Mini Split sizing questions?

PNW_Steve · 09-03-2017, 01:53 PM

Hey Everyone,

I need some input from the HVAC Guru's out there.

I am working on my HVAC plan that includes a pair of mini split A/C's.

I had originally thought that I would simply install 2 x 12k Btu units and mimic the pair of 12k Btu roof air's installed on many similarly sized RV's.

As I got to looking at specs and power consumption I started to consider the idea of one 12k and one 9k instead. As I looked over the detailed specs on the 9k unit I saw that the cooling capacity is variable from 3.5k to 11.5k Btu's.

Then I found an online "sizing calculator". I did my best to approximate bus conditions and that resulted in total cooling capacity requirements of 10k to 13k Btu's depending on whether I chose "fair insulation" or "no insulation"

Given that the paired capacity of two 9k Btu units would be 7k to 23k Btu's I have to wonder if 12k's would be over sized?

I also read somewhere that the higher SEER units are more fragile.

What are your thoughts on sizing and durability?

Am I better served with the 9k Btu units? Are the 16-17 SEER units really more durable than the 21-22 SEER units?

Thanks

S.

Edit: Here is the 9k unit: https://www.highseer.com/ductless-mi...EaAl1FEALw_wcB

somewhereinusa · 09-03-2017, 04:43 PM

I have nothing to go on as far as calculations and charts. I can tell you my experiences. I have a 40 ft RE. Walls and floor have about 2 inches added insulation. Roof, as of right now, has just factory fiberglass. I have 2 12,000 btu mini's mounted in front and rear. We just spent a week in TN sitting in the sun in above 90 deg weather. And, while I didn't look at a thermometer we were ALWAYS comfortable. Well actually SWMBO was always comfortable, I slept in my sweats with a couple of blankets.

I have never felt the need to run the AC while driving but, driving into the sun on a hot sunny day will seriously over tax your AC. You are sitting in a big fishbowl.

For the difference in price I would go with the biggest 120VAC unit you can get. Labor to install is going to be exactly the same. I don't think operating costs are boing to be any real world differance.

PNW_Steve · 09-03-2017, 05:52 PM

Thank you for the info.

If I were always plugged in to shore power I would go with a pair of 12k but I expect to be spending much of my time dry parking and power consumption becomes a big consideration.

I spent 6 years traveling in my last bus and know that a pair of 12k won't keep the bus cool while traveling in hot weather so we are also working on an OTR solution.

Bottom line: If a pair of 9k's will do the job, while parked, then I will likely go that route to save purchase $ and reduce power consumption.

The big question is: will two 9k's cool my bus while parked?

PNW_Steve · 09-06-2017, 10:16 AM

Has anyone tried the 9k's? Or has everyone running mini split gone with 12k?

family wagon · 09-06-2017, 12:54 PM

Sorry I can't contribute to the sizing. I would (eventually will) attempt to do heat gain/loss calculations as one would do for a residential building with Manual J for example.

A couple years ago I did a heat loss experiment which involved a pair of 240 volt 2250 watt baseboard radiator style electric heaters laid on the bus floor overnight when the outdoor temperature dipped to about 10° F. Knowing the ∆t and the heat input we should be able to figure its resistance to heat transfer and translate that to a cooling scenario. There'll be a few other pieces that have to be added above and beyond that for cooling: direct solar gain, people and cooking activities inside, and removal of moisture from the air. I'm not sure how to model those things yet.

As for over the road cooling: I was quite surprised during my re-skin project at how much opening there is allowing air to come through the dash area while driving. I hope that sealing those up, plus better gasketing around the windows and doors, will bring the OTR cooling demand much closer to the parked cooling demand.

PNW_Steve · 09-06-2017, 02:50 PM

I had spray foam insulation in my last bus. Things were fairly well sealed up front as well.

I made several trips through 100F weather with 2x 12k Btu roof airs blasting and I ROASTED....

By far the worst times were when the sun was lower on the horizon and shining in through the windshield.

One thing in my favor is that, this time, I intend to "go where the weather fits my clothes"..... And this time I mean it!!!!

Last time on the road I found myself in Las Vegas in August and Detroit in February. Not gonna do that again if I can avoid it.

family wagon · 09-06-2017, 03:23 PM

Roasting in a well-sealed bus with a pair of 12k running full tilt doesn't bode well!

It makes sense that it'll take more A/C work when the sun shines through the windshield, but I'm not following why sun shining through the window would be much worse when underway vs parked. Is that because on the road the windshield has to face forward and that might be looking into the sun, whereas parked you would position the windshield a more favorable direction and avoid that solar gain? Or is there something more subtle -- heat gain is simply greater for an unknown reason when the bus is rolling down the highway?

brokedown · 09-06-2017, 03:54 PM

Hey Steve, here's my suggestion as someone who is unlucky with AC:

If you have 2 roof mount AC units, use them as the edges for where you put walls (or even thermal curtains like what we have!), so you always have the option of turning on the maximum level of AC to manage the smallest area possible. If you guys are just watching Long Way Round on the TV (did you start it yet?) you don't need to cool the unused areas... And you can open the curtain/door/etc when you're doing other things.

PS - check your PMs!

Defjr333 · 09-06-2017, 04:32 PM

One note on inverter mini splits.. They are at their optimum energy efficiency at 50-80% of rated BTUs. So a 9K btu MAY cut it to keep you cool, but its going to be running at max with max amp draw. Whereas the 12k will be running at 60% its btu rating and not pull as many amps due to efficiency. These new mini spits "spool" up to meet what is needed to cool to your selected temp. One last note, if you have 2 12k btu units running and its still hot, you need to open a ceiling vent to remove the hot air pooled at the roofline. I have seen it many times. An RV running ACs at max and still roasting. Open a vent to remove all that excess hot air at the top 6" to foot makes a HUGE difference. My ambulance was like that. Ran a 9k btu with little effect. Opened the roof vent and went from 92 to 71 in under 30 mins!
Doug

PNW_Steve · 09-06-2017, 04:38 PM

Quote:

Originally Posted by family wagon

but I'm not following why sun shining through the window would be much worse when underway vs parked.

The reason is quite simple..... I have yet to figure out how to drive with 32sq. ft. of refletix in the windshield......

In hot weather I would strive to find shade and at a minimum, try and park the bus so that the afternoon sun does not shine into the windshield. Also, I keep the windshield covered with refletix.

There is also more (hot) air infiltration when driving. We were pretty well sealed but not perfect.

PNW_Steve · 09-06-2017, 04:43 PM

Quote:

Originally Posted by brokedown

Hey Steve, here's my suggestion as someone who is unlucky with AC:

If you have 2 roof mount AC units, use them as the edges for where you put walls (or even thermal curtains like what we have!), so you always have the option of turning on the maximum level of AC to manage the smallest area possible. If you guys are just watching Long Way Round on the TV (did you start it yet?) you don't need to cool the unused areas... And you can open the curtain/door/etc when you're doing other things.

PS - check your PMs!

Good idea. But.... The two roof airs were in my old bus. I am planning on mini-splits in the new one.

The will definitely be "zoned". One in the living room and one in the bedroom. I will have the ability to close off the two area so that I don't have to heat/cool the unused space.

brokedown · 09-06-2017, 04:45 PM

Quote:

Originally Posted by PNW_Steve

The reason is quite simple..... I have yet to figure out how to drive with 32sq. ft. of refletix in the windshield......

Install a FPV camera for a drone and drive with a pair of FatShark goggles!

Totally possible but totally illegal!

PNW_Steve · 09-06-2017, 04:56 PM

Quote:

Originally Posted by brokedown

Install a FPV camera for a drone and drive with a pair of FatShark goggles!

Totally possible but totally illegal!

Awesome idea!!

Try it out and let me know how it works out...

cadillackid · 09-06-2017, 06:30 PM

just saw this... one thing to remember is that minisplits are variable speed as you noted.. but they are Ineffecient when run at more than 80% of their max... so a 12k unit may push out 13500 or 14000. but it wont be efficient there.. I have 3 Modified minisplits in my house and i watch the energy usage factors when they run their respective zones.. its true that they will ramp up to max and cool an area quick.. but you want it to spend as little time in that high-ramp state as possible... being that they are variable speed it doesnt hurt to oversize them.. they will ramp down when needed to control humidity and be efficient at low speeds... undersizng will cost in efficiency..
-Christopher

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