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Author Topic: Vent size  (Read 1492 times)
Busnut83
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« on: October 13, 2006, 10:16:14 AM »

New task??? I have an onan 4.0 generator in the rear dr. side.... I normally leave the door open when running the gen.... If I left the door closed what size and where should the vents be??? I have the exhaust running threw the floor and a tail pipe just 4" outside the tire,,, the compartment is not divided,,, 1983 mc-9 njt...

Thanks for all suggestions
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Nick Badame Refrig/ACC
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« Reply #1 on: October 13, 2006, 11:41:00 AM »

Hi Busnut83,

My suggestion would be, Make 2 vents, one high and one low for proper intake and exhaust.

As for the size, Make them as large as you can....

Nick-
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« Reply #2 on: October 13, 2006, 02:21:43 PM »

I have a wrico 13k, Kubota diesel.  Dick recommended two vents, one with a fan on it and one without.  His recommended vent size for my setup was 80 square inches for it to run the best.  He said I could go as small as 40 sqauare inches per vent.  This should give you some idea as to the max size you might need.
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« Reply #3 on: October 13, 2006, 02:36:53 PM »

Most Onan's vent out the bottom! So if your intake vent is a little bigger than your exhaust vent you should be good.
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DavidInWilmNC
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« Reply #4 on: October 13, 2006, 02:40:41 PM »

Is your generator an RV style or a regular, portable generator?  The reason I ask is that many of the Onans have a blower that will exhaust through the floor.  This is the type that I have.  Somewhere, I've read that my model (6.5kw) needs 48" of air intake.  I'm not sure where I read this, but I'll go through my manuals and see if I can find it.  If yours is a portable model sitting in a bay, it'll be entirely different. 

David
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Busnut83
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« Reply #5 on: October 13, 2006, 06:54:35 PM »

Yes it is a rv gen... taken from my 1986 southwind 21'... I dont remember if the floor was open or closed.... the side door was maybe 26" x 26" ....with the vent at the top of the door...
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TomC
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« Reply #6 on: October 13, 2006, 11:18:10 PM »

Measure the exhausting vent that is through the door and put a vent in that's twice the size to feed it since the exhaust vent is running at pressure.  The usual way is to have a bulkhead on top of the generator with the vent above going across to the opening in the rear so to shield the noise from the outside.  Good Luck, TomC
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« Reply #7 on: October 14, 2006, 06:18:38 AM »

When we installed a 6.5 Onan gasolene generator in the AC condenser compartment, we started by leaving the entire mesh panel on the door open. We installed a RV Safety Systems alarm system that monitors ambient air temperature in the generator compartment. The ambient temp was 105-115 degrees. We started blocking off the mesh panels until we had an opening of 8"X8", Temps were still 105-115. We noticed a considerable reduction in generator noise with the smaller air opening. The opening was at the top rear corner of the door.  Hope this helps, Jack
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« Reply #8 on: October 14, 2006, 07:42:11 AM »

From the Onan Installation Manual for the 4.0 CCk which is of that vintage:

" For the CCK set running at 1800 rpm, the amount of air discharged is 750 cfm. The minimum free air inlet with no filter or restriction is 140 sq. in."  HTH
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Busnut83
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« Reply #9 on: October 14, 2006, 11:06:21 AM »

Stan---- does that mean the vent should be at-least 10x14" HuhHuh

Thanks
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« Reply #10 on: October 14, 2006, 11:56:30 AM »

Busnut83: That is the way I read it, and as they note, that is unrestricted. area. If you put a grill in the hole, then the hole has to be larger to compensate for the space taken up by the grill.
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