Air Conditioner Duct

[FLCapt]

I found something interesting on my '09 347 THT. I had the cover down off the A/C inside, as I was investigating the installation of the heat strip I pulled off my old camper when I traded in. On the plenum (cold) side of the A/C - where the fan blows the cool air into the vents, there was a hole about the size of my fist in the duct tape - leading into the area between the ceiling and the roof. No vent attached here, just a hole into the void under the roof. All vents are hooked up and working properly. I simply taped it shut with some duct tape I had handy. Shortly after, while camping, it was in the 90's and very humid. We had the thermostat set lower than we should have ( I think!) and the evaporator coil iced up completely. Once we got it thawed out, we just set the thermostat higher for the duration of the trip, and had no further icing issues. I was wondering about the hole into the sometimes very warm void between the roof and the ceiling. Is it possible that someone put this there to keep the evaporator from icing? Doesn't make sense to me. Has anyone experienced anything similar? The hole looked as if it was cut in the tape on purpose. Thanks for any input!

[PARAPTOR]

A reason for evaporator icing is inadequate air flow across it. By you patching that hole you are now relying on the air ducts in the rig to pass enough air to prevent icing of the evaporator. I would check and make sure all your ducts are open and not clogged.

Looks like that hole had masked over a problem. My two cents not a AC expert

[Bob Landry]

The hole you patched was not by design, it was a screw-up during the installation. The other thing that will cause icing is for discharge air to be leaking directly back into the return plenum. Drop the plastic cover from the return side and stick your hand in the cavity with the air running. If you feel ANY cold air leaking into that chamber, break out the duct tape and get to work. You should feel NO cold air.
Pull your ceiling registers and check that the ducts are sealed around the cutouts so that you are not pumping air into the ceiling where it will do you no good.
Finally use some kind of material to close off the ducts just past the last register in the string. The theory is that the discharge air in the duct will back pressure and be forced out of the register but it only works that way for engineers on their drawing boards. It's better to stop it right there and force it out the register. These procedures will work. I'm cooling an Outback 277RL with a single 15K BTU unit in the Texas heat and am quite comfy with the unit cycling off and on like it is intended to do.
Solar screens are great also if the heat really gets up there. You can make a complete set of snap on solar screens for around $300, not a lot of money considering the overall price of your rig and your comfort factor. I also added an additional 30A service to my trailer so I can run the A/C non-stop if need be without having to shed loads, but that's another story.

[hankpage]

Since it hasn't been mentioned ..... the coil will also ice up if the fan speed is too low. If hot and humid, always run fan on high speed. Also, if you used regular duct tape it will not hold for long. Get the metal foil faced tape with the peel off paper backing and it will seal better and last much longer.

[JRTJH]

On Airxcel air conditioners, there is a low temperature probe that is placed in the evaporator coil. If you open the filter housing and shine a flashlight up into the air conditioner, you'll see a "probe" that is stuck into the evaporator fins. The purpose of that probe is to prevent evaporator freeze-up conditions. If it is not in good contact, is loose or partially pulled out of the evaporator fins it will not function properly and the A/C will freeze up.

There are a number of other issues that could also cause the A/C to freeze, but the first thing I'd check is to make sure that probe hasn't been pulled out, vibrated out or was never installed into the evaporator coil.

You can find some information on it at: http://www.rvcomfort.com/rvp/pdf_documents/1976307.pdf

Here's the "gist" of the explanation:

"All air conditioning equipment is subject to freeze up when
evaporator air flow is sufficiently reduced. Ducting of any
length creates potential for reduced evaporator air flow and
system freeze-up. To protect both the installer and Airxcel,
Inc. from conditions that promote reduced air flow and system
freeze-up, Airxcel, Inc. has equipped the ceiling plenum
compressor control circuit with a low temperature probe. The
low temperature probe monitors the temperature of the air
conditioner evaporator coil. When the temperature of the
evaporator coil drops below 28 degrees F, the switch will
open, stopping compressor operation. Compressor operation
will resume once the evaporator warms to 55 degrees F."

[FLCapt]

Thanks for the replies, I really appreciate it. I'm reasonably sure the A/C is in good working order - just iced up due to air flow problems as stated above. The vents right out of the unit itself were closed as well. ( Wife doesn't like the noise ). I will check to make sure the other vents are all hooked up tight, and go to town with the duct tape if needed. I will patch up the hole - there is a bracket for the A/C mount right behind the hole - I bet the hole was punched to install it! I do have plenty of the good aluminum tape. It is almost 100 degrees here in East Central Florida today, so I should have my answers fairly quick!

[Bob Landry]

The temperature probe is actually called a freeze sensor and is also used in Dometic units. They are only used on units that utilize a wall thermostat and not on the ones with the knob controls on the unit.

[Larry1013]

Freon level low will also cause the evaporator coil to freeze up.

[Bob Landry]

Originally Posted by Larry1013

Freon level low will also cause the evaporator coil to freeze up.

Yes, it will, however, on roof units, it's very unlikely. These units are charged trough a process stub which is cut, crimped, and soldered. Unless the unit was incorrectly charged when manufactured, the only way this would be the case would be if an area of the copper tubing had contacted another surface and worn a hole in it. These units only use around a pound of charge, so any leak would result in total loss of charge and no cooling at all. The most likely cause is going to be blocked air or discharge air leaking back to the evaporator through the return air plenum.