what temp should I set my attic exhaust fan on? right now it's set at 120, the daytime highs here are 95-100

 

Attic fans are a waste of money. They burn more energy than they save you. The motors can also be a fire hazard.

You should cool your attic passively. That would include ridge vents and lots of eave vents. This allows the heat to naturally rise out of the attic.

 

I had one in Florida and set it at 90. It help keep the house cooler and the AC didn't have to work as hard thereby extending the life of the AC. I also was told it extended the life of the roof. I'm thinking about having one installed here.

 

======== This is true very costly to run and usely has a motor life of 2 or 3 years Passive is the way to go.

 

A passively cooled attic will rise to as high as 140 deg F on a sunny 95 deg F day. If you have R40-R60 insulation on the floor of your attic, the AC load can be minimized. If your AC evaporator is in the attic space, it will work harder: the coolant lines run through this space and pick up this latent heat. A properly sized attic fan will keep that same space at 90 deg F if the eave vents are numerous enough. One that is rated for 1170 cfm (7000 cubic feet attic) has a 1/4 hp motor (~ 300 watts). 1500 cfm (9000 cubic feet attic) has a 1/3 hp motor (~500 watts). That is maximum draw/load. They do not actually burn this much. A typical 2 ton AC unit runs at 1800 watts. The math is simple.

I have a 2 story/2 AC unit house and if I don't run an attic fan, the upstairs AC unit runs 8 hours a day continuously. and it is impossible to go up in the attic for more than 1 minute at a time. My last house had the same roof fan after 15 years. My current house is going on 6 years. Works fine.

My neighbor helped me move some stuff into my attic 2 summers ago. he was impressed enough that he had one installed a week later.

Mine is set to 90 deg F.

 

You are the exception and there is always one!

 

Being in the trade, attic fans help, they fail they make noise and they can overheat, though fires are very rare.

The attic needs to be cooled to suppress the heat load of the a/c system amd to EXTEND THE LIFE OF THE ATTIC PLYWOOD THAT AGES FASTER WITH HEAT OF 150 AND HIGHER. Try using an Infared gun to gauge the actual temperature. I do.

The fan should be set high enough that it shuts off by morning and does not make a noise all day and night. 95 is a start point and the 45 watts is nothing compared to a compressor, 3 tons is about 3 horsepower.

I PREFEER the passive fans that are called turbines and go with the wind. THey allow heat to go out when there is no wind and really help with a breeze. They take no energy, can not burn, dont get stuck, stall, last a long time and are even available with thermostatic vent controls that shut in the winter, THOUGH you want ventilation in the winter to keep the humidity down.

BTW The air going out needs to come in and you need to confirm that the house has vents in the soffet to allow air IN THE ATTIC also.

I was at COSTCO today looking at the attic fans that are solar, neet and a nice try, but dont work in the evening when the heat is starting to fall off outside, so they are CUTE and expensive.

Iam all about turbine fans.

OH BTW did I tell you that we are using LEAK STOP for air conditioners now. WOO WOO it seems to to work, I have been testing it for 2 years on my truck fleet, office heat pump and now on my customers systems with good results to KEEP AN OLD SYSTEM RUNNING A few more years. Though some really need to be discarded and the old R22 freon is gone at the end of the year and its time to go with the newer R410 freon.

IF you got a leaker or car that seems to go down, the leak stop is one item I can recommend, bought 5 cases so far this season.

ENJOY, W DY

 

Or two

 

You hit it pal. Passive is fine but active saves money in the long run. Hot air hangs in the attic and radiates into the lower levels, even with insulation, so your AC unit works harder in the evening trying tocatch up.

Meanwhile your roofing cooks as well as the framing. Remember that hot air, and susequently hot lumber, absorbes more moisture. Heat and moisture are the two worst enemies of your frame, composite shingle roof. You are in SC. It gets very humid there, yes?

Re did my neighbor's 32 year old roof last fall. He had very inadequte passive venting despite three noisy turbines halfway up the pitch. The rafters, at the ridgeline, had cooked so brittle that you could break them with your hand.

My attic fan has worked for 32 years. I replaced a belt once because it thumped.

Set your attic thermostat at 90 degrees and see how much it lowers your electric bill. Add a roof ridge vent as soon as you can because there will always be dead air trapped at the peak, even with a fan.
And, as usual, Woody is right. Your intakes need to be sufficient and keep the venting on in winter. It is acually more important because of condensation on the super cold surfaces in the occasional wam winter day.


roadster with a stick

 

thanks for all the advice I switched it to 90 from 120, my insulation is 7-8 inches thick not sure the r-value.

 

There are a lot of good points posted here. I was a little quick in my response and I apologize for not providing more detail. At 7-8 inches of insulation, you are probably at about R-30...depending on the type of insulation.

As an architect, I have a few points I'd like to address about what has been posted and a few things that haven't been addressed. Unfortunately, I don't have the time tonight to write it up but will try to post it tomorrow.

 

I don't doubt there are better ways to keep attics cooler but I don't want to redesign my roof

 

This I gotta' hear.

 

I just removed my storm windows last weekend & threw on the screens! It looks like we may have our first taste of 80 degrees this up coming week, nyah, nyah, nyah......

 

93 deg outside. 68 inside. Free air conditioning at work!

Double nyah-nyah!

 

Like Bill. F I set mine at 90 and it does help the AC. Also helps if you set the fan on your furnace to ON. That also helps.

 

Fran ,you need to bring your fan down here to Texas ,it is going to be hot in the shade

 

============== Some areas on fan may be ok. In humid climates and spaces maybe not, On fan will add the moisture from the e coil and drain pan back into the air stream when compressor is not in operation . So just be aware, also many of the larger ac units can use several amps 10 are more. Which could cost you some dollars at constant fan. ============================= I may reconsider and try a electric attic exhast fan, never really like them because They do not last very long around here. But if some one can do it themself which i can, could be a good deal. Around here it cost 500 to 800 $$$ to have 1 installed by a contractor. I already have R-42 insulation,and R-8 on my air ducts, but would help with the plywood , roof,ac equipment etc. I now have turbines and would keep in place . But i thinking about getting a fan with a humidstat and t-stat to help keep humidity in check.

 

Had my house built 10 years ago.
I replaced the attic fan once so far and it runs by day and is off at night.
I don't recall the temp setting I have it set at but it seems to be doing the job.
Usually only runs when the outside temp reaches 80 degrees or more.

 

Can I watch, Max?

Spent a life-time as a construction manager listening to the engineers battling the architects. Once in a while common sense would actually prevail.

Regards.