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Author Topic: How many rooftop a/c's?  (Read 3364 times)
MikeH
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« on: May 10, 2010, 05:08:53 PM »

Hello everyone,

This is my first official post to the board here. I appreciate everything that is shared and hope someday I will have something to contribute besides questions. However, at this point I have a lot of them, so I am going to start posting some.

I am in the thinking/planning process. We don't currently have a bus, just dreams of having a bus. It will be for pretty-much full-time use by our family of 6 as we travel around the country singing Gospel bluegrass music. Starting in the back, there will be a bedroom for my wife and me, then a bathroom, then bunks for the kids, then a living area, and last but not least, the driver's area. We will probably end up with a 40' bus, most likely a 96", but possibly 102. We don't know where or when we will be traveling at any given time, hopefully not in AZ in Aug or in MN in Jan, but you never know, anything's possible, and I'd rather plan for the worst (AZ in Aug), than not and regret it. Given that limited amount of information, how many rooftop air units should I plan on putting in? How many BTU's will it take to keep the bus cool, and will I need to have zones? I don't really know how zones work, does one unit cover a single zone (i.e. bedroom and bath), or do they all fire at once and direct the output to the zone that needs it? Does each zone have it's own thermo?

Thanks in advance for your help.

Mike
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« Reply #1 on: May 10, 2010, 05:25:12 PM »

Mike, first off welcome. I was in your spot a few months ago. You are asking like you want to build a bus? I would suggest looking at an entertainer for what you need. That’s what I did only for the reason I needed to sleep about 10 people at our camping event twice a year. I do have the back as a bedroom and the middle bathroom and 9 bunks. You could take some out in your case for more closet space. Then up front I have all the comforts of a living room and galley. I have 3 roof airs one for front, middle and back. The middle closes off to where it is total darkness for late sleeping and keeps everything cold. If you shop it out right you can really find the deals out there. A leasing company has to move out the old to keep up with the stars. They have made their money with them and it is not for the retired couple touring the country. They make a great base to start with and the deals are out there.
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« Reply #2 on: May 10, 2010, 10:50:42 PM »

I have a AMGeneral transit with very large lightly tinted windows.  I also have 2.25" sprayed insulation (this is the key).  I have 3-13,500btu Colemans-one up front to blow on me, one in the middle, and one over the rear bed.  In 108 degree weather while driving (the hottest we've been in) just two running (front and back) will keep the bus in the mid 70's.  Running all three, and my wife has to put on a sweater.  Once again-good insulation, tight fitting tinted windows and no other leaks are the key to a cool bus.  Good Luck, TomC
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« Reply #3 on: May 11, 2010, 02:43:41 AM »

Mine has 3.  I have nit needed all three but I have not been in 109 degree heat with it yet.
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« Reply #4 on: May 11, 2010, 06:35:38 AM »

I think the number of adult (or full sized) people in the bus has a lot to do with it as well.  6 is a lot for a converted bus.  I would start with three and go up from there, if you are talking rooftops.  Look at a big generator installed to run all the time as well.  Or just look for a used entertainer bus.

Brian
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« Reply #5 on: May 11, 2010, 07:43:05 AM »

Thanks for the replies so far. I don't think an entertainer coach is really an option. We would end up taking a lot of the conversion out to make it work for us. Entertainers rarely have a rear bedroom, a shower or any kind of workable kitchen, all of which would be important for us. I think a better option is to start with a bare shell and build it up the way it will work best for us.

Assuming I need 3 rooftops, how big a generator will I need?

Thanks,
Mike
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« Reply #6 on: May 11, 2010, 08:04:42 AM »

Three roof airs should be doable (depending on other loads) with an 8KW generator as long as all three don't start at the same time.  This assume propane water heater.

If you want to not do any power management and run anything you want at any time you'll need to go to 10KW or even 12.5 KW.  If you buy the larger generator for the worst case scenario you'll need to remember that it will burn more fuel all the time.
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« Reply #7 on: May 11, 2010, 08:07:11 AM »

With my three Coleman 13,500's, I have a 10kw Powertech.  With my truck conversion, I'm using a 12kw-a new model that Dick Wright made up for me with a 3 cylinder Kubota D1305 engine and a newage alternator end.  The whole generator is 35.5" long!  Good Luck, TomC
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« Reply #8 on: May 11, 2010, 09:01:59 AM »

I use a 15 KW powered by a Kabota fuled by the main tank.  My wife turns everything on all the time.  Never had a power problem.
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« Reply #9 on: May 11, 2010, 01:10:56 PM »

Hello everyone,We don't know where or when we will be traveling at any given time, hopefully not in AZ in Aug or in MN in Jan, but you never know, anything's possible, and I'd rather plan for the worst (AZ in Aug), than not and regret it.

I would plan for the worst that you actually expect, rather than the worst that might happen.  If it gets worse than that, it would only be for a short time and it might be more cost-effective to just go into a motel to cover those.

Most folks seem to go with either 2 or 3 roof aircons. 

However, you might be better off using the "portable" aircons that you see at the big stores this time of year.  There are several options for where to put the heat dump vents (out the side, through the roof, through an open window).  They need no installation (thus no roof leak issues), you can store them during the rest of the year, they are cheaper than rooftop units and you can replace them pretty much anywhere if one goes bad.
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« Reply #10 on: May 11, 2010, 01:15:32 PM »

Remember, Sean is trying to trade a 15KW generator for an 8KW because it is just too big and sucks too much fuel.  To be fair, I suspect Sean and Louise are pretty adept at power management plus they have a load sharing inverter that can help.
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« Reply #11 on: May 11, 2010, 01:55:10 PM »

Welcome aboard, Mike.

The issue about roof airs depends on everything. We have five 15K roof air conditioners. They are powered from our 20KW generator.

However, what color your bus is, how much insulation, etc. You can probably do great with two or three. WE have just done an overkill on most of our things (because when we travel, we are on a schedule, and can't be down for any length of time).

FWIW

God bless,

John

I am also sending you a personal message.
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MikeH
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« Reply #12 on: May 11, 2010, 04:49:22 PM »

Thanks for the replies so far. Please keep 'em coming. So far the consensus is that three is a good number of rooftop AC units, but maybe we start with 2 and add another later if needed.

Belfert: We won't be going with anything propane. Not a risk we are willing to consider. We will probably go with an Aqua-Hot for hot water and heat (subject of future posting).

BG6: I admit to an interest in the portable units from the standpoint of no possibility of a leaky roof. Are these like window a/c units you would put in a bedroom window? Any more info on them I'd appreciate it.

John316: Insulation will probably be a big thing. I haven't gotten to planning that yet. I pm'ed you back. Thanks for the note.

Mike
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« Reply #13 on: May 11, 2010, 05:21:03 PM »

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« Reply #14 on: May 11, 2010, 05:22:25 PM »

Mike

If you are converting the bus then consider the following:
Insulation  - spray foam is best

Windows -  dual pane. Yes expensive but in your case needed due to noise and insulation value.

Floor - insulation to cover the drive axle area. Accept the hit on clearance and add at least 1/2", I'd prefer 1", to the floor.

Webasto heating system - Although Webasto is getting expensive it is the best. Consider also Hydro Hot and Proheat. You can build your own system with MSR heaters and baseboard.  AquaHot is just a Webasto in a fancy case.

AC units -  Get three 15K BTU units and send the air to a central duct. Ducting will reduce the noise a lot. Your people load is going to drive the heat loading up. That's why three 15 K AC units. The control system for maintaining temperature is beyond my knowledge base but it is doable. High end conversions did it before computers.

Bus - A 102" will serve you much better than a 96"  The 6" is in the aisle as the furniture and appliances are standard depth.

Good luck
Bill

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« Reply #15 on: May 11, 2010, 05:28:52 PM »

Welcome..we all have different needs ....intrest...financial limits...so you have alot to consider..40 ft coach with all the glass still in =3 or 4 ac..take half glass out drop to 2to3...must insulate insulate...insulate...consider mpg also if your going a lot of miles...also roof tops come up to 15000 and heat pumps are available also to take care of your heating and ac with one unit..jut a few thoughts  Bob
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« Reply #16 on: May 11, 2010, 05:42:45 PM »

...also roof tops come up to 15000 and heat pumps are available also to take care of your heating and ac with one unit..jut a few thoughts  Bob
Bob, thanks for the input. "roof tops come up to 15000" what? Dollars, KW?

I think I am trying to keep out of the basement so we have more storage space down there. I am guessing that a heat pump will end up down there?

Thanks,
Mike
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« Reply #17 on: May 11, 2010, 06:36:49 PM »

Us the Over The Road HVAC when traveling.
When parked 1 Roof Air can keep up, 2 is all you will need. Cool
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« Reply #18 on: May 11, 2010, 06:36:57 PM »

... "roof tops come up to 15000" what? Dollars, KW?


BTU's per hour (BTU/h).  The other common RV size is 13,500 BTU/h.

BTW, unless they are doing calisthenics, each additional human only adds about 300 BTU/h of heat load.  So if you are getting advice about what works for a couple, you only need worry about another 1,200 BTU/h of heat removal for your family of six, or less than a tenth the capacity of a standard roof unit.

The vast majority of the heat will come from the external environment, as has been noted.  Glass is the big killer, followed by inadequate insulation and dark external colors.  Paint the roof frost white, and the lighter you go on the sides the better off you will be.

Quote
I think I am trying to keep out of the basement so we have more storage space down there. I am guessing that a heat pump will end up down there?


Heat pumps are also available in rooftop package units.

Rooftops are more efficient and generally easier to service and replace than basements.  Also, basement units are getting hard to find -- see the extensive thread on this subject elsewhere.  The principle reason to stay off the roof would be aesthetics.

-Sean
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« Reply #19 on: May 11, 2010, 06:44:21 PM »

Us the Over The Road HVAC when traveling.


That's some very subjective advice.  Unless you spend many more hours traveling than parked, which would describe very few of us, the OTR system is a huge maintenance expense, space hog, and weight burden that very, very few RVers can justify economically.  If it floats your boat, go for it, but I suspect that with six souls aboard, you will need every cubic foot of space and pound of weight for other things.

Quote
When parked 1 Roof Air can keep up, 2 is all you will need.


Again, very subjective and probably true for very few people.  I can tell you unequivocally that we need three units running full-tilt when the outside temps are over 110°, and we have an extremely well-insulated and efficient bus with only two humans and three ~15lb pets on board.  That's in a 40' coach, which I would also guess would be the minimum length you would want for six folks.

I would frame and wire for three units.  You can always put them in one at a time when you better understand your needs.

FWIW.

-Sean
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« Reply #20 on: May 11, 2010, 06:48:43 PM »

Bill B Bus is spot on.

I agree about the floor insulation. We have 1.5" foam under our half inch plywood. That has helped alot.

We have the Oasis heating system (like Aqua hot) and it is great.

And everything else that Bill mentioned hits the nail right on the head.

God bless,

John
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« Reply #21 on: May 11, 2010, 07:31:45 PM »

see we all have opinions..I was speaking of the 15000btu heat pump roof air...I don't use anymore because I like the Basement air...I have found it to be more efficient..again a point of discussion...comercial units are getting hard to find..home engineered units take a little thought,but can be done..I recently posted pictures of how I did mine.  how I did homemade basement air...planning locating new system in old condensor compartment....Roof air is very simple...the coach I am doing now had 4 roof airs on it before I got it,was a 6 bunk entertainment coach...
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« Reply #22 on: May 11, 2010, 08:34:28 PM »

see we all have opinions... I like the Basement air...I have found it to be more efficient..again a point of discussion...


It may be a "point of discussion," but there is no debate.  This is a simple matter of physics and it can be proven beyond doubt -- the thermal efficiency of a package unit on the roof will always be higher than the thermal efficiency of an identical system below the floor.

Different systems from different manufacturers with different designs, will, of course, vary.  So it is possible to have poorly designed (or engineered) roof units whose performance can be surpassed by a well designed and engineered basement unit.  But that's true of anything.  The general case is not a matter of opinion, but fact: rooftop package units are, generally speaking, more efficient than basement units of the same effective capacity.

Unlike traffic laws and even my coveted codes, the laws of physics can not be violated.

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« Reply #23 on: May 11, 2010, 09:34:25 PM »

Sean, why are the roof top units inherently more efficient?
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« Reply #24 on: May 11, 2010, 10:23:01 PM »

Give you a cost difference- a 13,500btu/hr Coleman with heat strip (they do work) about $500.00.  A 15,000btu/hr Coleman heat pump about $800.00.  Good Luck, TomC
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« Reply #25 on: May 11, 2010, 11:15:33 PM »

Dometic calls the "heat strip" a "chill breaker".  It is not adequate for heating....not at least in my coach.  As I understand the instructions it is to be used when warming things up after a cold soak and in concert with the furnace.  I would skip the expense but Tom is known to be correct with frightening regularity and that gives me pause.  Get the heat pump AND the wabasto system so you can get hot water and warm the engine.  There is only one instance where flexibility is a real detractor and HVAC isn't it..

I urge duplication in systems.  Wabasto and heat-pump and your OTR.  3 roof air so you can normally run two and keep the whole coach comfy. Three on to pull the temp down quickly after a heat soak.  One breaks and you still have little to worry about.  OTR that cools the whole coach using an efficient modern auto compressor and the OTR is a back-up for the entire roof AC system that can be taken out by one electrical problem.  Lose AC in 90 plus and you cannot stand the heat in the coach...literally.  Seems like over kill in a lot of ways but in the planning stages it isn't all that much of a chalange or expense......doing it yourself, of course.

I think you are off to an excellent start.  Good luck to you and your family and your mission.

John
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« Reply #26 on: May 12, 2010, 12:56:09 AM »

Sean, why are the roof top units inherently more efficient?


The short and simple answer is because heat rises.

The longer version:

Air conditioners are inherently inefficient machines.  They take heat from one place, and move it to someplace else, but they also use a lot of energy in the process, much of which is dissipated as waste heat.  Lots of the waste heat is discharged through the condenser, but not all of it.

Take as an example a pair of idealized systems.  We will use package units for our example but the same reasoning applies to split systems (most RV units, whether roof or basement, are package units).  One system will be a roof system and the other will be basement.  They will use identical parts, IOTW the same evaporators, condensers, fans, compressors, and other items.

Let's further assume that either of these systems would have ducted supply and return vents located in exactly the same spot in the coach, mid-way between the floor and the roof, so the duct lengths would be the same in each case.  In addition, we will also duct the supply and return condenser air from an identical location midway up the outside coach wall, again same duct length.

So to be clear, the only difference between these two systems is one has all the working parts on the roof, and one has all the working parts under the floor.

At a bare minimum, whatever internal heat is generated by the working parts that is not discharged through the condenser rises into the coach in the case of the basement unit, and rises away from the coach in the case of a roof unit.  So at a bare minimum, you have this additional heat load that will cause the basement unit to have to work harder than the roof unit to achieve the same result.

Of course, our "idealized" system is far from practical.  We theoretically located the evaporator and condenser supply and return in an impractical place, just for the sake of proving that, no matter what you do with them, there is always a penalty for units under the floor versus units on the roof.  As a practical matter, things are actually even more lopsided.  Nobody ducts the condenser air this way, and in practice, hot condenser discharge from a basement unit ends up under the coach, where it ends up heating the coach even more, whereas hot condenser air from a roof unit is discharged above the coach where it will rise away from the coach.

Likewise the most efficient arrangement for evaporator discharge (cool air into the coach) is near the ceiling, and it requires less ducting and therefore less fan horsepower to supply air to ceiling registers from roof units than from basement units, which must first be ducted all the way from the basement to the ceiling to achieve the same effect.

So for units of identical cooling capacity, roof units are more efficient than basement units.

Now where this breaks down is when you start getting into larger capacities.  The largest roof units are about 15,000 BTU/hr, whereas basement units could theoretically be built in any size, and in practice come in capacities up to 30,000 BTU/hr.

If you need a full 30,000 BTU/h of cooling, it will almost always be more efficient to get it from a single 30,000 BTU/h unit that from two 15,000 BTU/h units, and so in this case, a 30kBTU/h basement unit might edge out a pair of 15kBTU/h roof units.  However, this equation flips around completely if you only need half that amount -- running a single 15kBTU/h unit at full capacity will be more efficient than running a 30kBTU/h unit at half capacity.

Over the full range of most usage profiles, a mix of capacities is required, and slight efficiency differences due to this size/efficiency disparity tend to even out.  If your usage profile is such that you mostly need only 15k or so of cooling most of the time, perhaps moving that cooling from the salon to the bedroom and back as needed, then two 15k units, irrespective of whether those are basement or roof, will be a more efficient solution for you, whereas if your usage profile is such that whenever you need air conditioning, you need 30k of it, then a larger unit may be a better choice.

HTH,

-Sean
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« Reply #27 on: May 12, 2010, 04:56:17 AM »

Welcome MikeH!   I'd like to mention one other factor in the AC selection that I don't think has been stressed thus far....   A big factor is how you plan to operate the cooling system. Let me explain:

On our family's 4104 (a 35 foot bus), we have only a single 13.5K BTU roof unit.  With 15+ years useage on this Coleman, I'm sure it has lost some efficiency.  Nevertheless, we still find it adequate for our needs. We have been on multiple week long trips to places like Alabama and Florida in the heat of summer, with 5 occupants.  The key is we use the AC "smartly".  We try to stay ahead of the heat curve by turning it on before the bus gets too warm (or we run it 24/7).  When parked, we take full advantage of window shades, awnings, and tree shade.  Overall, it works well for us, and my desire for a second rooftop unit has been steadily dropping on my priority list.

However, our usage does not match everyone else.  Some folks need AC systems that can handle extreme situations.  Take for example a bus that is left sitting in the afternoon sun on a 95 degree day, with everything shut down. The interior could easily reach 105. At 4 pm, everybody piles onto the bus and gets readily to leave.  You want the bus cooled down immediately!  That is going to take serious BTUs.

My point?  .... How you plan to use the bus is as much a factor as the insulation, paint color, window glass, etc.
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« Reply #28 on: May 12, 2010, 05:29:05 AM »

so as you see it turned into a debate..ser factor never came up though...so to finish my coment it comes to cost efficiency...over the road air is a major expense and headache to maintain...would be idea but could cost you more in the long run.ck to see what 30 lb of refrigerant installed would cost.? My basement air units discharge hot air out the side where it rises to the sky not the bus..I do use a 20 inch fan to bring heat off the ceiling to mix with air and also my ac vents are discharging cold air up at a 45 degree angle..have experience with monthly elect bills at RV park (apples to apples) compared with sticks and staples 'same time same month..my bill was 91 dollars a SS friend was 142 and another 240..they both had roof top air....my coach is a 40 ft with 2 slides with 37000 BTU ac capacity..total initial cost under 900 dollars...haven't lost a basement unit to bridge or tree yet...so will agree to disagree...I'm sure my ideal is not as efficient but feels good inside our coach.   Bob
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« Reply #29 on: May 12, 2010, 05:35:54 AM »

Mike, welcome!  A couple of answers here.  Entertainer buses usually use many more roof airs than a basic family conversion.  Most buses are broken down into three main area's, living room/kitchen, bathroom, and the bedroom!  Duotherm makes air conditioners, and heat pumps for roof mounting!  Roof tops, are definately more efficient, easy to work on and replace!  Duotherm systems can be zoned together allowing each area the benefit of the need in that area,  with remote sensors in each area they can operate independantly of each other depending on the zone or owners need!  Another feature of Duotherm is it allows the addition of a full heating system remotely located and capable of running on the same thermostat, all of these units can be operated in a zoned environment, as the owner wishes.

Like Sean said, basement units are becoming rare, which was in my opinion caused by a downturn in the economy!   A basement unit will take up a lot of storage area in a bus usually an entire compartment.

Roof airs by many are considered an eye sore.  It depends on you the owner and your pocketbook.  I'm not a fan of window air conditioners in a bus but many use them and seem to be satisfied!  Remember this, one day you will sell the bus.........and what you build into the bus today will determine the price of the bus when the day to sell comes!

Again good luck and ask many questions before starting or buying............

« Last Edit: May 12, 2010, 05:40:04 AM by muddog16 » Logged

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« Reply #30 on: May 12, 2010, 05:55:12 AM »

Welcome Mike! 
It is always better to overbuild in the first go round than to try to add later.  Roof airs almost always require cutting the roof and roof supports and welding in new supports.  Not very easy or desirable to do that after you put in a nice new ceiling!  I have seen as many as 5 roof airs on some Prevosts!  I figure one roof air per living area.  1 for the driver and front living area, 1 for the kitchen /rear living area, one for the bathroom/bunk area and 1 for the bedroom.  Definitely have it spray foamed.  That was some of the best money I spent!  With as many people as you are going to carry, and since you could face a great range of temps, you should build for the worst mother nature can throw at you.  Go for 4 rooftops.  This also means redundancy!  If one goes out, you're still probably fine.  Don't forget that when you think about heat.  I have 2 propane furnaces in my bus.  It gives me the option to run smaller furnaces in the places I want heat.  Good luck and keep asking questions.


Glenn
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« Reply #31 on: May 12, 2010, 06:08:07 AM »

There are a few other factors that will determine how many ACs you will need. How well is the coach insulated? Poorer insulation allows more heat to transfer through the walls/ceiling. The ACs have to remove this heat.  What color is the coach painted? Darker colors absorb more heat, which has to be removed by the ACs. How much glass and/or awnings. The more light that comes in through trhe glass, the more radiant heat that the ACs have to remove.  Jack
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« Reply #32 on: May 12, 2010, 11:56:40 AM »


BG6: I admit to an interest in the portable units from the standpoint of no possibility of a leaky roof. Are these like window a/c units you would put in a bedroom window? Any more info on them I'd appreciate it.

No, these are a standing unit with a length of dryer duct to pump the heat outside.  You put them where you want them, move them from room to room, etc.  They are about 24 - 30 inches tall, 18 inches wide and a foot deep.  I don't see any reason that you couldn't hang them on the wall if you wanted to.

I've been seeing them at my local Costco for a month or so for something like $400 each, and they are at the big hardware places.  I wouldn't be surprised if they are also at Walmart, so just call around and you'll find them.  DO NOT say you're putting them into an RV -- if one goes bad, you don't want someone saying that you caused the problem.

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« Reply #33 on: May 12, 2010, 04:29:19 PM »

I believe it was Jerry Liebler who said that the portable units are grossly over rated in terms of BTUs.  Do you have enough space for a number of these?

I have two roof tops and I need one more up front.  I have tons of glass up front and one roof top isn't cutting it.
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« Reply #34 on: May 12, 2010, 06:52:24 PM »

Go with 4 if you are going to be in warm climates. I have 3 and wished for one more. Live in florida No less than 15kw on genny. Marc
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« Reply #35 on: May 12, 2010, 07:55:48 PM »

Welcome to the group!

We were getting advice to put on only two a/c units.  Soooo glad we had 3 installed instead.   Been very happy w/ that setup.  We have a 10 Kw Kubota from Dick Wright ( Wrico ) - excellent unit.

Kind Regards, Phil
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« Reply #36 on: May 13, 2010, 05:47:23 AM »

I realized another big variable. What are you used to? I have noticed that the older you are, the less cooling you need. 75-80 feels good for some. However, others are used to 65 or less. That is the way with us. When we are running the air, we want it to be around 65 up to 70.

We can do that, because when the air is running the genset is running. If we were running a couple roof airs off of our inverters and batteries, then we could do with less. At night we often need just one to keep things cool.

So what are you used to? Cooler, or warmer?

God bless,

John
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