Thoughts on adding a 3rd rooftop AC

[adonh]

Originally Posted by pasdad1

The Advent is made by the same company that produced carrier RV air conditioners a few years back.... Seems well made. 2 year warranty.

Here is a link to the Advent AC.
Advent Rooftop Air-Conditioning Units

[gemini5362]

Originally Posted by pasdad1

Need to get a "dual hose" portable. The single hose units are everywhere.....skip those and find the dual hose model.....I'll tell you why...

The single hose models suck air from inside the RV (air conditioned air) and use it to cool the condenser coils, blowing the hot air outside through the single hose. This creates a negative pressure, and so hot outside air is drawn back in.....not good.

The dual hose models bring in outside air through one hose, cool the condenser, and exhaust the hot air through the other hose......much more efficient.....no negative pressure created, and you aren't continually taking cool inside air and blowing it outside.

Thank you for the information I will start researching those. Hopefully with the end of the air conditioning season I can find one for a good price

[gemini5362]

Originally Posted by Peralko

But the ACs don't use outside air, they recirculate air from inside the coach, so it is a closed system. Even with 120 degrees outside, you can still get into the 70s inside.

Actually that would be true if the coach was in a vacuum bottle. AS you cool down by 20 degrees the cooler air in the coach is heated up by the outside temp radiating through the walls, windows etc. At some point the outside air is radiating more heat than the Air conditioner can lower to 20 degrees below incoming. When this happens your temp in the motor home keeps rising. The obvious solution is more insulation and covering windows with insulation or some thing like that. My A/C is working up to about 95 degrees it will cool down with no problems but above that it does not have the ability to cool the warmer air. I an not talking about outside air i am talking about inside air that has been warmed because of heat radiation into the coach.

Allegro makes a very good coach and i am very happy with mine. If you watch one being made they start with 2 inches of styrofoam. Then a NMC router routes a path for air ducts, and wiring runs. Those paths now have less than 2 inches of styrofoam insulation. If i understand right the air ducts are just cut into the styrofoam and have not other insulation. If the air duct is 2 inches then it only has an inch of insulation between it and the roof. It is my understanding that most coaches are made this way. What suprises me is that the upper tier 5th wheels seem to have 4 inches of foam in the roof and more in the walls. I do know my 5th wheel was a lot easier to cool and heat.

[Peralko]

I don't remember mentioning a vacuum bottle! Of course there is heat gain from the outside when you're in hot weather cooling the coach. Country Coach made one of the best insulated coaches around, and ours has reflective coating on all the windows. They also used a denser foam for the walls. We actually camped in Las Vegas at 122 degree daytime temperature, and the temperatures inside the coach never went over 78 degrees.

[Cat]

We use a portable two hose 12000 btu a/c on our 38v Bounder along with the two 13500 AC that came on the Bounder. In the summer time in Tucson Az I can set and hold 74 with no problems. We used ours for 2 summers in Tucson. The AC will run off a plug inside the coach. The Two 13.5 ACs will run all day and the portable ac runs periodically just supplementing the other 2 AC in the heat. The down size to having the portable is the room it takes up and taking in and out the front door when seasons change. Weight 90lbs I think. But works really well, but You do want the 2 hose portables. And you need a window to be able to use for the 2 hoses.

[gemini5362]

Originally Posted by Peralko

I don't remember mentioning a vacuum bottle! Of course there is heat gain from the outside when you're in hot weather cooling the coach. Country Coach made one of the best insulated coaches around, and ours has reflective coating on all the windows. They also used a denser foam for the walls. We actually camped in Las Vegas at 122 degree daytime temperature, and the temperatures inside the coach never went over 78 degrees.

And for those of us that do not have a country coach. . Sometimes there is issues with heat. I am glad you can have your coach cool in 122 degree heat. For those of us who cannot do that we are looking at our options. I can tell you personally one of my options is not going to be to rush out and buy a country coach. They are fine units but I like my coach. But thank you for sharing with us how good you can cool your coach down in hot weather.

[TQ60]

Not an air handler engineer but...

A 2 x 8 duct does not seem very large then make it long it seems smaller.

Granted an ac is at each end but it seems that the ductwork may be a limiting factor.

Increasing btu of ac may be lost in limited air flow.

There are ways to measure back pressure to assure no restrictions in what seems to be a small duct.

Regarding heat calculations...in central Ca the rough number is 1 ton or 12000 btu per 500 sq ft.

2 x 13.5 kbtu units is 2 ton and change so it should be able to do 1000 sq ft and the mh is no where that big.

Mh has heat gains or losses caused by lesser insulation and maybe dark paint.

It seems the units are not working as well as they could be.

Not an engineer in this field but just thinking out loud...

[Cat]

The heat gains on a motorhome are much greater than house. Most home are going to have a least a R 16 ( in 4-6 in thick walls) in the walls maybe r30 in the ceiling. Motorhomes have approx 2" to 2 1/2 " thick walls so there not a lot of room for insulation - and steel frame components, so maybe r7-r9. In Las Vegas it was a 105 f day --the outside skin of my motorhome in my tan painted areas were 155' my black painted areas were 178f
My inside walls were 116f where the internal framing of the wall was. So that heat --was being radiated in to the motorhome interior. Then you also have the heat that comes from underneath which a house doesn't have.

The coaches that have more weight capacity = more expensive too- have more insulation mostly in the roof and floors which help a great deal as do the better tinted dual windows. But even those coaches carry three A/C on most. And it seems like more and more of 38- 45 are being built with 3 A/C or at least it's an opition.
Of course we make harded to it cool by using darker colors- mine has black swirls on it- it looks good though

[akeyzoo]

Originally Posted by powerboatr

i just replaced my died on the roof carrier 13000 btu a/c in the rear bedroom with a coleman 15500 btu unit
WOW what a difference in cold air movement in the rear half of the coach
it has a bigger air intake and moves much more air, at near the same amp draw.
i had recently thought to add a 3rd in the kitchen using the fantastic vent opening...but when the rear a/c went south i upgraded btu capacity and it cured the heat issue
just waiting on the front one to bite the dust now....any day i expect.

Was this a direct vent or ducted application?

Chris

[gemini5362]

Originally Posted by TQ60

Not an air handler engineer but...

A 2 x 8 duct does not seem very large then make it long it seems smaller.

Granted an ac is at each end but it seems that the ductwork may be a limiting factor.

Increasing btu of ac may be lost in limited air flow.

There are ways to measure back pressure to assure no restrictions in what seems to be a small duct.

Regarding heat calculations...in central Ca the rough number is 1 ton or 12000 btu per 500 sq ft.

2 x 13.5 kbtu units is 2 ton and change so it should be able to do 1000 sq ft and the mh is no where that big.

Mh has heat gains or losses caused by lesser insulation and maybe dark paint.

It seems the units are not working as well as they could be.

Not an engineer in this field but just thinking out loud...

Not sure what you base your calculations on for determining the units might not be working correctly. Yes 2 tons would heat a 1000 sq ft brick house probably but a lot of difference between that and a motor home. If you look at all of the posts on here you will find that there are a lot of similar stories about MH not cooling well with 2 units. It is interesting to note that 40 ft motor homes have two units but a 5 ft longer one has three usually.

[TQ60]

I stated not working properly only to indicate that the mh as is may not have the heat load more than the ac.

A clogged vent or dirty filter could hamper the existing units.

Or the units new were too small.

Need to be sure everything is working correctly before spending money on additional units is all.

[Jupiter]

Interesting thread. I have one of those coaches with two rooftop units that will not adequately cool the coach in hot, direct sun. Blinds closed, etc. 2013 36M Journey with some dark colors and extremely noisy Coleman rooftops. The Colemans simply work too hard in hot weather.

But, let me explain what an A/C unit is designed to do. Doesn't matter if it is rooftop, basement air, portable, or even a refrigerator! They are designed simply designed to REMOVE HEAT... they DO NOT add cooling. I won't get in to the entire refrigeration cycle or latent and sensible heat loads, but suffice it to say that they ALL work essentially the same way when they are working correctly (clean coils, clean filters, full charge of refrigerant, and adequate air flow across the coils which is about 360 to 400 CFM p/ton.)

When the infiltration of heat into the coach (direct or otherwise) is greater than the ability to REMOVE HEAT with your A/C's, you will experience interior temps higher than desired.... period. And yes, your 13,500 btu A/C works and cools (removes heat) precisely the same as any one else's 13,500 btu A/C. The difference is the infiltration of heat.

In my case, I'm going to add insulation in cabinets and shield the windshield from the outside, along with window awnings.

Hope this helps folks understand that A/C's don't "add" cooling. They simply remove heat and moisture. Oh, and heat pumps simply reverse the process. They remove heat from the outdoor air and transfer it indoors via the refrigerant gas. Yes, there is heat even in cool weather!

cheers,
Joopy

[TQ60]

Cool description...pun intended.

Air flow is almost as important as btu.

If the air does not move with the vorrect volume thrn you may have less but colder air but it will not transfer the heat the same way as less hot air by volume is being "processed" into cooler air.

there are many ways to mittigate heat load...simply making sure top is clean...dirt is dark and is hotter than white or light clean surface.

Ours has silver reflective windows that reflect a lot.

We made stand in barriers for behind the front windows to block direct sunlight.

Wonder why folks pay more money for full body dark paint only to have to pay more to cool it.

Some have had success modifying and cleaning out the central duct system to improve the air flow which would be one of the first on the list to look at as it is the lowest hanging fruit.

Measure temp drop across the ac at multiple points to see how much heat is gained or cooling lost in the ductwork.

All may be perfect and you will need to add capacity or you may be almost close to balance of loading to where minor work could be enough or your units are only running at 50% capacity.

The previous poster sounds like he does this for a living and post is good.

He may be able to provide some good advice.

I have someplace in a dark hole someplace a set "tools" we used for power calculations and heat load that included air volume so we could calculate when things would overheat upon ac fail.

The same formula or similar one can be used to estimate your shortage of capacity with imputing the time it takes for the temperature to rise a given amount when ambient and a few other things are known.

It may be possible to perform some measurements to determine your heat load and actual output or your ac.

You only need good thermometer timer and tape measrure and the formulas to use them.

Just passing some suggestions for consideration as we have seen a few instances where folks just toss money at problems and often do not solve it.

Take some time to make sure your ductwork is good and units working.

Evaluating the actual heat loads and actual best need and reserve capacity is best left to those who do that...we did same in a different environment for different reasons so I would not have accurate tools for that.

[gemini5362]

Originally Posted by Jupiter

Interesting thread. I have one of those coaches with two rooftop units that will not adequately cool the coach in hot, direct sun. Blinds closed, etc. 2013 36M Journey with some dark colors and extremely noisy Coleman rooftops. The Colemans simply work too hard in hot weather.

But, let me explain what an A/C unit is designed to do. Doesn't matter if it is rooftop, basement air, portable, or even a refrigerator! They are designed simply designed to REMOVE HEAT... they DO NOT add cooling. I won't get in to the entire refrigeration cycle or latent and sensible heat loads, but suffice it to say that they ALL work essentially the same way when they are working correctly (clean coils, clean filters, full charge of refrigerant, and adequate air flow across the coils which is about 360 to 400 CFM p/ton.)

When the infiltration of heat into the coach (direct or otherwise) is greater than the ability to REMOVE HEAT with your A/C's, you will experience interior temps higher than desired.... period. And yes, your 13,500 btu A/C works and cools (removes heat) precisely the same as any one else's 13,500 btu A/C. The difference is the infiltration of heat.

In my case, I'm going to add insulation in cabinets and shield the windshield from the outside, along with window awnings.

Hope this helps folks understand that A/C's don't "add" cooling. They simply remove heat and moisture. Oh, and heat pumps simply reverse the process. They remove heat from the outdoor air and transfer it indoors via the refrigerant gas. Yes, there is heat even in cool weather!

cheers,
Joopy

I put part of your post in Bold Letters. I was thinking about this subject today and this is a common thing in posts people posting about hot cabinets and putting insulation in them. I got to wondering why. The cabinets are on the wall of the MH. I saw those walls being built and installed at the TIFFIN factory tour. The insulation is the same for the whole wall. The reason the cabinets are hot inside is because the same heat migrates into them that migrates into your coach the difference being that air conditioning can not get into the cabinets to cool them down. How much gain will you get from insulating the inside of them. I am not saying it will not work just very curious about whether or not it will make a big difference.