Please help me size my mini-split for older RV

[daveemac]

I'm full-timing in a 1989 Toyota camper, about 21' long... the cab & over-cab areas are sealed off with thick blankets, so the area I'll be cooling/heating is about 100 sq ft, 12x8'.

I'm looking at the high efficiency EG4 units, debating between 9K and 12K, 115VAC.

The 9K is rated for "rooms up to 500sf"... but I know that old RVs don't behave like modern houses. My toyota has stock insulation (compressed foam mostly), stock window tinting (pretty good), a new roof w/highly reflective PVC membrane, + 900W in panels covering about half the total roof area...I'll be insulating the windows with warm window. The indoor unit will be mounted against the rear wall, & I have a maxxair fan right in front of it, so I can reverse the fan and blow conditioned air around the cab pretty powerfully. I have a 48V system with 5.1kwh battery and am open to adding another battery if necessary.

This will be my home for years probably, and I'll be doing a 3-month surgery recovery in it at some point, so I want to be comfortable....but I don't want to over-spec it either.

I'm in the Sacramento area most of the year --hot and dry in summer--but I plan on traveling a fair bit, maybe go around the country to more humid climes. I also would like to be able to heat the space comfortably when outside temperatures are in the 20s...

Is the 9K plenty for me, or would you go 12K?

Second question--If I moved into a cabin later could I transfer the mini-split, or is better to start fresh? I ask this question because even if 9K is enough, maybe I'd go 12K to potentially use it in a bigger space later.

I have very little income and this is a big purchase for me. I'd rather spend the extra $200 than be underpowered, but if 9K is more than enough I'd happily save the $200.

Thanks!

 

[sunshine_eggo]

IMHO, the 9K is enough. Many folks claim that mini split installation and efficiency makes them more effective than the claimed 13,500 or 15,000 BTU/h rooftop units.

HOWEVER, if I was faced recovering for 3 months in the thing, I think I would go for 12K. Yes, it will be more expensive and yes, it will use more juice; however, it won't be that much more. Part of their efficiency is operating at reduced output maintaining temperature vs. cycling on/off, so they will be pretty much the same when in "maintenance" mode.

IMHO, 5kWh of battery won't be enough for either size if you have heavy need, and the big limiter will be your PV. 4.5kWh/day with 900W of roof mounted solar is only enough for about 7.5hr/day of max A/C output before you start eroding your battery capacity. I'd look at deployable ground arrays to supplement your rooftop and/or max out the rooftop.

 

[wme]

I'm a member of the Toyota RV forum. As units get older and things fail a lot of the 12k btu unit AC's are being replaced with 9k btu roof mounts. They will maintain cool just as well as the larger units and the slightly longer run times helps with the cold clammy feeling you get from short cycling a large AC.
So IMHO the 9k btu mini split would be enough.
P.S. All bets are off in Death Valley

 

[Partimewages]

I don't think you are understanding.
A 12k unit run at 75% capacity is 9k and likely more efficient than the 9k run at 100%.
This will help extend your solar and batteries. Check your specifications for details.

 

[daveemac]

I don't think you are understanding.
A 12k unit run at 75% capacity is 9k and likely more efficient than the 9k run at 100%.
This will help extend your solar and batteries. Check your specifications for details.

Thanks for all the replies

Partime, this makes sense and would likely more than nullify the 1 point superior SEER rating of the 9K

Eggo, I think I can get 1200-1300 up there and still have room for a maxxfan...and buy another 5.1Kwh if necessary

WME--I took off the old AC yesterday and am glad to have it gone!

 

[sunshine_eggo]

I don't think you are understanding.
A 12k unit run at 75% capacity is 9k and likely more efficient than the 9k run at 100%.
This will help extend your solar and batteries. Check your specifications for details.

I don't know that I would agree with this as stated. Would the 12K be MORE efficient at 9K than the 9K? I really don't think so. Units are typically optimized for max efficiency at max output, and they tend to be less efficient at lower power settings; however, I think it will be very similar, but still more with the 12K @ 9K than a 9K @ 9K - but not enough to make a substantial operational difference for a scenario where both will meet the needs.

@Supervstech - I bet you can eliminate the guesswork here...

Thanks for all the replies

Partime, this makes sense and would likely more than nullify the 1 point superior SEER rating of the 9K

Eggo, I think I can get 1200-1300 up there and still have room for a maxxfan...and buy another 5.1Kwh if necessary

WME--I took off the old AC yesterday and am glad to have it gone!

1200-1300W helps, but I'd still look at a deployable ground array, especially if you're going to be stationary for awhile. Lightweight flexible panels mounted in DIY PVC frames are cheap and light:

Cheap Dual Flex Panel Mobile/Tilting Stand

Here’s a super lightweight and cheap adjustable stand design that I use for 100w flex panel pairs. Each unit costs

diysolarforum.com

In my own mini-split research, I have discovered something a bit odd. I get VERY suspicious when comparing units side-by-side and observe dramatically different SEER/SEER2 values and nearly identical EER/EER2 values. To me that says some sort of shenanigans are taking place.

IMHO, SEER numbers are wishywashy and may not reflect reality on a regular basis. Look at the EER/EER2 numbers.

That's (BTU/h) / EER(or EER2) = power consumed using a 95 °F (35 °C) outside temperature and an inside (actually return-air) temperature of 80 °F (27 °C) and 50% relative humidity at max BTU/h operation.

If you want a minimum "real world" max power consumption, the EER/EER2 number yields a defined power at defined conditions. While your actual average consumption may indeed be closer to SEER/SEER2 calculated values, one may not want to count on them at the outset.

EER2 is the same as EER except it has a higher static external pressure. EER2 is 4.7% lower than EER, so you can convert if you need to.

 

[Supervstech]

Oh, i have no idea.
Logic and actuality are often incompatible.

 

[Supervstech]

Yeah, a 9kbtu unit running variable up to the load of 9kbtuh will use less energy than a 12kbtuh unit operating at a varied operating load but never reaching 12kbtuh wouldnt be very efficient.
But it WOULD have additional heating and cooling abilities.
However, in humid climates it would be uncomfortable in all but the hottest weather...

 

[Supervstech]

I don't think you are understanding.
A 12k unit run at 75% capacity is 9k and likely more efficient than the 9k run at 100%.
This will help extend your solar and batteries. Check your specifications for details.

It is a very bad idea to install ac equipment larger that the building load at design temperatures.

Unless you live in a low humidity area... or you install dedicated dehumidification equipment as well.

No, the dehumidify mode of a minisplit is NOT the same.

 

[sunshine_eggo]

It is a very bad idea to install ac equipment larger that the building load at design temperatures.

Unless you live in a low humidity area... or you install dedicated dehumidification equipment as well.

No, the dehumidify mode of a minisplit is NOT the same.

I've seen first hand how paltry RV insulation and solar load from lots of surface area sucking up that 1000W/m^2 besides just the heat transfer from interior to exterior makes A/C very ineffective; hence, my recommendation to stick with the stock size even if a 9K mini-split might suffice.

I would take the advice of a pro over mine, so once @Supervstech actually gives his opinion, I'd wait.

 

[Supervstech]

I've seen first hand how paltry RV insulation and solar load from lots of surface area sucking up that 1000W/m^2 besides just the heat transfer from interior to exterior makes A/C very ineffective; hence, my recommendation to stick with the stock size even if a 9K mini-split might suffice.

I would take the advice of a pro over mine, so once @Supervstech actually gives his opinion, I'd wait.

Performing an actual load calculation to determine the energy needed to handle the load on a mobile vehicle is very difficult.
On a home, there is a portion of the structure that always points one direction, shading can be counted on, etc.

Mobile is difficult to determine...

YES is pretty much the size needed for mobile...

 

[liamlunchtray]

We have a 9k btu in our 27' class C. It's adequate although there are times when I wonder if maybe the 12k would have performed better. I was more concerned about short cycling and not being able to adequately remove humidity, so I didn't want to oversize. If you run the calculation using typical residential math even the 9k would rightfully be too large for the space. With a 21' I really think a 9k would be fine.

 

[sunshine_eggo]

We have a 9k btu in our 27' class C.

Is it a minisplit?

 

[wme]

Not applicable in this case, BUT Furrion has released a 18k btu AC roof mounted AC with an variable speed compressor.
Called a Chill Cube

 

[0truck0]

I think 9k is plenty enough as long as you are insulated, the glass being the biggest heat loss.
I have a 9.2k Coleman Mach roof unit, not mili-split, but is 9k range. It is in a cargo trailer and is way way more than needed. If they had a 4K I would use it. As is the compressor runs just minutes before cutting back off again, is kinda silly. If I had to do again Id get a 5k window unit or I would get the smallest mini-split I could find for this particular trailer. Yours is different, but still the 9k is plenty.
I do notice that where the panels are on the roof is much cooler at the ceiling than where the sun hits. As long as you have lifted panels 1/4 inch or more so air circulates under that will help keep roof cooler. Using awnings also can keep the side wall cooler. I am working on using plastic to cover the bare spots on the roof so that most of the roof is covered which will help the compressor go from running minutes at a time to seconds at a time - all joking aside, it will help keep things cooler.

**edit. I never thought much about the dehumidifying aspect. My 9k runs just a few mins, 2 mins, then cycles off for 10 or so. It is not pulling much water out of the air when the compressor is not running... Maybe that is why not a lot of water runs off the roof either. If I had a 13k then the compressor would run even less and pull even less water out. That is a thought.
My compressor has two speeds: Fully wide open, and fully off, nothing in between, so it is a 9k AC unit sometimes and mostly it is a fan.

 

[daveemac]

I think 9k is plenty enough as long as you are insulated, the glass being the biggest heat loss.
I have a 9.2k Coleman Mach roof unit, not mili-split, but is 9k range. It is in a cargo trailer and is way way more than needed. If they had a 4K I would use it. As is the compressor runs just minutes before cutting back off again, is kinda silly. If I had to do again Id get a 5k window unit or I would get the smallest mini-split I could find for this particular trailer. Yours is different, but still the 9k is plenty.
I do notice that where the panels are on the roof is much cooler at the ceiling than where the sun hits. As long as you have lifted panels 1/4 inch or more so air circulates under that will help keep roof cooler. Using awnings also can keep the side wall cooler. I am working on using plastic to cover the bare spots on the roof so that most of the roof is covered which will help the compressor go from running minutes at a time to seconds at a time - all joking aside, it will help keep things cooler.

Thank you. I think my roof will be pretty good with panels & highly reflective membrane elsewhere. My insulation however is pretty much 3/4" styrofoam type stuff, and I have big old-school windows as well...

I'm leaning towards the 12K currently. The install is reversible and I could put it in a cabin down the road....and be sure that I've got plenty of capacity on the RV. Still deciding though...

 

[0truck0]

I had a class C with the front half of box, about 7x7 foot area with bed over cab, and the 13.5 AC unit was still over-kill in a 120 degree desert. A 9k would have been plenty for that. When I opened the door and cooled the entire 15 foot box then the 13.5 was still fine.
I would put in the smaller AC in the camper, and when you get a cabin just buy the right AC for that instead of having to un-install an AC and then having the then install all over again an AC for the camper. Seems like a lot of redundant work.

 

[daveemac]

I had a class C with the front half of box, about 7x7 foot area with bed over cab, and the 13.5 AC unit was still over-kill in a 120 degree desert. A 9k would have been plenty for that. When I opened the door and cooled the entire 15 foot box then the 13.5 was still fine.
I would put in the smaller AC in the camper, and when you get a cabin just buy the right AC for that instead of having to un-install an AC and then having the then install all over again an AC for the camper. Seems like a lot of redundant work.

In this same RV I had a 13.5K roof A/C that kept me cool during 111 degree heat wave....but not with much to spare.

Yes, removing and re-installing is not ideal...but I may do it if budget is too tight to buy another mini