How to size an air conditioner?

[ReluctantTraveler]

I've found a handful of threads on AC units in the forum archives (here, here, and here), but none of them seem to address how to actually choose an appropriate size unit for your rig.

I'm planning out a flatbed camper, 12x8 box plus a king-sized cab-over (so about 150 squarer feet) with 6.5' high ceiling. Four people will be living in it, two adults, two kids.

I'd like to be able to run our AC on battery + solar if possible. We'd like some off-grid capability, travel with a dog, and sometimes need to live the furball back in the rig while exploring. Our current 16' Forest River Wolf Pup travel trailer has a Coleman Mach AC, and it's definitely overpowered for the size of the rig (I'm not sure if it's 12,800 or 15k BTU).

Since the threads I found were posted, Dometic released their RTX 2000 12 VDC AC unit. It's only 6,800 BTU, though, and I'm worried about how well it can cool the space.

I plan to add a DC-to-DC charging setup to charge the batteries from the truck, and could run the truck on idle for a bit to juice up the batteries if needed. Hopefully, these negates the need for a generator. In an ideal world, that would be an automatic on/off thing like some camper vans have, but I think I lack the skill for that kind of setup.

 

[2.ooohhh]

There's a lot to consider that goes into that. One is the insulation of the box you intend to condition, the other is the amount of power you can contain(battery reserves) and regenerate via solar setup. then there's how efficiently you can power an hvac unit. Those dometics are generally designed for an idling over the road truck cab, that said they will require a hefty house battery for daily use without a motor running.

 

[1000arms]

Make sure to look at mini-split (heat pump) air conditioners. They can be used for cooling, and for heating (down to certain outside temperatures). The lowest outside temps they are still efficient at varies, check each unit you might be considering, but you might be able to use one for heating too. A mini-split might also be quite useful between trips to keep things from freezing or roasting.

Ductless Mini-Split Heat Pumps

Installing this kind of heat pump in your new or existing home can save money while improving the efficiency of how you heat, ventilate, and air co...

www.energy.gov

 

[ReluctantTraveler]

Those dometics are generally designed for an idling over the road truck cab

Is that right? From what I've read, they're intended use is specifically to prevent the need for idling as more states pass anti-idling laws.

The Dometic RTX 2000 in particular has some features to reduce battery drain, and shut off automatically if the battery gets too low.

they will require a hefty house battery for daily use without a motor running.

I'd imagine! I'm comfortable adding a bigger battery bank and solar array to support it if need (up to say 1200 amp hours), but I'm try to balance size of the AC and effective cooling places where the heat might be 90-95 degrees against energy needs.

Planned insulation on the camper is 1.5" thick rigid foam board sandwiched between 1/4" marine plywood. There will be about 5-6 large double paned awning style windows with built-in shades.

 

[ReluctantTraveler]

Make sure to look at mini-split (heat pump) air conditioners. They can be used for cooling, and for heating (down to certain outside temperatures). The lowest outside temps they are still efficient at varies, check each unit you might be considering, but you might be able to use one for heating too. A mini-split might also be quite useful between trips to keep things from freezing or roasting.

Ductless Mini-Split Heat Pumps

Installing this kind of heat pump in your new or existing home can save money while improving the efficiency of how you heat, ventilate, and air co...

www.energy.gov

That's how a lot of folks did it before 12v DC air conditioners were a thing, right? And they're arguably still the more energy efficient option? Every time I've looked at them, they've seemed to involve a fair bit of hacking to make work.

 

[1000arms]

That's how a lot of folks did it before 12v DC air conditioners were a thing, right? And they're arguably still the more energy efficient option? Every time I've looked at them, they've seemed to involve a fair bit of hacking to make work.

No hacking/maiming/scarring needed. Just follow the instructions (some states/counties/... may require "professional installation"). Just look them up on the Lowes and Home Depot websites to get you started. (You might be thinking of people cutting up a typical 120 V window air conditioner.)

You might find an efficient 12 V DC heat-pump air conditioner, and maybe at a price that you will be happy with, but please be aware that a 120 V mini-split might work well for you.

 

[Darwin]

Have you ran the AC in your trailer in hot HUMID conditions? The humidity 'wet bulb' temperature will really require more than if it's a 'dry' heat. The amount of insulation and windows etc. also play a major role, lots of factors like that to consider, and without more information, hard to answer your question.

 

[ReluctantTraveler]

Have you ran the AC in your trailer in hot HUMID conditions? The humidity 'wet bulb' temperature will really require more than if it's a 'dry' heat. The amount of insulation and windows etc. also play a major role, lots of factors like that to consider, and without more information, hard to answer your question.

Yes, pretty close to our max "we'd camp in this" temperatures: high-80's in the swampy Orlando, FL weather May. Not nearly as bad as its get there in the summer, but we wouldn't be camping that far South that time of year.

We ran our Coleman Mach AC at mid-cool the whole time and were always quite comfy.