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Author Topic: Dometic Roof A/C  (Read 2945 times)
usbusin
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« on: July 15, 2010, 12:43:48 PM »

Does anyone know if the Dometic roof-top air conditioner can be set on "recirculate"?

In your automobile you have the choice of cooling the fresh air or the recirculated air. 

What is the maximum temperature differential between outside air temp and the inside cooled air?  Twenty degrees?

What about recirculated air? 

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Gary D

USBUSIN was our 1960 PD4104 for 16 years Ustruckin' is our 2001 Freightliner truck conversion
bevans6
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« Reply #1 on: July 15, 2010, 01:14:32 PM »

Rooftop AC's. at least the ones I've seen including my dometic, can only recirculate.  They draw all air to be cooled in from the interior of the bus through a screen and send it out the duct vents.  No fresh outside air at any time.

Brian
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« Reply #2 on: July 15, 2010, 02:34:57 PM »

Brian is right on! That's how ours works, and works great!

Paul
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« Reply #3 on: July 15, 2010, 02:43:16 PM »

Same as Brian and Paul.  I've not yet seen a roof air that brought in fresh air.  Maybe they have been made, but I have never seen them.

As far as external temperature range - I don't think there is a realistic limit.  But like all AC's they have a finite cooling capacity.  Usually 13,500 or 15,000 BTU's.  How well that cools vs. outside temps will depend largely on how well insulated the bus is, what color paint (white, light or dark) and how much window surface area it has.  Most conversions seem to have 2-3 roof airs.
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« Reply #4 on: July 15, 2010, 03:03:21 PM »

 3 roof tops are cutting it today in Az we just got back from lunch with friends that have a light colored H-45 Vantare with a white roof and  factory foam they had to unplug and are now running the generator sitting in a RV Park lol 50amps in this heat will not run all 4 and even with 4 running it is not real cool in their bus.
Would you believe they are going to head to North AZ 120 degrees is just to much for the pansies lol
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« Reply #5 on: July 15, 2010, 03:33:15 PM »

I've been runnin one of ours on low half the day. When the wife gets home, I turn on one more and it keeps us cool. 97 degrees until further notice! Grin We have three, and switch back and forth depending on what part of the coach we are in.

I'm glad we don't have four, it would be a freezer in here! Roll Eyes
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« Reply #6 on: July 15, 2010, 03:53:07 PM »

Thanks to all of you for the responses.  I was under the assumption that they took in outside air.  (As the bird colonel told me, "don't assume; it only makes an A-- out of U and ME)

I now feel better about traveling across the Mojave desert through Las Vegas and on across the country on I-70!!  We aren't leaving until the 26th and I sure hope we get a cooling trend by then.  LOL

Thanks, again.
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Gary D

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« Reply #7 on: July 16, 2010, 07:20:19 AM »

I dont know what route you are going across the mojave desert, but if you swing by Ridgecrest give me a hollar and I will have ice cold drinks and a dump station at my home you can use
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« Reply #8 on: July 16, 2010, 08:09:54 AM »

Thanks, happycamperbrat, I appreciate the offer, but we are on a tight schedule this trip, at least on the out-bound leg.

We are traveling highway 58 to Barstow, then to Vegas and on to Indiana over I70 and I80.

Maybe we can catch you later on our way to Arizona next winter.
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Gary D

USBUSIN was our 1960 PD4104 for 16 years Ustruckin' is our 2001 Freightliner truck conversion
JohnEd
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« Reply #9 on: July 16, 2010, 11:36:52 AM »

Does anyone know if the Dometic rooftop air conditioner can be set on "recirculate"?

In your automobile you have the choice of cooling the fresh air or the recirculated air. 

What is the maximum temperature differential between outside air temp and the inside cooled air?  Twenty degrees?

What about recirculated air? 




If you take liquefied freon, as it is out of your compressor, and cool it, as it is out of your condenser, and spray it into a evaporator, as you do in you AC, and blow air across it you will find a absolute max low temp you can achieve.  That is only germane if you can put enuf air thru that evap unit to absorb all that "cold" and blow it out.  Few design can do that and there is a cut off thermostat that is set to 35 degrees or so cause to go lower will freeze up the coils and without air flow you get nada for cooling.  The more perfect your insulation, the closer to 35 degrees you can come.  Here was how I got some idea of just how much temp I could tolerate.....Turn on one AC unit to MAX and let it run for hours.  Measure the temp inside and subtract that from the outside temp and that will tell you how far you can pull the temp down.  Do the same with the other unit and that will tell you what it can do and which is the stronger unit.  Finally turn on both and let them run for a long time.  Take that number....say 20 degrees and at 100 degrees your inside temp will be 80 degrees.   All that I have said is incorrect...cause the efficiency of the units changes with outside temp and you can make a AC unit work miracles by spraying water on the condenser fins.  Did you hear me say "SOME IDEA"?  If you try this at 80 degrees ambient I think you can safely assume that the cooling ability will go down a smidgen at higher temps.  Nick or Sean could refine this procedure a little, shoot hoes in it, discount it all together and provide alternative procedures you nor I could comprehend....let alone apply. Careful!

This can also let you know what you R factor is for the entire bus and could eval the benefit of improvements as you build it or make mods to an existing coach. What did I get out of isolating the rear? How much did carpet change things?  Stuff like that.

HTH,

John

PS: This procedure works with your space heaters and your furnace(s)  How low can you go?
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« Reply #10 on: July 16, 2010, 02:31:22 PM »

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« Reply #11 on: July 16, 2010, 06:10:14 PM »

Dallas,

You can feel free to quote me on matters "Bus".  I'm sorry you are having problems....truly, I am.  As you were told by the moderators, use PMs for the personal things such as "love" and whom to.  I will PM you this evening on personal matters about the bus posts.

Be well Dallas....Best regards to Cat,

John E. Grabe
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« Reply #12 on: July 16, 2010, 10:01:59 PM »

I dont know what route you are going across the mojave desert, but if you swing by Ridgecrest give me a hollar and I will have ice cold drinks and a dump station at my home you can use

We're going to Vegas next weekend. But I was in Ridgecrest last Sunday and it was 112 degrees! Makes my 103 in Bakersfield seem downright cool!
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« Reply #13 on: July 16, 2010, 11:19:42 PM »

Songman,

You have a Eagle with three roof tops....correct?  Which AC units do you have?  Make and size?  How did your bus do at keeping cool in that 112 temp.  What is the performance you experience in what temps.  Do you have spray in foam in the ceiling and floor and walls?

Thanks for sharing,

John
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"An uneducated vote is a treasonous act more damaging than any treachery of the battlefield.
The price of apathy towards public affairs is to be ruled by evil men." Plato
“We can easily forgive a child who is afraid of the dark; the real tragedy of life is when men are afraid of the light.”
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« Reply #14 on: July 17, 2010, 09:30:39 AM »

Nope, John. Must be thinking of someone else. I had the Eagle with the natural air conditioning.



But I sold it and am busless at the moment. Whenever I buy another Eagle I will probably pick one up that is already converted. That seems to be the way to go in this economy.

Actually, for my Ridgecrest trip I was in my Jeep Wagoneer. But it gets about the same mileage as a bus. I get about 7 mpg no matter how or where I drive it.
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« Reply #15 on: July 17, 2010, 09:55:55 AM »

Songman,

Good pic.  That certainly is natural.

I had a Land cruiser with the inline six and it never got more than 15  or so.  I dearly loved that FJ55 but economical it wasn't.  What is it with inline six 4X4's?

I agree with what you said about the Eagle conversion being a hot ticket.  Had I known more about that one advertised for 29K recently I might be a Iggle owner by now.

I learned that that AC unit you picked up with the metal exhaust appendage....that metal is very soft aluminum ad is intended to be simply bent and pushed back up into the duct to allow for it to lay flat for shipping.  Whoo Knu?

Hope you can get your bus soon.

John
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"An uneducated vote is a treasonous act more damaging than any treachery of the battlefield.
The price of apathy towards public affairs is to be ruled by evil men." Plato
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—Pla
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« Reply #16 on: July 17, 2010, 10:28:07 AM »

My Waggy has the big ole 360 V8. I'm keeping a lookout for a diesel donor for it.

That A/C unit is sitting in my garage until we can find someone headed East to take it to TN. I straightened that duct out and like you say, it was very pliable.

We've still got plenty of room on our forum for Eagle owners so grab you one and come on over!  Cool
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« Reply #17 on: July 17, 2010, 10:39:16 AM »

JohnEd, that center fold Eagle may have not been a good buy for you somebody was looking out for you I heard through sources the person that bought it lost the engine in less than 400 miles  

good luck
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« Reply #18 on: July 17, 2010, 02:43:13 PM »

Clifford,

I read  your post very carefully.  You were guardedly optimistic and said it had been "out of your sight" for a couple years.  That was my read, anyway.  I pondered that awhile and got bad vibes from the either.  Nothing you said.  Even with a new engine it would not have been a bad deal for someone that could afford it.  Looked well done and I think you confirmed the quality of the construction.

Thank you for your comments and help,

John
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The price of apathy towards public affairs is to be ruled by evil men." Plato
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« Reply #19 on: July 17, 2010, 03:05:43 PM »

Songman,

I had that very engine in my Jeep and a Turbo 400 trans was behind it.  J19 and it got around 20 mpg and went like a bat.  Try a spread bore "Thermoquad" or "Quadrajet".  If you have that FORD carb get rid of it....Autolite, I think.  If you don't have a spread bore intake manifold then that is another item to pick up.

Drain the "tar" out of the differential and replace it with synthetic.  $15.  Replace the oil in the trans with syn and the transfer case as well.  Don't go UP a notch on the viscosity but stay with the thinnest the book calls for.  Your distributor is shot unless you changed it.  Get a speed shop item that is curved for off road.  Mallory, MSD, whatever.  VAC advance with cent. Silicone plug wires.  Filter minder....you keep those when you trade or junk but till then you make sure the system isn't being choked off and never waste a dime on an unnecessary filter change.

I think you can do the oil for less than $50.  Minder for 27 on EBay.  Distributor for $25 used and the Mal and such are rebuildable but should go 50 Kmiles before needing service.

MAKE SURE your vac advance is connected to the correct port on the carb.  Manifold vac will chop 30% off your MPG.  That was a great engine.  Not what I would choose to rebuild but still a little torquey power house.  The 400 Cid version is a direct bolt in replacement when that time comes.

Vac gauge will verify the carb mix at different throttle settings and alert you to tune problems as they are building.  I once caught a wheel bearing going out because the vac was reading 12 when it usually read 13 so mine paid for itself many many times over with that one heads up.

I like my gauges and the data they let me mull over.
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"An uneducated vote is a treasonous act more damaging than any treachery of the battlefield.
The price of apathy towards public affairs is to be ruled by evil men." Plato
“We can easily forgive a child who is afraid of the dark; the real tragedy of life is when men are afraid of the light.”
—Pla
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« Reply #20 on: July 17, 2010, 04:33:25 PM »

Thanks JohnEd. That is some good info. I have several of these old FSJs. The J10 truck has the exact setup you described. This particular GW we are talking about has a 727 trans. I will follow your guidelines for both to see if I can improve my mileage. Thanks.
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