mini split size for HOT HOT HUMID CLIMATE

Hello. I am looking for opinions from the many here who have experience with a decision I am currently making.

I have a 31 foot Bluebird.... big front window, side windows, decent insulation.... it lives in an environment (the rainforests of Belize) which is hot and humid day in and day out. Figure 100 degrees, no shade and 90+ humidity and some engine heat. It has two roof airs (old, inefficient 13K and 15K BTU's) which cannot handle the load.

I plan on putting a mini split single unit in the front blowing straight toward the rear. There are no obstacles. I can fit anything from a 12K to 36K BTU single unit. Fan will max out at 800 cfm.

I am not worried about powering any of the above 12-36 BTU options (plenty of power from generator, alternator, shore power and solar).

I also plan on running the mini-split full time 24/7 set to the mid-low 70's.

The rubber is about to reach the road.... so...

Here is my question: is 24K just right or should I go for more? Advice and opinions are sought.

Thank you ALL very much.

Rob

I do not have direct experience with your scenario, but I'm reasonably proficient in numbers.

13K/15K is cooling capacity, i.e., 13000 and 15000 BTU/hr, respectively. It doesn't matter whether it's a window unit, an RV rooftop unit or a mini-split. A 12K unit will transfer the same amount of heat. The efficiency is what's different between them.

You've already established that 28000 BTU/hr isn't sufficient to keep up. If you're using the original glass, it has horrible insulation value and is likely the main culprit in the heat transfer.

Unless your original units are severely underperforming due to poor maintenance or low refrigerant, I can't see how 24K would work for you.

Ultimately, you could calculate the tonnage needed via the R-values of your exposed surfaces.

So. If it's an inverter style variable speed unit it shouldn't really matter I believe as it'll just drop its output to stay running and thus keep up with the humidity issue regardless.

If you already know the two units can't keep up then you should just confirm they are both still putting out their rated cooling, though this is difficult without a means to measure air flow properly.

Are you willing to leave the existing ones in place as supplemental AC for the really hot days?

If so, you'd probably be fine with the 24k unit.

If you wish to entirely avoid the rooftop units (other than backups) to cut back on energy a bit you may need to go with 30k or more to replace the 28k they are giving you, assuming they work fine.

Instead of adding more cooling you might want to look at windshield ceramic coatings for the windows (some are available that are legal for the windshield) and a dehumidifier. The film has high light transmission, but low heat transmission.

I'm in lower Alabama, so I know your pain with the humidity and run my roof ACs almost year around on my Bird too. I use a stand alone dehumidifier for winter months, summer the AC typically takes care of it. I was looking at marine units that I could just mount into the ducting that use absorption discs but the price break out was silly high and I just ended up going with a compressor unit. Not super efficient power wise but works fantastic to keep the interior dry. I've seen where some people put the dehumidifier into the old ice maker cabinet and ran the continuous drain out to the bottom of the coach. If it is really humid, don't route it to your tanks unless you want them to be full all the time.

Weru12 said:

Instead of adding more cooling you might want to look at windshield ceramic coatings for the windows (some are available that are legal for the windshield) and a dehumidifier. The film has high light transmission, but low heat transmission.

I'm in lower Alabama, so I know your pain with the humidity and run my roof ACs almost year around on my Bird too. I use a stand alone dehumidifier for winter months, summer the AC typically takes care of it. I was looking at marine units that I could just mount into the ducting that use absorption discs but the price break out was silly high and I just ended up going with a compressor unit. Not super efficient power wise but works fantastic to keep the interior dry. I've seen where some people put the dehumidifier into the old ice maker cabinet and ran the continuous drain out to the bottom of the coach. If it is really humid, don't route it to your tanks unless you want them to be full all the time.

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A variable speed mini split solves the humidity issue pretty well usually because they just stay running constantly at a lower output.

Depending on the design of course.

You also have to remember that a dehumidifier is also a heater and will add heat to the space. We run one almost constantly in a die vault here at work and it's virtually always 80F in there.

I've installed two mini splits in my home and while rural Pennsylvania isn't the tropics, we do get days of low 100's with high humidity in the summer. A 24k BTU mini split easily cools the majority of my 1900 square foot home. I added a 9k unit to a basement office because it's physically isolated from the rest of the home.

Thank you all for your help. Has anybody here tried the window ceramic coatings??? Please share your experience.

apcorob said:

Thank you all for your help. Has anybody here tried the window ceramic coatings??? Please share your experience.

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I've ceramic tint my i3 car - the difference is night and day. From getting toasted in Florida to hardly feeling anything coming through the glass.
That film is really thick, nothing for DIY get a professional doing it.


Can't you get a shade structure above the RV? Carport? Shade Sails?

apcorob said:

It has two roof airs (old, inefficient 13K and 15K BTU's) which cannot handle the load.

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Yeah, roof A/C units are notorious inefficient. I got a 8000 BTU Inverter unit outperforming a 13000 BTU Conventional A/C

In Mobile home (900SQFT) I ripped out a 3ton unit (36.000 BTU) Conventional Split and installed a 8000 and a 12000 BTU Inverter - cools much better now.

My opinion, Inverter BTUs are not comparable with regular BTUs. 1 Inverter BTU = 1.5 Regular BTU.

sunshine_eggo said:

13K/15K is cooling capacity, i.e., 13000 and 15000 BTU/hr, respectively. It doesn't matter whether it's a window unit, an RV rooftop unit or a mini-split. A 12K unit will transfer the same amount of heat. The efficiency is what's different between them.

Click to expand...

From on past installation experience I'm disagreeing with that statement.

I have no idea how manufacturers are coming up with the "BTU" but every Inverter of smaller size I've installed outperformed a higher sized conventional unit. I don't care about BTU, I care at what temperature split from the room to outside can be achieved in daily life.

When a conventional 3 ton (36k) unit fails to deliver 72 degree in 98 weather and a 20.000 BTU even manages it do get to 69 with same ambient - what does that tell me?

apcorob said:

Thank you all for your help. Has anybody here tried the window ceramic coatings??? Please share your experience.

Click to expand...

They work great. I've used it on my current and prior three vehicles. My current ride is an F-350 with black leather seats. The truck has a ton of glass and I had ceramic tint applied everywhere, including the windshield. On the windshield I went with the lightest tint and it still helps a ton.

You might also consider using reflectix between the window shade and glass. Even better are retractable awnings over your side windows for when you are stationary. Or, make some awnings with flexible solar panels and get some extra power generation and window shading when you're stationary.

I replaced a 15k BTU rooftop unit on my 28' travel trailer with a 12k LG mini-split and it works well. It's much quieter, uses about a quarter of the power, and cools about as well as the old rooftop unit. I do find that I have to run a dehumidifier on the most humid days when we go in and out of the trailer throughout the day. While it dries the air, it also adds to the heat load inside, which is counter-productive in the Summer and beneficial when it's cold out. But you likely won't have that issue if you go with a 36k unit. In your case, I'd suggest you go big to deal with the humidity.

I agree with exodus. Every mini split I've installed replaces a unit with larger capacity. We just sized one last week for a project with a 24-26k heat load, and decided to go with an 18k Mitsubishi. They are super quiet, very efficient, superior in every way.

In my home, we replaced a 36kbtu unit with a 9k and a 15k. Most older a/c units are oversized, and don't take into account the advantage of continuously running at slow speed, instead of wild swings, cycling on and off. In a home, significant advanatge comes from eliminating the duct work losses, which you likely won't have anyway. Unless you use it for heat and cool. MIne run down to about -12f.

In a coach, you also have the option of putting the compressor/head unit someplace other than the roof. Look into that possibility, and consider whether that real estate might be better for some solar panels.

K8MEJ said:

You might also consider using reflectix between the window shade and glass. Even better are retractable awnings over your side windows for when you are stationary. Or, make some awnings with flexible solar panels and get some extra power generation and window shading when you're stationary.

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Order of best way to reduce heat gain on windows. - From outside to inside

1. Shade: trees, awnings
2. Exterior window tint
3. Double pan windows
3. Factory tinted windows
4. Interior window tint
5. Reflectix
6. Shades
7. curtains

awnings are much better then reflectix,

In a humid environment a lot depends on how much humid air is allowed to enter air conditioned area. It takes a lot of btu's to condense moisture from air. Starting out from initial turn on, 50% of A/C btu's is consumed by humidity removal. Once humidity is brought down more of the available btu's is put into air cooling.

Many mini-splits have a 'dry mode' for humidity removal. What is does is slow down the blower to allow more time for air moisture to condense out on the evaporator coils. Too fast a CFM air flow across evaporator will eject out humid air before it has a chance to condense out moisture on evaporator coils. For a traditional cycling A/C never leave blower continuously running. Doing this will evaporate water in evaporator area and put it back into ejected air when compressor is not cooling evaporator.

If you have a lot of door openings, continuously going in and out of RV, it allows not only warm air in but a lot of humid air to enter A/C'd area.

eXodus said:

From on past installation experience I'm disagreeing with that statement.

I have no idea how manufacturers are coming up with the "BTU" but every Inverter of smaller size I've installed outperformed a higher sized conventional unit. I don't care about BTU, I care at what temperature split from the room to outside can be achieved in daily life.

When a conventional 3 ton (36k) unit fails to deliver 72 degree in 98 weather and a 20.000 BTU even manages it do get to 69 with same ambient - what does that tell me?

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I agree, but in a slightly different manner.

In my case, I fully believe the RV AC Companies (Dometic, Coleman, whatever) are 100% lying about their BTU rating. I have 3x 15k BTU Air Conditioners in my RV, and there is no way 45k BTU's total are coming out of the vents. I believe 1x 30k BTU Mini split will far surpass the performance of what I currently have.

RCinFLA said:

Many mini-splits have a 'dry mode' for humidity removal. What is does is slow down the blower to allow more time for air moisture to condense out on the evaporator coils. Too fast a CFM air flow across evaporator will eject out humid air before it has a chance to condense out moisture on evaporator coils. For a traditional cycling A/C never leave blower continuously running. Doing this will evaporate water in evaporator area and put it back into ejected air when compressor is not cooling evaporator.

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I'm sure with a mini split/ Inverter Unit - which has 3 different parameters to play with does a lot more things then only dry mode.

Lt.Dan said:

In my case, I fully believe the RV AC Companies (Dometic, Coleman, whatever) are 100% lying about their BTU rating. I have 3x 15k BTU Air Conditioners in my RV, and there is no way 45k BTU's total are coming out of the vents. I believe 1x 30k BTU Mini split will far surpass the performance of what I currently have.

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There is probably a testing protocol to get those seasonal energy rating - which also gives a certain BTU rating.

It's just garbage for hot and humid climate. The protocol might work for a temperature climate where you got 2 cooling months a year. Otherwise I can not explain. I'm in South Florida, there is not a single month of the year where wo don't need A/C

For my usecase the Inverters are just much better. - Even a non-split Window A/C inverter unit is better then a regular conventional split unit.

In Europe they started selling RV-Style Roof Inverter Units. Wonder how they perform in comparison with regular Roof A/C. Or if the packaging format leads to the inefficiency.