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Author Topic: Rooftop AC  (Read 3156 times)
Tikvah
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« on: May 18, 2011, 09:40:18 AM »

I realize this could open a can of worms.  But, easy answers often are never easy.

One of the first things I want to do in my new conversion project is get the rooftops installed.  One, so I know where the wires need to run, and secondly, it will be hot working in there without them.
I don't plan to spend my summers in Arizona or Florida.  But, I don't like heat either.  So, here's my questions:

Do I need two or three rooftop units?  My current plan is to install three; one over the driver, one about mid-way, in the kitchen area, and the third in the rear bedroom.
What size should they be?  Maybe this depends on insulation.  I am not raising the roof, so I'll have about 3" of insulation in the roof, including both dow foam board, and some bubble-reflective.
Also, I'm a firm believer that air conditioners are better slightly undersized than oversized, especially in the humid areas. 
Does anyone use the heat strip that is an option with these?
Do you have any brands that are preferred or brands that I should stay away from?

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« Reply #1 on: May 18, 2011, 09:49:46 AM »

Three should be plenty.
I have heat strips in mine but can't see what good they are. Kind Of like P$$!NG in the wind if you know what I mean.

John
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« Reply #2 on: May 18, 2011, 10:20:58 AM »

I gotta tell you I was really surprised when I had my driveway dwellers from Alaska recently. They had a 35' coach and said the insulation in it was just a layer of that foil bubble wrap. While they were here it was 105 one day. Their bus was parked with the windshield facing west and no shade. They hung a heavy blanket in the windshield and only had one roof top air. It was very comfortable inside their coach! I dont know what brand or whatever the roof top was, but it was older and it worked very very well even here in the  HOT
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« Reply #3 on: May 18, 2011, 10:29:07 AM »

I have to tell you... one of my companies sells that bubble foil.  I've come to realize that the benifits of this stuff is huge.  The "R-value" isn't great, but it is hard to argue that it reflects an incredible amount of energy.  It is amazing in steel pole barns that gain heat.  I've never seen it used in a bus, but I certainly plan to use it, both in the roof and the walls.  Reflecting the sun's heat out in the hot summer sun is the biggest part of this game. 
I did a work shop recently for my previous home.   It had 4" walls.  I simply stapled this bubble/foil/bubble to the walls with a 3-1/2" fiberglass inside the studs.  It turned out that the shop could be heated with very little energy.
Enough of my sales pitch Smiley
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« Reply #4 on: May 18, 2011, 10:29:54 AM »

I like having 3. I do not think undersize is better than oversized. In your house it is important because it runs off a thermostat. If you bring the temp down faster than you remove the humidity yes it will always feel damp and sticky eventhough it is 75*. In your coach it is a bit differant. They will need to run all the time removing humidity. At least here it does. It takes about 5 min for people start to complain about the heat if the genny gets hot and shuts down. The only way I can keep it going in 95* weather is to close the front lounge door and turn the back two off. Get what you can afford. You don't want to be buying six of them to fix the heat. You can always turn one off.
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« Reply #5 on: May 18, 2011, 10:42:03 AM »

I would put more A/C capacity towards the front as that is where all the windows usually are.  My front living area has lots of glass and my one 15K BTU unit up front can't handle the heat load especially when in the 90s.  I am going to be installing another A/C unit up front.

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« Reply #6 on: May 18, 2011, 10:42:44 AM »

I have to tell you... one of my companies sells that bubble foil.  I've come to realize that the benifits of this stuff is huge.  The "R-value" isn't great, but it is hard to argue that it reflects an incredible amount of energy.  It is amazing in steel pole barns that gain heat.  I've never seen it used in a bus, but I certainly plan to use it, both in the roof and the walls.  Reflecting the sun's heat out in the hot summer sun is the biggest part of this game. 
I did a work shop recently for my previous home.   It had 4" walls.  I simply stapled this bubble/foil/bubble to the walls with a 3-1/2" fiberglass inside the studs.  It turned out that the shop could be heated with very little energy.
Enough of my sales pitch Smiley


Good to know! I have a lot of "dimples" inside the RTS and plan on filling them with cut to size foam board, then a complete layer of foam board over that, then another layer of that foil bubble stuff. I will only be using the foil bubble stuff in the back where the engine is.... maybe 2 layers of it there, not sure. But Im sooo impressed with this stuff for both fire prevention, fumes, and now the actual insulation value too! Time and time again in here I read the most important thing to keeping a coach comfortable is the insulation...
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« Reply #7 on: May 18, 2011, 01:55:37 PM »

I went overboard with the AC's but I dont regret it. I have 5 Coleman Mach 15s with the heat strips.
Plus I have the original bus AC in case the generator breaks down.
I think they are a God send!

I've used the heat strips when the temps were in the high 30's and they worked very well. It takes about 10 or 15 minutes to warm up but when they do, they get the bus nice and toasty.

I haven't tried them out in freezing temps yet. I guess I'll find out soon enough.
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« Reply #8 on: May 18, 2011, 02:05:34 PM »

I have three 15k Colemans installed right in the same locations you originally mentioned and when it's really hot I need the front and middlle units on to cool.  The windows are the issue, but my family likes the view...
I have heat strips in the front and rear, but I went with a heat pump for the middle, if it's really cold I like the heat that my built in propane furnace supplies.  Can't go wrong with at least three units.
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« Reply #9 on: May 18, 2011, 02:26:00 PM »

I too have three units positioned like you said.  Insulation wise-when I stripped the interior, I installed 1x2 fir strips going lengthwise then had insulation sprayed in and ground down to the fir strips.  I have 2.25" of sprayed insulation that works well.  Single pane Peninsula glass with light tinting.  On a over 100 degree day, I can run two out of the three Coleman 13,500btu's and the bus is a comfortable 75 degrees inside.  Run all three, and my wife has to wear a sweater.

As to the heat strips-all three of mine have them.  The trick is to close the vents to slow down the air speed, and they do work-but take about 5 minutes to warm up, so in the meantime have cold air swirling around-which can be irritating.  I rarely use them-usually have two electric space heaters that just about heat the bus completely.  Then also have my 35,000btu ducted propane furnace that does a really good job at heating.

I would suggest in this day and age (and this comes from Camping World) you use the Dometic Penquin.  If you go into a Camping World service bay, you'll see lots of Coleman and Carrier A/C's that are broken.  They said they have the least problems with the Penquins.  Use the 15,000btu since it has a more powerful fan.  And they have a three speed fan, rather then just 2 speeds with the others-so it is quieter at night on low. Good Luck, TomC
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« Reply #10 on: May 18, 2011, 02:32:03 PM »

The Mexican made Dometic Penquins are having their share of problems now and have ever since moving to Mexico


good luck
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« Reply #11 on: May 18, 2011, 03:04:36 PM »

As to the heat strips-all three of mine have them.  The trick is to close the vents to slow down the air speed, and they do work-but take about 5 minutes to warm up, so in the meantime have cold air swirling around-which can be irritating.

Tom
I will have to try this. I think I got to the Irritating part very quickly.

Thanks John
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« Reply #12 on: May 18, 2011, 05:51:06 PM »

Be aware that the 15K Penguins are only available for use with a wall thermostat or the Dometic Comfort Control Center.  This means they do not have a model with the thermostat on the ceiling unit.

This may or may not be an issue for folks.  I would just as well not have a separate thermostat, but I got my Penguins cheap.
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« Reply #13 on: May 18, 2011, 06:05:34 PM »

I have 3 unitsa as well and that keeps it arctic cold in here.  My wife likes 62 degrees while sleeping under 2 blankets Huh  The heat strips are about as useful as lipstick on a pig.
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« Reply #14 on: May 18, 2011, 07:20:10 PM »

I used 2 15,000 dometics with the heat strips, The bus is spray foamed and insulated windows and as far as air conditionig the warmest place we have been is Myrtle beech in July and it was hot and they kept the bus a cool 70 and still cycled on and off. The heat strips we also use all the time and keep it toasty down in to the 30s. I used the comfort control that belfert mentioned and it works great you can controll  every thing independent from each other plus it controls the furnace and has a separate temperature sensor in the back for the rear unit. Jason
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« Reply #15 on: May 18, 2011, 07:49:51 PM »

My first coach I built before I was a member here. With very little guidance. A mic 8 with little more than stock insulation. Two 13,500 btu roof top ac were marginal at best. That was 3 conversions ago. Would go with 15,000 heat pumps if budget allows with smaller in bedroom area which should be easiest  to heat being smallest area. Box heaters make good back ups when it gets to cool and power is free. Propane furnace?. If you go with bigger and one lays down you may just get by with one less without terminating your trip. Or driving home in your underware Shocked  Bob PS Just looked at your profile and saw you in northern Michigan. I realize your summers are short but even if you come the 500 mile south to Southern Indiana we get about 4 months of hot miserable humid weather.Just a comment for future travel considerations'
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« Reply #16 on: May 18, 2011, 07:53:09 PM »

Heater quit on me once and I had to dig out the longjohns Bob!!!
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« Reply #17 on: May 18, 2011, 10:28:34 PM »

Dometic Penguins.  Everything breaks and they are more reliable than the others. (OR?)

Go with HEAT PUMPS.

Avoid the heat strips.

Forget that bubble wrap.  R factor is the only way to eval insulation.  The rest is smoke and mirrors OR it performs well in a particular situation.  Now if that is you situation you might consider it as augmentation. R FACTOR.

The spray foam is the most insulation for your thickness you can buy.  It used to be cheaper than anything else in total.  The guys that spray it usually have tools and energy to go through the bus after applying the foam and level everything for you.  might cost beer and then it might cost?  Putting up the fir strips is a superb idea.  Foam also quiets the interior to the extreme.  Some have said they can no longer tell if the engine is running by listening.  Makes the shell stronger....but who knows what that is worth.  You should add a layer to the top of the bays to get a R10 boost for the floor.   You can make this a thermos bottle.  Make sure you are getting the correct type foam. 

Seal up every friggen crack and hole in the front....wire chases....screw holes....everything.  That is another benefit for the foam as it seals up the walls and roof for air penetrations.

Be anal with this part of the conversion.  What you accomplish now will determine your comfort for many years to come and many miles as well.

Incorporate a propane furnace into your design.  Maybe not install it yet but plan for it.  Redundancy and backup.  You will find yourself in the winter hooked to a 30 amp pole and if you are electric heated you have a problem.  All around, propane is the best heat you can do for the money.  heat pump if power is available.

Double pane glass that is the heat and uv rejection.

Two thermostat controlled roof exhaust fans.

Good luck,


John
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« Reply #18 on: May 19, 2011, 02:42:26 AM »

I dont know.... the aluminum foil stuff "might" not work so good in cold weather, I just dont know. But I do know it worked great out here on a real hot day and that is saying a LOT
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« Reply #19 on: May 19, 2011, 03:10:30 AM »

. . .Forget that bubble wrap.  R factor is the only way to eval insulation.  The rest is smoke and mirrors OR it performs well in a particular situation.  Now if that is you situation you might consider it as augmentation. R FACTOR. . . .

I installed some common home-center reflective  "bubble wrap" in an antique car that previously had been like riding in a hot-house and snare drum all in one. Installing just one layer of the insulation on the floor, cowl, and above the headliner  made a terrific improvement. Noise level dropped to modern car standards and the previous high level of heat radiating to the car's interior dropped to an acceptable level.

True, it's probably difficult to scientifically evaluate where it will work and how well, but "bubble wrap" definitely has merit.

Ted
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« Reply #20 on: May 19, 2011, 04:50:46 AM »

Just for fun, R-Value is measurement of the thermal conductance of a material.  Spray foam is probably one of the greatest at slowing thermal conductance.  Plus the advantages of being an excellent sound deadner.  However, the advantages of bubble/foil/bubble can't be dismissed.  Here in the Grand ol' USA we don't consider radiant energy as much as we should.  Radiant energy can easily be reflected by aluminum foil or other such products.  However, aluminum foil by itself conducts heat energy very quickly.  Hence, the use of the bubbles on each side of the aluminum.  The bubbles only prevent the aluminum from conducting heat directly by contact transfer.  So, the products doesn't even attempt to get involved in the R-Value argument, but to simply reflect heat energy. 
As a side note, heat energy can be reflected in both directions.  In other words, it can keep heat out by reflecting the radiant energy from the outside, or keep heat in by reflecting the radiant energy from the inside. 
I wouldn't use the bubble/foil/bubble by itself.  We need the R-Value of some other kind of insulation, but I wouldn't try to cool any tin box without it either.

Thanks so much for all the input of the air conditioners.  That's exactly what I needed to hear. 
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« Reply #21 on: May 19, 2011, 06:52:27 AM »

This winter, I tracked my energy costs running electric heaters IN FLORIDA, and using a heat pump.   There are a lot of factors. 

In general, no large electric draws, just house refrigerator, pumps, and battery charging   $3/day.

All the general uses plus heat pump above about 40 degrees   $5/day

All general using resistant electric heaters 50-20 degrees  $6-10/day

This last winter sold me on the use of heat pumps.

Ed Roelle
Flint, MI/Williston, FL
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« Reply #22 on: May 19, 2011, 08:37:15 AM »

Just for fun, R-Value is measurement of the thermal conductance of a material.  Spray foam is probably one of the greatest at slowing thermal conductance.  Plus the advantages of being an excellent sound deadner.  However, the advantages of bubble/foil/bubble can't be dismissed.  Here in the Grand ol' USA we don't consider radiant energy as much as we should.  Radiant energy can easily be reflected by aluminum foil or other such products.  However, aluminum foil by itself conducts heat energy very quickly.  Hence, the use of the bubbles on each side of the aluminum.  The bubbles only prevent the aluminum from conducting heat directly by contact transfer.  So, the products doesn't even attempt to get involved in the R-Value argument, but to simply reflect heat energy. 
As a side note, heat energy can be reflected in both directions.  In other words, it can keep heat out by reflecting the radiant energy from the outside, or keep heat in by reflecting the radiant energy from the inside. 
I wouldn't use the bubble/foil/bubble by itself.  We need the R-Value of some other kind of insulation, but I wouldn't try to cool any tin box without it either.

Thanks so much for all the input of the air conditioners.  That's exactly what I needed to hear. 

T,

You have pretty much summed this argument "for and against" up into its salient points.  And did it well.  Search on this subject and you will re-read your words over and over.  You get the nod of the wisdom of this collection of true experts and experienced Knuts.  I am only repeating them, in part, as well in my comments.  Nothing new here.

Continuing, and not contradicting a single word, consider a common tarp as an insulator.  It has not a single digit of insulating R value.  String it up across the bus on a 80 degree day and the bus will go from sweltering to comfortable due to the bus now being parked in the shade.  That tarp would be the greatest payback on performance for the buck we could make. For sunlight heat rejection alone.  Not practical to "carry" our shade so its out and further more it won't do a single thing to help us out when we are dealing with "cold".  Seeking a spot on the north side of a tree or building while in the 110 degree Phoenix sun is a no brainer.  Along those lines is color: white is the best performing and, like shade, it has incredible heat reflectivity.  A pure white roof is essential to efficient cooling in direct sun and, here again, does nothing for heating. 

Were you to install two inches of foam around you in the perfect bus, one where only the skin was a consideration for heat loss /rejection, I don't think you could find the improvement that bubble or even white roofs would contribute.  You could paint your roof black and sit in the sun and three ac units would do fine.  As a previous post noted, ac costs were $10 a day in very cold weather so the message there is do every thing you can at the construction phase as energy costs are an ongoing thing and investment in insulation gives you the biggest bang for the buck. And like the Energizer Bunny, it just keeps on keeping on.    And, like luggage, it lasts forever.

For me its awnings on every window for the max shade I can carry with me.  White roof coating of the highest grade automotive paint I can find.  2 inches of foam in the walls and 4 + inches in the ceiling and floor and reflecting metallic interior surface to the insulation or bubble wrap, double pane reflecting glass in all windows.  All the insulation in the world won't overcome significant air permeation any more than you can compensate for an open door or window.  Every improvement adds something....all of it does.  You just have to decide where you are going to put your money and sweat.

Don't think for a moment I am talking down to you or anyone.  Have to consider the education and experience of all the readers in an answer.  If you can.

Thank you for bringing this up again.  There is an obvious need to rehash the subject periodically.  Notice the posters have a short history.  There must be 20 people here that have more experience with this topic than I and have credentials.  They fatigue of the exercise, understandably, but given the history of the topic and the knowledgeable new Knuts that keep bring it up....it needs to be aired
 periodically.

John
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« Reply #23 on: May 19, 2011, 10:12:29 AM »

JohnEd,
Very well said. I am one of those new bus knuts that wants to talk about this subject.
For example, when I started my conversion I was not aware of this forum.
I had no knowledge of foam insulation. I would have gone that route had I known.
Instead, I used Rollboard on the floor, walls and cieling.
Im happy with the results but I would have prefered foam insulation.
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« Reply #24 on: May 19, 2011, 11:32:39 AM »

It is to bad Carrier left the market they made the best roof top of any of the brands IMO

good luck
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« Reply #25 on: May 19, 2011, 05:13:56 PM »

We used one layer of Lowes 1/2" foil backed styro over the sprayed in foam-worked great. Take one of those foil backed panels in the sun and reflect the sunlight at you-you'll feel the heat!
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« Reply #26 on: May 19, 2011, 07:57:43 PM »

JohnEd,
Very well said. I am one of those new bus knuts that wants to talk about this subject.
For example, when I started my conversion I was not aware of this forum.
I had no knowledge of foam insulation. I would have gone that route had I known.
Instead, I used Rollboard on the floor, walls and cieling.
Im happy with the results but I would have prefered foam insulation.

Seville,

It is always a secret dread that I will disappoint someone for whom these opinions and advice are OBE.  There have been precious few that posted here that didn't show a similar concern.  Mine doesn't show all that much to most.  Thank you for your sensitive and encouraging comments.

Your friend,


John 
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« Reply #27 on: May 19, 2011, 09:22:12 PM »

JohnEd,
You very welcome sir.
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