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Old 03-17-2012, 10:40 AM   #1
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Best way to vent garage and attic


Hi,
This is my first post on here and I am looking forward to all of the knowledge everyone has, I will try to keep this as short as possible while providing details.

I live in Phoenix AZ, just moved into a newly built home and want to vent the garage via a roof fan to keep it cooler in the summer and also avoid super heated air sitting against interior walls. The home I just moved out of had one and it seemed liked it cut the temps down 30 degrees in the summer. So the garage is 4.5 car, about 1200sq. ft and above 3.5 of them is living area. So the one car bay of the garage(about 300 sq ft and connected to the others) has a traditional truss roof(uninsulated) above it but is only about 4' high at its highest point in the attic area so its not a lot of room up there. In the roof is a gable vent as well as what I think we consider a ridge vent here in Phx, its a single vent near the top of one of the tops of the roof. I have seen where some people say you should not have a gable AND a ridge vent as it defeats the purpose. That leads me to my idea.

I had bought a couple of solar operated attic fans a year ago and figured they would be good enough for this space I have. I was going to install one mounted against the a hole I will cut in the drywall on the ceiling and it will draw hot air up and out of the garage space into the area above the one car garage I mentioned. Then I was going to mount the second fan either against the gable or ridge vent to blow the air out of the attic space. So a few questions, the gable is easier to work with in this small area but is it better to mount the fan on the ridge vent so the warm air blows straight up and out(the natural path of warm air) or sideways out the gable? Does it even matter in this smaller application? Do I need to seal off the ridge vent if I put the fan in the gable as not to draw air from outside and to keep it pulling from the garage? There is also a small soffit vent(similar to the ridge vent but closer to the bottom of the roof) and should that also be sealed off?

My last concern is, I have a gas water heater in the garage but its about 25' from where the vent would be in the drywall for the fan. The garage has two vents going outside near this whole 1 car vent/project(per code in having a gas appliance in the garage) and I was going to just use this for my air inlet for the first fan mounted in the ceiling. I was told by a contractor that I cannot put a fan in the garage w/ a gas water heater as its a fire hazard. The suction of the fan can make the flame jump and cause a fire. These fans seem to be on the smaller side and from what I have read, solor fans dont move the same cfm as an electrical fan and the fan would be about 25' away like I mentioned. Is this a real concern and any ideas about it? Ok, I think thats it, thanks in advance for any feedback. Mike

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Old 03-18-2012, 07:57 AM   #2
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Best way to vent garage and attic


The combustion air for the water heater should be your main concern. But you should not be guessing here. Use a manometer and check the existing pressure of the garage with respect to outside air. Then run the proposed fan to calculate the effect on the water heater. you need at least 3pa of air pressure to be safe. But check for gas spillage and check with a gas combustion gas analyzer also.
The fan in the ceiling is not needed anyway. by using a fan in the attic space you will be depressurizing that area which will draw air out of the garage anyway. Just use a grill in the ceiling, but place it far away from the water heater and from the attic fan.

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Old 03-18-2012, 10:49 AM   #3
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Best way to vent garage and attic


I like that idea Bob, thanks!! Do you still suggest closing off the ridge and soffit vents in that attic space to allow the fan to pull the air only from the garage and out of the gable rather than allowing those other vents to be open and pull in outside air? Thanks again.
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Old 03-18-2012, 12:11 PM   #4
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Best way to vent garage and attic


Just trying to be the devil's advocate here. Have you thought about what is going to happen in the future when your power vent fails? (and it will)
Sealing off the passive venting you have and not noticing that the power vent is not working
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Old 03-18-2012, 01:17 PM   #5
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Best way to vent garage and attic


the soffit and ridge vents should remain open. If adding insulation, (which is what you should be doing) to control solar loading in the garage be sure to baffle the eaves to not clog the air vents. Air moves from high to low pressure. air enters the soffit vents and exits the ridge vents. This natural venting should be all you need. Power vents are not a good solution for many reasons... one just mentioned here.
Another solution is reflective film over the insulation. blown cellulose is the best for your project. The garage wall is another place to use a small exhaust fan. This is to depressurize the garage so no polluted air is flowing into the house.
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Old 03-19-2012, 12:11 PM   #6
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Best way to vent garage and attic


I think I liked your first idea the best Bob, lol. I was going to insulate the ceiling of that 1 car garage and actually found a very interesting company here in Phx that does things a bit different but actually uses that foil and the average person is dropping 60% off of cooling bills so I am really going to look into that. In fact they demonstrate how well it works by building a home w/out insulation and just the foil and in some cases is better than insulation! It really keeps the heat from reaching the living areas and allows the AC to run a lot less and people are dropping the ton rating on new AC's using the foil.

I could put a fan in a wall in the garage, it just seemed like moving hot air up and out of the garage and attic would be better than moving air out of the garage alone. I really did like your idea of one fan in the attic just pulling the garage air through a vent, up and out and the fan is further away from the w/h than it would be in a garage wall as well. But then I think if you dont seal up the other vents around the gable(ridge and maybe soffit) you end up drawing in air from outside and not the garage as much. So I am back to square one.
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Old 03-19-2012, 04:22 PM   #7
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Best way to vent garage and attic


you will not draw air from the other vents.... do no worry about this. foil without insulation is a scam. In fact the owner of the company did this to his own house and it have to be completely stripped and done with insulation in the walls. Foil over R38 or better insulation will help reflect some heat. About 10%. Check out my site, you will see I do know what I am talking about here. www.homedoctorofamerica.com
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Old 03-19-2012, 05:21 PM   #8
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Best way to vent garage and attic


I would install the fan on the ceiling and leave the ridge-vent open. I dont think there is any need to install a fan on the gable, the ridge-vent will be enough.

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Old 03-22-2012, 10:48 AM   #9
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Best way to vent garage and attic


The ridge vents they install here in Phoenix are not what most home have, they are a small vent that fits under a roof tile, maybe a foot across and I have one in that small area, so not much surface area to the vent itself. I like both of these ideas you guys have with either putting the vent in the fan in the ceiling and allowing the attic space to naturally vent or putting it in the gable and allowing the fan to pull from just a vent installed in the ceiling. I checked into the code on the water heater and its a non issue as they are sealed units so that is good. Thanks for the great ideas.

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