Do I really need the lower AC unit?

Diesel Dan · 04-09-2010, 09:59 AM

I've noticed that many of the used rooftop air conditioners sold on craigslist do not come with a lower unit. I wonder what hassles and trade-offs I might be in for if I just don't use a lower unit. I'd rather not have anything extending below the level of the ceiling because I will surely be banging my head on it. Could I just have a remote on/off switch and thermostat, and install a fan in the ceiling opening under the AC and wire it to come on when the AC does? I know it would only blow straight down, but I'll have other fans in the bus to move air around. I suppose I'd also have to have some way of closing off the hole in colder weather so as to keep my heat in...

Any ideas/feedback?

baadpuppy · 04-09-2010, 10:06 AM

The problem you'll run into is that the cold side and the hot side air both go through that 14" square hole. You don't need the interior piece however, if you decide to duct the air yourself. Basically, you need to make sure the warm humid air from inside the vehicle can get into the air conditioner without much obstruction, and that there's some insulation between that and the cold dry(er) air returning from the air conditioner so that you keep condensation to a minimum. You can duct the cold air to where you really want it.

Many models of RV A/C units can have either the standalone inside piece, or a ducted inside piece, and the only difference is what part number you order for the inside piece.

hope this helps,
jim

baadpuppy · 04-10-2010, 06:41 PM

Smitty,

The rooftop A/C units have all the blower stuff in the part that is on the roof. The inside piece contains the ducting with separation of hot/cold air, the controls, a thermostat, and possibly a heat strip.

There is a single electric motor (not considering the compressor) with a shaft out both sides. It's a lot like a window shaker, with a removable face, where the controls and face ductwork is detachable.

(click the pictures to see the larger version)

Showing the fan motor:

Showing the control piece:

Showing the 2 air plenums and the wire harness between the main unit and the controls:

hope this helps,
jim

baadpuppy · 04-10-2010, 08:44 PM

Quote:

Originally Posted by Smitty

Gotcha! Sounded like he was talking like there was no evaporator blower. So why couldn't a curb be built, with the part that normally intrudes into the cabin (grille & controls) above ceiling level? If it takes a 14"X14" hole for things to fit through, and we'll just say the grille is 20"X20"...bump-up a 24"X24" square curb si the grille recesses into the ceiling?

Think skylight curb, but A/C setting on it instead of a bubble.

Smitty

That would work as long as the bump-up was large enough that the air could be deflected back down into the cabin area instead of into the return on the A/C unit.

There are lots of ways to tackle this situation. Coleman's installation docs include descriptions of the proper way to install such a unit with ductwork, and I've seen somewhere a method of installing 2 with shared ductwork in such a way that they don't interfere with each other (static pressure, etc... something I'm sure you know much more about than I do).

One advantage of using external controls is that you have control over the compressor... and if you have 2 units, you can do staggered starts on the compressors so that they both can't kick hard at the same moment and trip a breaker, or you can even have them take turns running, or whatnot.

jim

GoneCamping · 04-10-2010, 09:56 PM

I didn't have room for the drop-down piece either, so I cut out the control unit and used some vent covers from Lowes (I had to duct it myself with some styro-foam and metal tape). It came out like this...

The big square vent cover is the air intake, the narrow vent is the cold air output, that particular vent sends it foreward and aft as the vent angle splits in the middle.

browncrown · 04-12-2010, 02:00 PM

Nice job on your installation GC

Diesel Dan · 04-12-2010, 02:45 PM

Thanks for the replies everyone! I wasn't sure if the fan was in the upper or lower unit. But since it's in the upper unit it sounds fairly simple to just duct the intake and output vents like GC did, and just have a remote thermostat and on/off switch.

Diesel Dan · 08-20-2010, 02:03 PM

Okay, so here we are 4 months later, and I now have an A/C unit I got off craigslist. It's a like-new Carrier 15,000btu roof unit, but does not include the inside ceiling assembly. This is perfect for me because I want to set it up like Gone Camping has done in his bus. I found a manual for this A/C unit, so I should be able to figure out the wiring. I will probably buy the thermostat from Carrier to control the A/C.

But I have a much more basic, and probably stupid question: how do you attach the thing to the roof? It is not clear to me where the attachment points are located. Can anyone give me some suggestions about how to mount one of these things? I see four threaded holes inside the 14" weather-stripped square, but it seems like these might be for the inside ceiling unit to bolt on to. Even if they were for the purpose of bolting down the upper unit, I'm not sure what I would attach the bolts to on th bus, since that will be a big empty hole... oh wait a minute... does the lower unit normally hold the upper unit on by bolting to it from underneath?

JakeC · 08-21-2010, 11:26 PM

Yea, that might be a slight problem. That is the other purpose for the lower part...it is a brace that the bolts from the upper unit go through, and then as the nuts are tightened it pulls the upper part down against the roof, sandwiching everything together. I'm sure you could run some braces across, though.

*edit* I just read your last sentence again. I could have just said..."yes, you are correct"...haha...

Locutus · 08-24-2010, 12:35 PM

Not to hijack DD's thread or anything like that, but I have a question for your consideration (the answers to which might help DD also):

How about suspending a rooftop A/C under the floor and ducting the airflows up behind the cabinetry, perhaps branching the conditioned air portion to several ports just above the overhead cabinets so that the ductwork is hidden, and perhaps the return air intake hidden in the cabinetry toekick area? Since cooler air sinks, this would flow the A/C from ceiling to floor. I would like to cut as few holes as possible in the roof, and this way the A/C would remain hidden from outside, making the bus more stealth for Wallydocking. I've noticed that some of the shuttle buses have an A/C unit mounted under the floor with a grate on the side just behind the front wheel.

Is this even a feasible mod to a rooftop A/C? Or would I be better off modifying a window unit in similar fashion? I've seen window units for sale at Home Cheapo and Wally's from $99 to $499.

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