Air Vent inc. continuous soffit vent. If all the specs are the same but one has higher NFA then would that mean the perforations/louvers are larger and could potentially allow bugs in? Most of the louver/screen opening sizes i have read about have been 1/8".


----------



## PerpetuallyRepairing (Apr 7, 2008)

Hi Eballer. That was Lomanco. The ones from Air Vent Inc. look about the same in their pictures. The opening on the Lomanco's were about 3/32". I had Lomanco's on there before. No issues with wasps or other bugs. The wasps just make their nests outside the soffit vents. I got wasps in the ridge vents however.

I just ran a computer simulation of the airflow over my cathedral ceiling. Having 30 feet from the ridge vents to the gable vents on a low sloped ceiling requires alot of venting to prevent ice dams. It looks like I will need to quadruple the Lomanco vents in the area. I don't know why someone hasn't created design guidelines for this. It is not a lot of work. 

Perpetually.


----------

