Venting hot air from AC unit

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Venting hot air from AC unit

Postby bobwhite215 » Wed Dec 07, 2016 4:51 pm

I am placing my AC unit internal in my TD. Am building an enclosure off the back to catch and exhaust the hot air coming off. I am exhausting it thru the floor. I am planning on placing (2) 3" computer fans to pull the hot air out. My original thought was to have them both running as exhaust fans. However, it occurred to me I might be better off running them in opposite directions and creating an airflow inside the enclosure.

Any thoughts??

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Re: Venting hot air from AC unit

Postby Shadow Catcher » Wed Dec 07, 2016 5:17 pm

No where near enough CFM, I would hazard a guess you can find the numbers necessary.
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Re: Venting hot air from AC unit

Postby bobwhite215 » Wed Dec 07, 2016 5:23 pm

They are about 30cfm. The total volume if my exhaust duct is less than 1 cubic foot.

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Re: Venting hot air from AC unit

Postby edgeau » Thu Dec 08, 2016 2:22 am

If you have somewhere to let air in use both to extract. If not you will need to do as you propose. Also remember got air wants to rise so you are fighting the natural flow. Heat will build up on the top of the area if there is nowhere to escape.

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Re: Venting hot air from AC unit

Postby noseoil » Thu Dec 08, 2016 8:31 am

A lot of people get in trouble with exhaust air in teardrops. You will need completely free airflow if the unit is to work properly. It needs cooling air from the sides & top (behind the fan), then the exhaust air will want to rise as was stated already. Cool air in from the bottom, exhaust air out to the top is a normal configuration, but now the "box" is getting bigger. I suggest rethinking your plan to allow for free airflow at the back of the unit. It will run more efficiently if it can stick out somewhere, as it was designed to do.

Otherwise you're making something simple into something complex with little to gain, more space taken & more moving parts. Just my $0.02 worth......
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Re: Venting hot air from AC unit

Postby aggie79 » Thu Dec 08, 2016 10:42 am

Take a look at what Mark did for his HVAC: http://littleswissteardrop.com/air.html. Both the intake and exhaust are passive - no fans - and duct to the bottom of the teardrop. His setup has worked well for years.
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Re: Venting hot air from AC unit

Postby bobwhite215 » Thu Dec 08, 2016 11:22 am

aggie79 wrote:Take a look at what Mark did for his HVAC: http://littleswissteardrop.com/air.html. Both the intake and exhaust are passive - no fans - and duct to the bottom of the teardrop. His setup has worked well for years.

Thanks aggie. I have seen that before, and it is pretty much what I am planning. Just thinking about adding the fans under the hood to help the airflow.

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Re: Venting hot air from AC unit

Postby bobwhite215 » Thu Dec 08, 2016 11:28 am

noseoil wrote:It will run more efficiently if it can stick out somewhere, as it was designed to do.

Otherwise you're making something simple into something complex with little to gain, more space taken & more moving parts. Just my $0.02 worth......


I've seen lots of builds with external ac units. I really don't want that. For one, I don't want to have to setup an ac system everytime I pull in. Secondly, I just think it looks kind of bad. I realize lots of people do it, and it works for them, but I am really going for a clean look, and want to keep it inside. Thanks for the advice.

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Re: Venting hot air from AC unit

Postby bam1028 » Thu Dec 08, 2016 7:38 pm

What formula would be needed to calculate the cfm needed to circulate the exhaust?
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Re: Venting hot air from AC unit

Postby linuxmanxxx » Sun Dec 11, 2016 10:26 am

You need to wall off around the sides of the AC unit behind the intake louvers for the exhaust fan or you will have thermal shutdown of the compressor. Feeding the inner plenum in front of the wall with fresh air is your concern as the AC fan itself will force the backside flow to happen. I did what you want but vented out there rear of the box through metal mesh cover on the back painted to make it more aesthetic. Your two concerns are that cool air plenum and condensation drainage. I was quite creative for both. I sloped the bottom floor to a rear corner and added a drain. Sealed the floor and about half the sides with the black stuff used on the exterior bottom of the camper. I drilled holes in the bottom of the AC floor pan at the rear to drain out due to now models sling the water with the fan and don't drain. Not good in a closed box in a camper. The cool air was pulling from inside my galley. Here's the creative part so in extreme heat I could run it with the galley closed up to secure things while sleeping. I took a cheap bathroom vent fan installed in the galley and with flex duct fed the cool plenum and got attic fan thermal relay to run it when the closed galley reached a certain Temp set on it. With proper passive venting in place and the plenum built correctly the fan only ran closed or when external temps were above 90s. I'm in Texas that happens a lot here. I think there are pics on my MicroCampers Facebook page.


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