Venting hot garage air into attic with ridge vents...

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Old 07-15-20, 06:15 PM
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Venting hot garage air into attic with ridge vents...

I have a central Florida garage with a dehumidifier in it that drains its condensed moisture into a utility sink so the humidity in the garage does not rise above ~50%.

I am considering installing an exhaust fan in the ceiling that will vent the hot, dryish room air into an insulated, hot attic. The roof has ridge and soffit vents. The exhaust fan would turn on only when the temp is above 90 degrees at ceiling level.

I read cautions on this forum about the problems of MOIST air, like from a bathroom exhaust fan, being sent into the attic causing mold/mildew, etc. I would not be sending "moist" air; I would be sending hot, dryer air into the attic. In fact, might that 50% RH exhaust actually lower the relative humidity in the attic?

Related to that, I read what to me were counter intuitive warnings about sending hot room air conditioner exhaust air into the attic. The presumption was that a room AC expels moist air. Does it really? Isn't it true that an AC acts also as a dehumidifier and expels condensate room moisture down a separate drain line, and dryer, hot air out is expelled out its rear exhaust? Wouldn't it then make sense that the exhaust air would also be hot and dry like that of a dehumidifier. If that's the case, why would expelling that air into the attic be a problem in the same 50% humidity room environment?
 
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Old 07-15-20, 06:33 PM
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Welcome to the forums.

Basically a dehumidifier and an A/C are the same thing. A dehumidifier discharges the hot air back into the room while an A/C discharges it outside.

The only problem with discharging warm DRY air into the attic is that you'll be bringing in warm WET air as makeup back into the garage.
 
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Old 07-16-20, 04:47 AM
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Why not just vent the new fan it out the roof or side wall?
 
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Old 07-16-20, 06:05 AM
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The only problem with discharging warm DRY air into the attic is that you'll be bringing in warm WET air as makeup back into the garage.
^^^ This ^^^^ You'll be defeating the purpose of the dehumidifier.
 
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Old 07-16-20, 07:11 AM
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"The only problem with discharging warm DRY air into the attic is that you'll be bringing in warm WET air as makeup back into the garage."

That statement made no sense to me. Where is my "warm wet air coming from?" The moisture from both the dehumidifier and the AC are being expelled down a drain or to the outside. Hot air is being expelled into the attic that has ridge vents, with hot air going up and out. What do you mean by "makeup back onto the garage?"
 
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Old 07-16-20, 07:15 AM
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"Why don't I expel air out side walls?"

I don't have any side walls I can use. All three garage walls have interior space on opposite sides.

Also, there is no need to vent out the roof because the roof is already vented via the ridge vent/soffit vent system. And theoretically, if the air being vented into the attic is not moist, in fact is DRYER than the outdoor air, there should not be any moisture problem, either.
 

Last edited by Gerald Mucci; 07-16-20 at 07:33 AM.
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Old 07-16-20, 07:20 AM
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"Defeating the purpose of the dehumidifier."

How so? Dehumidifiers are often used to supplement the dehumidification performed by ACs.

The biggest concern I have heard expressed about expelling air conditioner venting into a ventilated attic is that it may increase moisture in the attic creating mold and mildew. Since my garage humidity will never exceed 50%, and the AC will reduce humidity further, there will be relatively dry air going into the attic - perhaps even dryer air than what is already in there.

The primary purpose of a dehumidifier is to remove moisture from the air. A byproduct is the expulsion of warmer air. That is beneficial in cooler rooms, but not helpful in already hot rooms.

The primary purpose of an air conditioner is to cool the air. A byproduct is reducing humidity, which in a humidity neutral room leans toward being beneficial. The expelled air from the back of an AC is also warm, like the dehumidifier's expelled air. It is also logical that the expelled air would be no more moist than the ambient humidity of the room. In fact it should be less moist than ambient room humidity because it condenses additional moisture away in the form of water that drains outside.
 
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Old 07-16-20, 07:54 AM
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To help nail down my specific concern about expelling moisture into the attic with the exhaust of a room AC, I have this scenario and question:

Assuming an ambient room temperature of 88 degrees and room relative humidity of 50%, with the room air conditioner set for 76 degrees, what is the relative humidity of the expelled exhaust air out the back of the window AC unit likely to be?
 

Last edited by Gerald Mucci; 07-16-20 at 10:20 AM.
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Old 07-16-20, 08:15 AM
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Putting your air conditioner question aside (since they are not really the same thing, as far as CFM is concerned), go back to your original question. You want to put an exhaust fan in a garage with a dehumidifier.

I'm baffled by how the phrases "central florida" and "dryer outside air" can even belong in the same sentence. If the outside air is dryer, what do you need the dehumidifier for?

If you expel air with an exhaust fan into the attic, you create negative pressure that pulls in outside air. Garage doors are leaky so that's where the majority of the makeup air would come from. If the outside air temperature is 85F and has a dewpoint of 72F, the Relative Humidity of that air is 62%. (Baffled by why you think this is dryer air) Introduce that same air into a cooler area that is, let's say 78F and your RH inside the garage goes up even more. I'm not sure what part of that you don't understand.
 
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Old 07-16-20, 08:19 AM
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How so?

You are removing air from the garage so it has to be replaced by air from somewhere else.
That will be outside air which has higher humidity and therefore will partly defeat the purpose of the dehumidifier.

Humidity build up in the attic will not be a problem for you.

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Old 07-16-20, 09:34 AM
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On the topic of pressure, you'll be positively pressurizing the attic with respect to the living areas. Do you really want to create a scenario where air from the garage (forget about its humidity) could enter the living space? Codes requirements are written to prevent that.
 
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Old 07-16-20, 10:10 AM
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"I'm baffled by how the phrases "central florida" and "dryer outside air" can even belong in the same sentence. If the outside air is dryer, what do you need the dehumidifier for?"

I checked through my statements above and don't find anything like what you [mis]interpreted. The closest to that verbiage I can find is "And theoretically, if the air being vented into the attic is not moist, in fact is DRYER than the outdoor air, there should not be any moisture problem, either." I think you missed the "than" in that statement.

As for negative pressure due to due exhausting hot air to the attic, there are several scenarios:

1) If I keep the door to the AC'd house from the garage closed. The path of least resistance for infiltrating air to relieve the negative pressure in the garage will be the leaky garage door. True. Just like closing the "recirculating air" control on your car's AC system. Some outside air comes into the car/garage the same way. That's why a car can be cooled off quicker when air is recirculating rather than coming in from the hotter outside. But the car is still made cooler by the AC, even with some outside air coming in. Cooling is just not as efficient.

2) If I keep the door between the garage and the AC'd interior of the house open a crack. Now that door becomes the point of least resistance, drawing some cooler AC'd air from the house into the garage. In both scenarios 1 and 2, the doors sort of act like the vent flap on a car's AC system - on/off or variable (at least they used to be variable).

3) Occasionally the outside air is cooler than the inside air of the garage. This occurs in the late afternoon and evening when the outside has cooled off but the garage still retained the daytime heat. It might be 82 outside but 90 in the garage. The garage door faces south. It is insulated, but not the best. In this scenario, cooler air coming through the leaky garage due to negative pressure in the garage is a good thing. The dehumidifier will take care of any increase in moisture.
 

Last edited by Gerald Mucci; 07-16-20 at 10:37 AM.
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Old 07-16-20, 10:17 AM
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"On the topic of pressure, you'll be positively pressurizing the attic with respect to the living areas. Do you really want to create a scenario where air from the garage (forget about its humidity) could enter the living space? Codes requirements are written to prevent that."

The attic is not capable of being "pressurized." It has open venting along 50 feet of its ridge line and over 100' of open soffits around the perimeter. The attic is sealed from living spaces. The points of least resistance are ridge and soffit vents much more than any potential of entering living spaces.
 
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Old 07-16-20, 12:56 PM
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So you've basically convinced yourself that this is a good idea, aside from everybody telling you it's not. You are doing 2 things wrong, pressurizing your attic space by forcing air into it (also a possible building fire code violation, your garage should never have an access to any portion of your home) and extracting air that's been "conditioned" by your dehumidifier. Just leave the garage door open, toss the dehumidifier and you'll accomplish the same thing while lowering your utility bill.
 
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Old 07-16-20, 06:29 PM
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"So you've basically convinced yourself that this is a good idea, aside from everybody telling you it's not."
Ahh, the elusive, creativity-killing "everybody." Not "everybody" understood the factors and objective, and one segment of that "everybody" doesn't make a whole lot of sense. If you made sense to begin with there would be no need to throw around the red herring "everybody."

For example:

What is your definition of "pressurizing the attic?" There is no "pressure" being created in the attic. It's free flow air. Perhaps that phrase is trade lingo that makes little sense in plain English in this context. If that's the case, I've learned something. If I pump air into a tire that has holes in a dozen places, I'm certainly not "pressurizing" it. It's still flat with no pressure. Therefore it isn't "pressurized", get it?

Also, when I vent my bathroom or kitchen, I'm venting "conditioned air." People vent "conditioned air" all the time.

The exhaust fan has a metal damper on it, so no air transfer into the attic when not on. So, if I open the attic hatch in the garage (which is covered with a thin board more flammable than the metal fan damper) that's a "code violation?"

A bit contradictory and contrary - "everybody" might agree.
 

Last edited by Gerald Mucci; 07-16-20 at 07:44 PM.
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Old 07-17-20, 06:27 AM
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What is your definition of "pressurizing the attic?" There is no "pressure" being created in the attic. It's free flow air. Perhaps that phrase is trade lingo that makes little sense in plain English in this context. If that's the case, I've learned something. If I pump air into a tire that has holes in a dozen places, I'm certainly not "pressurizing" it. It's still flat with no pressure. Therefore it isn't "pressurized", get it?

If you pump air into a tire that has dozens of holes in it and the air is escaping through the holes, you are creating a pressure differential. In other words, you are pressurizing the tire and the holes are allowing that excess pressure to escape. Get it?
Good luck with your project.
 
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Old 07-19-20, 03:15 AM
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So I checked in after a few days off. Tomf63 has it right.
 
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