Is there any way to get the ACs to cycle on high instead of low fan?

Rhetorical question but I had to ask (and I think I saw someone ask it previously but couldn't find it). Due to a change in plans I'm sitting in the Arizona heat and not leaving until it's nice and cool in the Fall (I know, backwards  : ). So with temps already hitting 105ish occasionally (and temps in the teens just around the corner  ), the forward AC in partiular runs almost continuously. The problem is, it has way more BTU capability on high fan, easily demonstrated by switching to high wherein it runs for awhile, brings the temp down and the compressor cycles off. In "Auto" the fan runs on low, thereby causing the compressor to run for a longer percentage of the day than it would if the fan was on "high". Since the compressor comprises most of the load and the load difference between low and high fan is probably insignificant, I'm thinking it is actually costing me more than it should (I'm on metered electric). Putting the fan on "high" causes it to run 100% of the time, even with the compressor cycled off. The big issue there is the heat being brought in to the cabin (when the compressor is off) by circulating room air up through the duct into the rooftop unit and back down - within 15 seconds of the compressor cycling off I can feel the heat. So leaving the fan on "high" isn't a solution.
I believe the thermostat (5 button Duo-Therm) programming is setup to only use "high" when there is a significant difference between selected and actual temp.

So the (hopefully not) rhetorical question: is there any way to get the ACs to cycle on "high" instead of "low"?

That is an interesting scenario Scott. I just checked mine and the fan speed is dictated by the position of the "HI-LO" fan switch. Then the AC cycles as needed when on auto.  If I turn the fan on while the AC cooling is in the off position, it runs at whatever the position the Hi Lo switch is set at.

You may have an issue with the wiring going to the fan. In my experience, it is generally better to run the fan on Hi when the AC is in cooling mode to avoid the condenser coils from icing up and restricting air flow. 

Scott, I believe ours operate pretty much like Sarges.  If you haven't experienced 120? Arizona heat in an RV you should consider window screens and insulation if you haven't already done so.  We have exterior window screens on all windows and while I haven't suffered through AZ 120? temps, it does help.  Haven't needed to use it yet, but I purchased a 100 sf roll of reflextix insulating material from Lowes for about $45.00.  It can be cut to size and inserted on the interior of the windows.  Careful about the kind that sticks to the window.  After a season or two it can be hard to remove (at least that's what I've read).  In any event, do what you need to do to enjoy the summer. 

SargeW said:

You may have an issue with the wiring going to the fan.

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When all else fails read the manual (attached)  : . Unfortunately it appears that mine is working per design, I just don't like the design! (I need one of our resident geniuses to come up with a "oh that's easy, just ________________________" solution!) 

garyb1st said:

If you haven't experienced 120? Arizona heat in an RV you should consider window screens and insulation if you haven't already done so. 

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I have spent time here in an RV in the summer, but only on a vacationing not fulltiming basis. Re screens and foil, check check. When I bought the coach I had exterior shade screens made for every single window - they help a lot, especially with sun coming in the window. On the windshield and every window bathed in sun I also have bubble/foil over the window. (I didn't do every window cause it would be too claustrophobic.) I do keep the "day" part of the day/night shades pulled and when I have checked there doesn't seem to be a bubble of hot air behind the shade. I've also covered the large shower skylight from the outside, and have inside insulated foil plugs for the two Fantastic Fans. And of course, the awnings are down.

Ah well. Who's plan was this anyway?  :

Is retrofitting a new thermostat an option? If it's simply thermostat programming and a newer, direct replacement model would give more control, I'd consider it.

Hard to say, because the thermostat mostly just says "it's warm in here" and the a/c does it's thing. Whether the interface between thermostat and the a/c controller provides for multi-speed control depends on how smart both of them are. You need to get into the specifics of how the a/c control on your a/c can be configured/wired and whether or not the thermostat can provide the inputs.

On my last rig, with an Atwood Comfort Control t-stat and Coleman Mach heat pumps, the AUTO mode used High & Low fan speeds depending on interior temps, but mostly used low fan (quieter) as much as it could.  Selecting Lo or Hi fan speed on the stat was not something I tried.

I run mine on HIGH FAN.. My Coleman does not have an "Auto" option, but my Other one does.

The advantage of fan always on is you get used to it and block the sound out.

Call the A/C maker tech service?????  If you get a techie instead of a robot reading from a script, they may be able to help.

Unfortunately, the thermostat and motor wiring both use plug in multipin connectors.  If it was an older A/C (like the Coleman in my motorhome) the connections inside the air conditioner itself used screw terminals, so you could literally just swap wires X and Y and get the high speed blower when the thermostat calls for medium speed and vice-versa.  But plug-in connectors means swapping pins around, difficult unless you have the proper tools.

I suggest getting a couple of small oscillating fans and setting them up to keep the air in motion inside the motorhome.  Moving air makes a big difference in hot weather and you may be able to raise the thermostat temperature and still be comfortable while reducing the compressor run time.

My Sunnybrook trailer has the 12 ft. superslide in the living room and kitchen/dinette.  I have an 8" oscillating desk fan on the front counter that sweeps back and forth across the room.  In 100+ degree temperatures it really makes a difference keeping the whole room comfortable instead of just letting the cold air fall to the floor.  That's the real difference between the medium and high A/C fan speeds - the high speed circulates the air better.  The BTU capacity stays the same.

Fans are cheap - $5 - $15 at WalMart and use almost no power.

Thanks Lou. Yes, I've got a couple fans and they do make a difference. It's not that I can't make it comfortable, was just trying to address the compressor run time. Example: at the temp I had it set (82) and fan in auto, the compressor ran continuously and the AC never achieved 82 to the point of shutting off the compressor. I tried turning the temp down 4 degrees to ensure the compressor stayed on and set the fan to high. Sure enough, about 20 min later the compressor cycled off. So while the AC in auto couldn't handle the heat load, it could with the fan in high. So the ACs work, just not a great setup when the compressor shuts off and the fan starts blowing warm air into the cabin. And yes you're right, if I could just swap a couple terminals I'd probably give it a try. But given the pin situation, and especially considering the communications issues I've heard about between the ACs and the thermostat, will just have to live with it.

Since the front AC carries the primary load, one thing I'm trying is setting the rear AC about 5 degrees higher and forcing the front to carry the full load, which then requires high fan and continuous compressor, but my rear AC only then comes on occasionally. I think this is a case of attempting to fine tune something with no way to test the results to know what's worked.

grashley said:

Call the A/C maker tech service? ??? ?  If you get a techie instead of a robot reading from a script, they may be able to help.

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Duo-Therm?s # in the manual is disconnected. I?m going to give Dometic a call and see if they have a direct swap plug and play upgrade that will play nice with the pins Lou described