Not a heat question,just hot air...

brad_4

i know this isnt a hydronics question,but i also know you guys know all... i have separate heating and cooling systems w/ one thermostat. hot water baseboard for heat and 5 ton coil in attic w/outside condenser for the A/C. the attic,approx 3200sq ft gets way hot in the summer thanks to my black roof. it has ridge vents along every peak and vented soffits around the entire house. all of the vents are clear and it is almost bright in the attic w/all of the vents. its just so freaking hot in the attic. i am an insulation freak (before dad passed away he had always said it would be the best money i would ever spend in my life). the house is 4600 sq ft and the 5 ton has no trbl keeping the the house at 64 degrees on 90+days. QUESTION:i bought two roof mount attic exhaust power vents. the instructions say to mount them as high and close to the roofline as possible. it just seems to me the path of least resistance will be from the RIDGE vents and the powered vents are going to be getting alot of "fresh" air from them and not pulling it from the SOFFIT vents. there is a boatload of ridge venting up there to supply the powered vents. my family/builder/A/C guy think im nuts for putting in the powered vents and to ignore the heat and get a life. any thoughts or suggestion are appreciated. i wanted to post this @ DIY.com but those people scared me when i read some posts where they had housewives going outside and discharging capacitors on compressors w/a screwdriver "and see if she starts now"... i am morbidly curious now to see if she lives to return to post her findings. thanks for any advice, bradley.
p.s. re:genos ongoing thread. if you want to see people that are uneducated and will risk life before they spend a buck calling a professional in,they are all there.

Tony_8

power vents

If you are getting air exchange you will move the heat also regardless of location, it may not be as quickly is all. Since they are thermostatically controlled they'll just run til it's cool enough to shut off. Put 'em 1/2 way up if you want to split it evenly. They are worth the money in my experience.

Eric Taylor_35

I'm not a plumber but.....

This isn't a plumbing question.

If your A/C ducts and the attic floor are well insulated, and the A/C has no trouble cooling the house, and your attic is already well ventilated, what's the problem?

I assume that your attic is unfinished and not part of the living space so you don't spend lots of time up there. Who cares if it gets hot?

Anyway, to answer your power venting question-- if you want to power vent and draw from the soffits then the ridge vent will cause a short cuircuit and defeat the purpose of power venting. You might be able to just block the ridge vent from the inside with something and it'll be like its not there.

The best way to cool it off up there is to reduce the heat gain. That foil bubble wrap stuff would be perfect up between the rafters. I plan to install it in my house and use it as vent trough too.(no A/C as of yet, soon to be re-finished re-insulated attic) Just space it 2" below the sheathing and connect the soffit vents to the ridge. It'll help manage any small roof leaks too.

Or just forgetaboutit, and go outside and play.

Eric

Joe_13

suggestion

I assume the attic is uniinished. I would mount them high just to isolate as much of the noise as possible when running. Keep the thermostat on them at least 120 so that they will be off durring the evening and morning.

kevin coppinger_4

in addition...

depending on where you are radiant barriers can also be an effective way to keep the heat fron=m geting in do to you black roof. It would involve using a foil reflective sheet to keep the radiant part of the heat out. It is more effective the more south you go in the US.
On another note it really does not make sence to keep the cooling in such a hot place. But this is done all the time. hope this helps. kpc

brad_4

THANK YOU

i just wanted to thank you guys for your assistance. half way up the wall seems like a good idea being that if convection is taking place there should be air entering the soffit vents to begin with. if layers of insulation are cumulative i should be @ r-59 in the attic and noise isnt an issue. thanks for not thinking im crazy and that the powered vents would be of benefit. i agree that putting the cooling eqpt in the hottest part of the house doesnt make sense but there turned out to be 21 registers,(each register is directly off the plenum) and three 16" returns and the plenum is approx 30 feet long on the supply side alone,(it gradually gets smaller in diameter as the ducts to the registers are taken off,i guess to keep the air pressure up?). there was just no other place in the house where the size and access could be accommodated. if u get a chance to read this, may i bother to ask that if the supplies are metal and wrapped and/or buried in insulation-i think good.. why are the three returns big flexible ducts and should they be wrapped in insulation also? anyway,thanks again for your thoughts and advice,brad.

Tony_8

Wrap

the returns. Heat gain is heat gain. Also, I would set the temp no higher than 100. Differential is only 5. Attics on sunny days w/80F ambients can and do get well over 110. There's a reason for those cooling vests guys wear when it doesn't seem that hot out.

brad_4

cooling vests..

will do... i noticed that although the A/C was installed in March (several years ago) the three technicians kept talking or calling out to each other approx every 30-45 seconds and i asked them why they kept checking on each other and they explained that it was out of habit and that during the warmer months it was a common occurrance for one or the other to PASS OUT from the heat they were working in! and that it wasnt so unusual for them to have to drag an unconscious coworker out of the hot attics they often worked in! at first i thought they were joking,but upon speaking to other folks it was not a joke. you actually can learn something new every day.

Steamhead

Brad, when you replace that roof

get lighter-colored shingles and watch how much cooler the attic gets.

Here in Baltimore, they're starting to paint those black flat asphalt roofs white to reflect heat in the summer. People say it makes a huge difference in comfort and reduced air-conditioning costs.

And if you haven't already done so, think about planting some trees to shade the house.

Finally, even though you say it's bright in the attic from all the vents, check their size against the amount of attic space to see that they're big enough. You can get this info from vent manufacturers.

I cool my attic with a similar gravity system, and it never gets hotter up there than 10 degrees over the outside temprtature. Why install fans and burn up all that energy when Mother Nature can do the work?

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bill clinton_3

how about

mounting those fans low and reversing them so they blow the cool air IN rather than the hot air out? Seems to me the cool air would tend to stay lower against the insulation and the hot air would exit from those ridge vents the way its supposed to.

Bill

Joe_13

Rain water, leaves could get sucked into the attic if the fans are blowing inward and you get a summer thunderstorm roll in. I also don't care if you had R-200 insulation, you get vibration through the rafters. My power attic vent bothers me when it's quiet outside. I also have a wall switch to turn off the whole unit when I'm away on vacation or durring the winter. The fan will come on durring a sunny mild winter day if you set the thermostat too low. Shame to to vent out that nature's radiant heat when it's free.

Dale

Independent research

Although the idea sounds good research done by several universities has shown no benefit at all for power vents. And, power vents can actually cost you more. The problem is that they end up pulling a very narrow air stream from whatever air inlet is nearest. If you have proper venting, meaning full proper vents above the walls or energy effecient trusses and the correct amount of static roof vent the power fan won't hurt, just won't help. And, if you don't have enough air inlets or if you have air leaks in the attic arround vent stack ect or ceiling cans for lighting the extra negative pressure pulls the cool air out of the house. Remember that fiberglass insulation isn't an air seal. What is really dangerous is the auto switch moisture sensor option where if the inlet gets blocked the fan runs on cold damp days and reverses the chimney to monoxide you. My company did an investigation on a case like this. I would of course be skeptical of this and any other free advice, do you own research on this.

brad_4

sometimes a black roof is just a black roof..

thanks for your help dale. after the electric and plumbing was roughed in the builder,a great guy,allowed me time to install the central vac piping,the alarm wiring,CO2,smoke detectors,ethernet,RG-6,cameras,sprinkler,blah,blah,blah...i was an installer for western electric in days of yore and was used to being a "cable monkey" climbing the cable racks of the telco CO's... while installing all the low voltage it gave me an opportunity to use a couple of cases of silicone II and expanding foam to seal any penetrations that existed. there were alot. all of the high hats are boxed,caulked and insulated. raftermate was used @ edges of soffits,we did the best we could. there are three 40,000 btu fireplaces w/sealed combustion,boiler w/sealed combustion. if anything, i think i could use some fresh air for the exhaust fans,BBQ,dryer and such. i just cant help but think that they make solar collectors black for a reason. i DO appreciate your advice. there is a switch in front hall w/a pilot lite ready for the fans so i will be able to control them manually. my original post alluded to the fact that i was concerned where the air was going to come from,ridge vs soffit. as you stated their exhaust comes from "a narrow air stream". as tony stated,i think placing them 1/2 way between ridge and soffit is a good idea,as steamhead stated in baltimore folks DO notice a difference after they change roof colors from dark to light. i would think that the combined 3200 CFM of the fans exhausting would have to be of SOME help,i could be wrong. my original post stated that i was told not to worry about it from the builder and the a/c contractor. you know what? i bought them,the wiring is there,its sunny out,i have the tools,i'll get off my a** and install them and perform my own research. thanks very much to all for their time and experience. (i still say they ARE going to make a difference;-)

Mike T., Swampeast MO

About the only way I know of to see if it is effective at reducing heat build-up in the ducting (your goal) is to measure temperature at supply/return at their outlet/inlet in room and outlet/inlet in the air handler.

Measure before and after the exhaust fans on days of very similar outside temperature/conditions.