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A couple days ago, Stef offhandedly posted a picture of me tinkering with the RV air conditioner on our Facebook page. Nothing new, she posts loads of pics of what we’re up to day to day there, and many of them involve me working on Lance.  I didn’t think much about it, but apparently, many of you left comments and were interested in what I did and if it worked.  The quick answer is: I adjusted the thermostat and yes it worked.

But before you go trying this yourself, here’s the situation I had:

  • Our new air conditioner, a Dometic Penguin II, works very well and produces a temperature differential (intake vs. cold air) of about 20 degrees.  So the air conditioner was not “broken”.
  • With the thermostat (the colder/warmer knob) turned all the way cold, the air conditioner would cool the inside of the RV to about 76 degrees.
  • Once the temperature was 76 degrees, the compressor on the AC would stop running.  The fan would still run, but it was no longer cooling the air.  When it warmed back up to 78, the compressor would kick on again and cycle with the temperature.

In my mind, 76 degrees is not “coldest”.  It’s not even “cold”.  In fact it’s uncomfortable.  If you’re sitting on the Ultraleather dinette next to a window in the front of the rig, and the temperature is at 76 in the back of the rig – you’re going to be sticking to the seats.  Not cool (ha!).  So, I decided to change the air conditioner’s minimum set point from 76 degrees to something considerably lower.


A Word of Warning

The air conditioner manufacturers have the minimum temperature set in the mid-70s for a reason.  They don’t want your air conditioner to “freeze up”.  That could prevent the AC from working, damage the air conditioner, or lead to a service or warranty call.  They don’t want any of these to happen, so they deploy their air conditioners at temperatures that will make you sweaty – but will guarantee that the unit never freezes up.

ca. 1602 --- The Maiden and the Unicorn by Domenichino --- Image by © Alinari Archives/CORBIS

Back in the 1600s, they had air conditioners that froze up.

I’ve always regarded the “air conditioner freeze up” as a myth.  I think it’s the same kind of reasoning that gives us RV shower heads that leak all the time on purpose – because there’s an extremely remote chance you could scald yourself if it doesn’t.  (By the way, I fixed that problem too, and you can read all about that here.)

Of course, I should point out that we live in Utah, where the relative humidity typically hovers in the single digits.  We don’t even get condensation on the outside of an icy glass.  So the chances anything would “freeze up” on me in the middle of summer are basically zero.  If you live elsewhere, you may want to take that into account.

But back to the AC.  There are four main reasons your air conditioner may freeze up:

  1. Insufficient airflow across your air conditioner’s evaporator coil.
  2. Your air conditioner is low on Freon.
  3. You are running your air conditioner when the outside air is below 62 degrees.
  4. There is a mechanical failure (kink in refrigerant lines, a blow fan is not running, etc.)

Number 1 is something you can prevent by keeping your air conditioner’s intake screens clean, so put that on your routine maintenance checklist.  Numbers 2 and 4 represent problems that I’d like to know about – and a frozen air conditioner seems like a good indicator.  And number 3 is something I’m not worried about because I’m not stupid.  So even if I did believe in air conditioner freeze-ups and Bigfoot, I think the problem is manageable at best, and just an indicator of a bigger problem at worst.


What I Did

So, if you think you have the same problem with cycling that I did; you think your air conditioner is unlikely to freeze up; you don’t have a separate wall thermostat for the air conditioner; you can use a screwdriver; and you’re unafraid of possibly voiding your AC warranty, here’s what I did.  I have tested this procedure on my own AC unit, which is a Dometic.  I suspect that others (Coleman, etc) may be similar, but I have not verified this.

First, obviously, make sure that ALL POWER to your coach is turned off.  No generator.  No shore power.  No inverter.  And I even shut the batteries down for good measure.

Then, remove the indoor air distribution box (ADB) cover.  You’ll wind up with something on your ceiling that looks more or less like this:

What you’re looking for is the inside part of the “colder/warmer” knob – the thermostat.  It will likely be inside another metal enclosure inside of the ADB.  In the picture above, I’ve loosened the cover on this interior box.  If you’re stumped, it will have a strand of copper leading into it.  The other end of that copper is on a probe in the intake air.

Once you’ve located the thermostat, do what you need to in order to remove it from the ADB.  In my case, I had to remove two screws.  But your air conditioner may have nuts or some other way of holding it still.

With the thermostat removed, you’re looking for an adjusting screw of some sort.  It probably won’t be labeled, because they don’t want you messing with it.  But it should be there.  On my Dometic, the screw looked like this:

Now, for the tedious part.  In small increments, start adjusting the screw.  You want to do this slowly:

  1. Adjust the set point screw ¼ turn.  (I don’t know which direction on yours!)
  2. Reassemble the ADB box (Safety first!  You’ll be turning on the power.)
  3. Run the air conditioner until the compressor cycles
  4. If it gets cold enough for you, you’re done.  If not  – lather, rinse, repeat.

In the end, I gave my screw about one and a half turns clockwise.  When I had finished, the “warmest” setting on the thermostat gave me about 80 degrees at the dinette, and 68 degrees at the dinette was at the ¾ mark.

It’s important that there is *some* realistic temperature where the air conditioner will cycle off, and that temperature is at 62 degrees or above.  So don’t overdo it!

And with that, I was done.

This took about 15 minutes to figure out, and about two hours to do with all the setting and checking.

Good Luck!