Master Flow Powered Roof Vent

acehMay 1, 2007

I had a new roof placed on the house after Hurricane Katrina. Roofers replaced my old Power Vent with a new Master Flow Power Vent and set the temperature at 90 degrees. Is this a "good" setting for the hot, humid and muggy summer climate of New Orleans?

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ron6519

I think the recommended setting is around 100-105 degrees. You want to keep the temps in the attic equal with the exterior temps. Set it to the expected summer temps.
Ron

    Bookmark   May 1, 2007 at 6:42PM
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aceh

Thanks. I just set it to 100 degrees.
Is there a reason for setting the attic temp equal to the outside temp? It seems as if you would want the attic to cool off BEFORE the outside temp got that high.

    Bookmark   May 1, 2007 at 9:24PM
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ron6519

Your not setting the fan to the outside temps, your setting it to an arbitrary temp based on region. You just want to set it so the attic doesn't get hotter then the exterior temps. Conditions vary as to roof color, shade trees around the building, etc...
The attic temps get that high long before the outside temps do. An unvented attic can get to 135 degrees on a sunny day even if the outside temps are only 75. You don't want that heat driving through the ceiling into the conditioned spaces. It's much cheaper to run a fan to keep the temps down in the attic then trying to cool superheated attic air.
You also need to match the intake with the outflow. If you don't have enough venting in the attic to draw in outside air, it will pull your a/c air out of the house through various gaps in the walls and ceilings. The instructions on the fan should give you a formular to figure it out. It will be based on the CFM produced by the fan.
Ron

    Bookmark   May 2, 2007 at 8:19AM
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aceh

I'm curious about this so please bear with me.

You mentioned the temperature is set to an arbitrary temp based on region. My Powered Roof Vent has a thermostat with actual temperature settings on it (i.e. 85, 90, 95, 100 degrees etc.) thus I assumed this had nothing to do with regions but rather actual temperatures.

In addition to the attic fan, I have 3 gable vents plus another vent.

    Bookmark   May 4, 2007 at 4:29PM
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ron6519

You set the fan to an actual temperature. It might be lower in places with less Sun and higher in places with more Sun. The instructions will tell you to set it around 100-105. Start out with that. You can adjust if you want later on.
The attic vents need to be sized to match the fan. Fans come in various sizes, loosely based on attic floor area. On the instruction sheet it will say how many square inches of venting is required for the fan. For the fan to work properly you need to have this as a minimum.
You need to know what size attic you have to buy the fan. And you need the correct venting sizes for it to work correctly.
It's like buying an A/C unit. You calculate the btu's you need based on your conditions and buy that size unit
Just measure the vents you have and see if it meets the required number of square inches. If not, add more vents. Most houses are under vented.
Ron

    Bookmark   May 4, 2007 at 11:59PM
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wbb557

Accept my apology for being slow on the uptake... but since I live in the north east - I should probably set my new fan to around 85 since that is the average high temp in the summer time or should I set it up to 90 because we get about 10 days a year in the 90's. It is an A/C'ed area up stairs that is a converted cape cod to a master bedroom, with lots of insulation - gable vents, ridge vent and soffit vents with a new architiectural gray shingle roof.
Thanks for any and all help!

    Bookmark   October 10, 2007 at 11:37AM
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ron6519

Set it to whatever temperature you want. The lower you set it, the longer it will run. I would set it to 100-105.
Ron

    Bookmark   October 11, 2007 at 2:54PM
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wbb557

Thanks for your help!

    Bookmark   October 11, 2007 at 3:28PM
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shorns

7/3/08
Ron - I bought a PR3 Master Flow Attic Vent - Had it installed. Electricals are hooked up. PROBLEM IS:
The fan won't stop running. Thermostat set up by mfg at 105 degrees. Attic temp per my temperature gauge (and it's a good one) shows 98 degrees. I then set the fan thermostat to 110 degrees - BUT - it didn't make any difference, i.e., the fan just keeps running. Have you heard of a new fan thermostat not working and/or what else would you suggest to maybe get the fan to stop for a while, i.e., set the temp to 120 or ???
Any suggestions, Thanks / J. Shorns

    Bookmark   July 3, 2008 at 2:54PM
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wbuckhiester

I'm having same trouble as J. Shorns, fan won't stop. I checked early this morning and the attic temp was down to 76 degrees, which I thought would be cool enough for the fan to shut off. I probably don't have enough vents, but there is one big one on opposite of end of house from fan, so there should be good enough cross flow. Could it be bad thermostat or am I just plagued by bad vents and will have to shut off power to let the fan rest at times?

    Bookmark   July 14, 2008 at 6:34PM
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wbuckhiester

Problem solved! Turned to be bad placement of the thermostat. When I angled the thermostat differently, it registered the temp correctly and now the fan shuts on and off as it should.

    Bookmark   July 21, 2008 at 5:29PM
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suburbanfan

Great explanation Ron !
In order to keep the super heated air out of the living space you have to remove it from the attic. It is OK for the attice to be at 100 degrees as long as the superheated air stays in that part of the house NOT IN THE LIVING SPACE ! www.suburbanfan.com

Your not setting the fan to the outside temps, your setting it to an arbitrary temp based on region. You just want to set it so the attic doesn't get hotter then the exterior temps. Conditions vary as to roof color, shade trees around the building, etc...
The attic temps get that high long before the outside temps do. An unvented attic can get to 135 degrees on a sunny day even if the outside temps are only 75. You don't want that heat driving through the ceiling into the conditioned spaces. It's much cheaper to run a fan to keep the temps down in the attic then trying to cool superheated attic air.
You also need to match the intake with the outflow. If you don't have enough venting in the attic to draw in outside air, it will pull your a/c air out of the house through various gaps in the walls and ceilings. The instructions on the fan should give you a formular to figure it out. It will be based on the CFM produced by the fan.
Ron

Here is a link that might be useful: suburbanfan

    Bookmark   August 3, 2008 at 10:02PM
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davewb3dwe

Our attic area is 3000 sq ft. It has been ventilated by two power roof ventilators. One burned out and I am replacing it with a Master Flow PR2D.
In Texas the temperature gets to 100+. Both fans are controlled by a single timer. I run the fans 20 hours in the summer, 23 hours when the temp is 100+. The attic temperature never gets down to ambient. I like to run the fans one hour a day in winter to prevent moisture build up.
The burned-out one is far from the stair in the garage. It would be _extraordinarily_ difficult to reach to reset the timer, climbing over ducts and under wires.
I sent email to GAF asking if I could bypass the thermostat by simply connecting white-white and black to black. Whoever answered did not understand and replied "Why would you want to do that?"
First there's no way I could crawl that distance to reset when the seasons changed.
Secondly if one is thermostatically controlled, the two fans will not work together.
I asked my air conditioning man and he replied keep it simple, ie. use the installed timer, which is easy to reset.

    Bookmark   August 18, 2008 at 6:05PM
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gene_2009

I resently purchased a Master Flow Whole House Fan from HomeDepot. When I got it home I took it out of the box to get some measurements. When I measured it I got two different measurements for the fan and the shutters.
The fan measured 27 3/4" X 27 3/4"(outside). When I measured the shutters I got 26 3/4" X 27 1/2". That is the measurement for the Hole in the ceiling for the shutters.
I would like to know if this is the right shutter for the S24 fan(WHFS24M). If it is not, can I purchase the right shutter for it.

Thank You for your time, Gene_2009

    Bookmark   February 25, 2009 at 5:30PM
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