mini-split sizing full size bus

[GTSailor]

So I am 90% convinced to do a dual zone mini-split. I am looking at two inside units (either 12 + 12 or 9+12) and a 36k btu outside (or even 24MBH outside but I think this limits it to 9 +12)

These are my reasons:

1. Two rooftop units are 12MBH to 15MBH each which was my plan, so that's 24k to 30k. not far from 36MBH
2. I am reading that the unit has really good turndown - 5MBH on 24MBH unit and 8.5MBH on the 36MBH model.
3. The price between the two 24MBH and 36MBH is $400. not cheap but would pay it to not be hot. Install is pretty much the same
4. The units operate ideally at less than 75% of full capacity so that is my reasoning for 36MBH (36*75% = 27MBH)
5. I am in Florida and only plan on having havelock insulation (still debating spray foam). Regardless.... it's still a bus and it's still Florida.
6. I could try a 24MBH but I really believe i will need a 12MBH indoor by my room (in back) to keep it cool enough at night. I like it cold for sleeping. That only leaves 4MBH front to stay under the 75%.
7. My goal would be to only use the full capacity for cooldown from "away" temps to "occupied" temp setpoints. normal usage while occupied would be to be under the 75%.
8. I want the option of very long stays on this bus (perhaps fulltime) so I am leaning towards comfort.
9. I am doing full window deletes by reskinning and putting back with double paned(not due to R value but mainly condensation/noise) so this could be an argument for smaller unit(s).

I understand this large of unit is only available as 240V. I think I have a good plan for that. Leaving that alone for a bit, am I crazy on the sizing? anyone have some real world large bus experience in florida? two units? one unit?

FYI i plan to install in between the frame rails just behind rear axle. I intend to do some ducting to direct exhaust from unit to rear and pull air from sides etc. but not box the unit in. my goal is to just discourage short circuiting of the discharged air(hot or cool) from being re-injested.

[cadillackid]

the multizone outdoor units are BIG.. the 12K outdoor units most install are much smaller in height and width than the bigger 18l / 24k outdoor units.. I know several people that have 2 12k outdoor units under their rear engine busses, thus also able to stay at 110 volts as the brand they bought are not requiring 220

[nolahusker]

Overall size of a multi zone outdoor unit is large. Will likely have to mount on the rear of the bus. That being said, be careful with over sizing the outdoor unit with respect to the indoor units. Work with an authorized dealer who has access to the sizing software for that unit and who can put in the actual refrigerant line lengths, elbows, and elevation change to properly match your indoor and outdoor units with the appropriately sized line sets.

[cadillackid]

Originally Posted by nolahusker

Overall size of a multi zone outdoor unit is large. Will likely have to mount on the rear of the bus. That being said, be careful with over sizing the outdoor unit with respect to the indoor units. Work with an authorized dealer who has access to the sizing software for that unit and who can put in the actual refrigerant line lengths, elbows, and elevation change to properly match your indoor and outdoor units with the appropriately sized line sets.

you can somewhat oversize the outdoor unit because they do ramp down when in use.. however there are definitely minimums.. if you use a 36K outdoor unit.. the minimum is usually somewhere around 9-11K running so that means any 12K indoor unit run by itself will have to run at a fairly high rate or the system will cycle and over-shoot quite a bit esp on heat.. with a 24K outdoor unit most manufacturers state a 5000-7000 BTU minimum run rate so you'll fare better.. maximum run is typically 15-20% over rated..



2 12K indoor units will cool a nicely insulated bus perfectly while parked.. no minisplit setup is likely going to give you that nice cool feeling going down the freeway on sunny days in the 90s..



each design case is different.. you have to look at how you plan to run them and the weather you plan to chase..

[nolahusker]

Originally Posted by cadillackid

. no minisplit setup is likely going to give you that nice cool feeling going down the freeway on sunny days in the 90s.. ..

I’ve been thinking about doing a 30k or 36k and having three evaporators- a ducted unit in the rear for ducting into sleeping areas and office, a wall cassette for the main living, and a ceiling cassette above the driver/passenger seat. Couple that with a couple of dash fans and I think I’d be okay.

[cadillackid]

ducted units and ceiling cassettes are pretty large.. ducted units could be made into basement airs in underfloor storage compartments but they cannot be in wet areas.. you are looking at i think 7 to 8 inches of height for most 12K BTU units.. if you are doing a roof raise then dropping the ceiling you could get away with above-ceiling stuff.. you could also mount a ducted unit under your bed .. floor mount it on rubber isolators and you wont have too much hum or vibration in bed..



I use the ducted units in my house.. in my house, I run the fujitsu commercial with wired wall thermostats (almost required for all cassette and ducted units).. the condensate pan on them doesnt seem real deep.. ive not played with how far they can be tilted and not overflow.. unlike the wall units ducted and cassettes dont typically have the provision for 2 drains..

[nolahusker]

There’s definitely a roof raise for accommodating the ducted units. I was counting on 12” lost, actually, though I hadn’t thought to put it underfloor. That’s not a bad idea.

[UFO pilot]

Originally Posted by cadillackid

2 12K indoor units will cool a nicely insulated bus perfectly while parked.. no minisplit setup is likely going to give you that nice cool feeling going down the freeway on sunny days in the 90s..

So........any hope for a poorly insulated bus in 100+ temps on long desert highways, and sitting in OHV staging areas without shade?
Could a ~30K BTU 3 zone mini do the job going down the road if the drivers compartment was walled/inslated off and had it's own ~8K evaporator?

[Dbacks2k4]

From my research dual zone units are impractical. 220 power while doable is much more complex/pricey. Ducting is complicated and takes up valuable headroom.

For me I'm just doing a pair of 12k 110v units. I have the real estate under chassis and that also gives me the option of just running one unit at a time if it's not ridiculously hot outside. Also reduces my line set distance by locating the indoor component closer to it's corresponding outdoor unit. Plus as an IT nerd, I like redundancy in everything I do. If one fails I'm not completely sol - sweating out of luck

[ism minerals]

I have two LG 12K units in my bus @120v. The condensers fit nicely under the bus with some room to spare. They work very well for my application, but The bus is very well insulated. I have no problem running one of them on the inverter 2800 watt and most likely could run both if needed. With one unit and the sun feeding the solar panels, i think i have an estimated 10h of run time with 1000AH battery bank at full capacity measured by the draw. Of coarse that assuming the sun is up for 10h other wise i figure it around 4-5h without the sun, before i need to start the generator to recharge the house bank.

[budbud7]

Originally Posted by cadillackid

you can somewhat oversize the outdoor unit because they do ramp down when in use.. however there are definitely minimums.. if you use a 36K outdoor unit.. the minimum is usually somewhere around 9-11K running so that means any 12K indoor unit run by itself will have to run at a fairly high rate or the system will cycle and over-shoot quite a bit esp on heat.. with a 24K outdoor unit most manufacturers state a 5000-7000 BTU minimum run rate so you'll fare better.. maximum run is typically 15-20% over rated..



2 12K indoor units will cool a nicely insulated bus perfectly while parked.. no minisplit setup is likely going to give you that nice cool feeling going down the freeway on sunny days in the 90s..



each design case is different.. you have to look at how you plan to run them and the weather you plan to chase..

I dont understand why a mini-split won't cool as well going down the freeway. I believe it could be issues with bumps and oil moving around when driving but it seems to me that if you have air flow to a properly charged unit, it should work. Please explain.

[nolahusker]

Originally Posted by budbud7

I dont understand why a mini-split won't cool as well going down the freeway. I believe it could be issues with bumps and oil moving around when driving but it seems to me that if you have air flow to a properly charged unit, it should work. Please explain.

Because you’re adding wind, which will lead to air infiltration. You’re also going to be sitting in the sun, so there will be direct solar gain to your body.

[GTSailor]

Originally Posted by nolahusker

Because you’re adding wind, which will lead to air infiltration. You’re also going to be sitting in the sun, so there will be direct solar gain to your body.

And because there is likely significant heat gain from engine directly into buss (via firewall). there are other posts about the BTU sizing of engine driven A/C --larger than mini-splits.

So its not that it won't work. it's that it won't sufficiently cool due to additional heat gain.

AC is all about heat gain/ loss. People often forget this when sizing units based on square footages. I replaced my unit in my house 3.5 ton to 3.5 ton. it short cycled like hell. ended up with 3 ton two speed. IN florida it rarely hits full speed. only the hottest days. the rest of the time it's essentially at 2 ton unit.

[Dbacks2k4]

Originally Posted by budbud7

I dont understand why a mini-split won't cool as well going down the freeway. I believe it could be issues with bumps and oil moving around when driving but it seems to me that if you have air flow to a properly charged unit, it should work. Please explain.

I had same question you had when I first started.

Cadillackid had a great write up here that was extremely helpful:
https://www.skoolie.net/forums/f9/th...a-c-27952.html

[nolahusker]

Originally Posted by GTSailor

And because there is likely significant heat gain from engine directly into buss (via firewall). there are other posts about the BTU sizing of engine driven A/C --larger than mini-splits.

So its not that it won't work. it's that it won't sufficiently cool due to additional heat gain.

AC is all about heat gain/ loss. People often forget this when sizing units based on square footages. I replaced my unit in my house 3.5 ton to 3.5 ton. it short cycled like hell. ended up with 3 ton two speed. IN florida it rarely hits full speed. only the hottest days. the rest of the time it's essentially at 2 ton unit.

I always forget that people have (relatively) uninsulated dog-nose buses. Rear-engine buses with spray foam really won’t experience much hear gain from the engine.

[Bigmikeclark]

I have two concealed units in my 45’ Prevost H3-45. I steel framed the bed in the rear bedroom and mounted one unit to it with the ductwork blowing straight down the hallway for better circulation. The other is mounted in a bay under the bus and ducted up to various places in the front.
Be sure the fully insulate the lines so that you don’t get condensation dripping and causing mold

[jjaj823]

I have a dual zone in my RE300. From living room/kitchen area is cooled by the standard “hotel” bulkhead type unit mounted just above the front windshield inboard of the driver.

The rear unit is a mini-ducted unit mounted along the centerline in my rear under storage. Vents come up through the floor into the rear bedroom as well as the bunk area, rear hallway, and bathroom. I also have a couple vents plumbed in at bed level in the bedroom too.

As for size, I went with an 18k outside unit from Senville. It was the largest I could find that still provided reasonable ground clearance. Both inside units are 12k. I was OK with that because I’m rarely to never using both sections of the bus at once, so one unit or the other is always off.

In the hottest part of the summer, mid-day in full sun, the bus would stay about 15 degrees below ambient. This is with the original roof panel in over the driver and no extra insulation in the door and stair area (so not really well insulated).

In terms of power usage, I can run the AC all day in full sun and still have a good bit of battery left at the end of the day. In crap weather, I get about 12 hours run time with the AC set at around 74. Most nights, I can switch from AC to dehumidify which uses waaaay less power but still keeps things pretty cool.

In terms of power, I have 240v split phase AC, 432 usable Ah @nominal 24v for 10,368wh of available power. I actually run 2 inverters (Growatts). Solar is 2000w roof mounted flat with no tilt. The cost of 24v equipment was more than offset by the decrease in wiring size required. And 24v stuff is still relatively easy to find. Breaker size is 25A for the outside unit and 10 for the insides.

Definitely doesn’t work while driving, but the unit itself has made it across the country twice with no cracks, leaks or other failures not related to me.

Also, overall cooling will be affected by the number of windows and if/how they are covered. I have blackout heat reflecting curtains that we switch around based on sun direction and time of day to keep the sunnier windows covered. Also, make sure you cover the front window really really really well…in full sun, I measured temps at the window at 30+ degrees above ambient.

This is just my experience, so YMMV…

[nolahusker]

Originally Posted by Bigmikeclark

I have two concealed units in my 45’ Prevost H3-45. I steel framed the bed in the rear bedroom and mounted one unit to it with the ductwork blowing straight down the hallway for better circulation. The other is mounted in a bay under the bus and ducted up to various places in the front.
Be sure the fully insulate the lines so that you don’t get condensation dripping and causing mold

This is the setup I’m looking at doing, but with an overhead unit for the driver.

[cadillackid]

Originally Posted by UFO pilot

So........any hope for a poorly insulated bus in 100+ temps on long desert highways, and sitting in OHV staging areas without shade?
Could a ~30K BTU 3 zone mini do the job going down the road if the drivers compartment was walled/inslated off and had it's own ~8K evaporator?

I run a 36K under-dash unit in my 1978 Superior conventional.. insulated the doghouse and sealed up the firewall.. clear shower curtain behind the 1st row of seats.. I dont sweat but I dont freeze in mid 90s heat...


in my 7 window DEV bus high headroom, conventional. insulated doghouse .. stock windows, stock bus interior.. non-tinted.. 100K BTU A/C system (engine driven, Dual compressors).. 104 degree texas heat on the highway in the sun.. 75 inside the bus.
.

[jimmythomas]

My 31' Thomas HDX has a Carrier 120k btu system, rear mounted evaporator with two ducts in the roof to the front. It was perfect this summer driving in Kansas, 105 F ambient and 65 degrees in the bus. Every time I stopped it dumped a few gallons of condenser water.