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Old 07-23-2017, 05:01 PM   #1
jaybird
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Roof Vents vs Hot Weather

I spent the last month in very hot weather and I noticed something. Many of my fellow RV's had one roof vent open during peak heat. At first I thought they forgot to shut them. Then I started thinking (possibly to much) that maybe they intended to have one open to release some hot air that rises. Is this their strategy, or was my first thought, they they accidently have it open correct?
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Old 07-23-2017, 05:06 PM   #2
DQDick
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Lot of different forces at play that will change that equation. We use vent covers and the little fuzzy pillows in the vents to keep the hot air out. Heat index in our area was 121 yesterday with the actual tem over 110. My inside tem was never higher than 84 with 33% humidity (we still cook for dinner and run the coffee pot) so I don't think I want to let any of the heat or humidity in. It also makes a difference if there is wind blowing. We had 10-15 MPH winds from the south which would have push air in my vents if they were open.
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Old 07-24-2017, 02:00 PM   #3
Mark N.
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I can't figure out how it would ever be a good idea to open anything at all to the outside if the outside temperatures were hotter than the inside temperatures! (Unless of course you are trying to warm it up in there!)
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Old 07-24-2017, 05:52 PM   #4
JABURKHOLDER
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How about when you first set up at your site in hot weather ?
I open the vents, turn on the a/c and push out the hot air. As it cools down inside, I close the vents and keep them closed the rest of the time at that site. Been dealing with triple digit temps the last few weeks and so far so good.
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Old 07-24-2017, 06:59 PM   #5
team bradfield
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Originally Posted by JABURKHOLDER View Post
How about when you first set up at your site in hot weather ?
I open the vents, turn on the a/c and push out the hot air. As it cools down inside, I close the vents and keep them closed the rest of the time at that site. Been dealing with triple digit temps the last few weeks and so far so good.
I do this also, it works well for us. And you are correct, The Chief is always right!
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Old 07-24-2017, 11:32 PM   #6
Mark N.
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Originally Posted by JABURKHOLDER View Post
How about when you first set up at your site in hot weather ?
I open the vents, turn on the a/c and push out the hot air. As it cools down inside, I close the vents and keep them closed the rest of the time at that site. Been dealing with triple digit temps the last few weeks and so far so good.
Only if you are in a situation where it's hotter inside than out. If the A/C units produced a positive pressure inside your trailer as they cooled, that would work. But, they don't. That is exactly how you use an evaporative A/C and it works (In fact you absolutely have to open windows with those because they do produce such a strong positive pressure), but it won't help a refrigeration unit. Actually, it hurts efficiency, just like in a stick home. Refrigeration type air conditioners don't pull air from the outside, then cool it, then push it into your trailer. They do not create a positive pressure on the inside like an evaporative air conditioner does. They simply pull the heat out of the air already inside the trailer, and vent that heat to the outside via the coils. Just like your automobile A/C, they recirculate the inside air, cooling it better than when that air mixes with any outside air.
Obviously, if the air temperature inside the trailer is hotter than outside air temperature, then opening the roof vents will allow that warmer inside air to escape, thereby pulling the cooler outside air in to replace it as it rises up and out. But, that simply can't happen if it's hotter on the outside. Not even with the A/C running. In that case (hotter outside than inside), close everything, block any solar heating you can, and turn on both A/C units and wait for the heat to get pulled out of the inside air.
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Old 07-25-2017, 09:48 AM   #7
DQDick
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Well said, except you forgot to mention the humidity. Part of the AC's work involves removing moisture from the air, hense the water running off the roof. If you let more humidity in the AC has to work that much harder.
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Old 07-25-2017, 11:34 AM   #8
Mark N.
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Well said, except you forgot to mention the humidity. Part of the AC's work involves removing moisture from the air, hense the water running off the roof. If you let more humidity in the AC has to work that much harder.
absolutely! That was just part of the equation I didn't want to get into. You are right, they pull humidity out of the air through condensation, then drain it away. Lower humidity allows more comfort at a lower temperature. 70 degrees "feels" cooler at 15% humidity than it does at 30 humidity. If you've been showering or boiling water inside the trailer, the A/C units will be dumping a lot of water off the roof for a while until the relative humidity inside the trailer drops back down. This is also related inversely to why evaporative A/C units ("swamp" coolers) won't work in regions where humidity is high.
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Old 07-28-2017, 12:50 AM   #9
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absolutely! That was just part of the equation I didn't want to get into. You are right, they pull humidity out of the air through condensation, then drain it away. Lower humidity allows more comfort at a lower temperature. 70 degrees "feels" cooler at 15% humidity than it does at 30 humidity. If you've been showering or boiling water inside the trailer, the A/C units will be dumping a lot of water off the roof for a while until the relative humidity inside the trailer drops back down. This is also related inversely to why evaporative A/C units ("swamp" coolers) won't work in regions where humidity is high.
I've seen the swamp coolers in North Texas and they work fairly well buy you will never see them in the southern part of the state because of the 90 and 100 percent humidity. They don't call Houston the air conditioning capital of the world for nothing.
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