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Old 05-17-2018, 09:19 PM   #1
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Tiffin Owners Club
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2018 34 PA AC Units

I just noticed this and seems a little strange. Apparently in my unit there is a single AC main duct line that runs all the way down the passanger side from the front passanger seat to the rear wall in the bed room. Both AC units blow cool air into this single channel. Therefore, either unit can cool the entire RV and having one or both on does not seem to add greater air flow from the vents with the exception that the front vents blow slightly harder with the front AC unit is on and the rear vents blow slightly harder when only the rear AC unit is on. I had originally thought that since there are two themostats the units could be run to cool the front side during the day and the other cooling the bedroom during the night for sleeping. So I was setting the front thermostat higher at night but I find my front unit shutting down and my smaller rear AC unit trying all night to cool the entire RV to the tempature set for the rear section only. Basically it runs all night without stopping because its sending most of it cool air to the front because there are only 2 vents in the bedroom and many more in the rest of the unit. Is this what Tiffin had in mind?

I unfortunately thought there was a separator placed in the AC ducting dividing the duct into a front section and a rear section. I guess I was wrong in my presumption.

Does anyone have thoughts on how such a divider in the AC ducting could be installed without major issues to the integraty of the RV itself?

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Old 05-18-2018, 06:15 AM   #2
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you could shut the vents closed that you want. I replaced any non closing vents with ones that close.

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Old 05-18-2018, 06:15 AM   #3
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Most newer, dual A/C campers have the shared "loop" duct system. It's designed just as you described. We run the front unit at night (bedroom is in rear) and rear unit during the day to reduce noise. Sometimes we run both. If you look at the inner, overhead "face-plate" of each unit, you should see a "slide" type vent device (or something similar) that will allow you to direct all of the cool air from that unit directly into the room you are using it in. This stops most of the air from traveling through the duct work. This will isolate the cool air to just one room and thermostat should take over after that. Sleep well!! Stay cool!!
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Old 05-18-2018, 07:35 AM   #4
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I have made a similar observation. I have a 37BH Phaeton. There are three units, three controls, three thermostats. But I hardly see the point? I don’t quite get it I guess. The thermostats for the “mid” is in the half bath which is always hotter, understandably. So I get huge (5+) degree temperature reading differences between the units. So to get one to come on in front or back I have to turn them WAY down. I am trying to figure out how this all is supposed to work best.
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New to all this....
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Old 05-18-2018, 07:43 AM   #5
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Tiffin has always used a shared ducting system that runs from the front to the rear. Cooling can be somewhat uneven depending on which way the front end of the coach is pointing. The windshield is a great solar collector.

I found that the best way to improve the air conditioning performance is to set the system so the A/C fans are On and circulating air throughout the coach and let the A/C units cycle on and off. Far fewer hot and cold spots.
Roger & Mary
2017 Winnebago Navion 24V
2014 Tiffin Phaeton 36GH (Sold)
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Old 05-18-2018, 08:40 AM   #6
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Just decide where you want to separate the system and pull down that vent outlet. An old t-shirt (or something about the same size) into the duct just ahead or behind the vent outlet and you now have a true zone heating/cooling setup. We've been using this setup in the last two Buses since 2008 and have never had an issue. Just remember, if you separate the systems this way, YOU are then responsible for setting temps the way you want it. Many people don't like having to go through this trouble, but the decision is then yours if you choose to separate the ducts.
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Old 05-18-2018, 09:23 AM   #7
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My 2018 36LA has the same arrangement.....the unitary duct is a plus for redundancy in the event one unit fails you still have central air from the second (or third) unit.

Since the outlet air flows aren't all that different I view the two units as staged cooling and set the units 2 degrees apart (first and second stage cooling). At night, for just the two of us, we run the forward unit (2 degrees lower) and just point the two outlets in the rear of the living area towards the bedroom door and we're good to go.

BTW, unless you block both the supply and the exhaust ducts you aren't completely separating the units. Take care in blocking ducts though, if you restrict the air too much you can have negative effects on the working unit by way of icing up the evaporator coil and slugging the compressor.


+1 on fans in "ON" mode.
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Old 05-21-2018, 06:28 PM   #8
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2018 34PA AC Units

Thanks all for your suggestions. Going out in 2 days and will have time to test them all.

Happy Travels

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