Condensation from our roof AC

[AJMike]

The roof AC on our RV works great out here in Arizona and keeps us reasonably cool even when the outside temperature is well above 100 F, but when we take our yearly trip to South Padre Island and get near the coast we start getting water dripping from the ceiling cowling of the AC. This has happened every summer trip we have made to south Texas, but never anywhere else.

I had the tech guy from the local Dometic repair shop here in Arizona take a look at it thinking that perhaps the drip pan was leaking or perhaps the drain lines were clogged, but he said everything was in good shape and that the problem was that the AC was oversized for such a small RV (the AC is, I think, 15,000 BTU and the RV is a 25' B+/C with no slides) and thus got so cold that condensation formed and ended up dripping out of the cowling. He said the only suggestion he could make is that we might consider replacing the unit with a smaller one and that if we had something smaller it would not generate such cool temperatures and thus we would reduce or eliminate the condensation.

This sounded strange to me as I thought that all AC units produced the same output temperatures and that the only difference in the BTU ratings concerned how much air they could move at that temperature, but then I don't know much about these units. The repair shop has been dependable and has not shown any tendency to oversell or overcharge, and when I spoke with the mobile tech guy before he came out he tried to help me with the problem without coming out so they would not have to charge me, so it does not seem like they just want to sell me stuff. But is this right? Does the size of the AC determine things like whether or not they might cool enough to cause condensation?

One other thing that might have some bearing. The AC unit works off of a Dometic thermostat and we can set the temperature. The compressor turns on and off so as to keep the set temperature but when we set the fan speed to low (it is not ducted so when the fan speed is high it is pretty loud) it runs for a while and then shuts down. I can cycle through the thermostat settings and restart it, but it shuts down again in another 15 or 30 minutes. This does not happen when the fan speed is set to high (and loud). The tech suggested that when the fan speed is low there is not enough air moving to prevent the coils from becoming iced up, but if I can restart the AC and get cold air just by cycling through the thermostat setting and restarting the AC that does not sound right.

Any helpful comments? Or suggestions?

[Old-Biscuit]

Fan continues to run in Low speed because Ice Probe is still COLD

YES you can have TOO MUCH A/C for size of space being cooled
Just like you can have TOO LITTLE A/C for size of space being cooled

Increasing the drip pan drain holes might help by increasing the flow of condensation.....whether that is possible would depend on size of drain hose

[AJMike]

Originally Posted by Old-Biscuit

Fan continues to run in Low speed because Ice Probe is still COLD

What is an ice probe?

When I have the thermostat set to low fan speed the whole thing shuts down, not just the compressor. In fact that is one of the ways that I know it has shut down. If the fan continued to run I would not know until the air output got warm, but I get woken by the sudden silence as the whole unit has stopped.

Originally Posted by Old-Biscuit

YES you can have TOO MUCH A/C for size of space being cooled
Just like you can have TOO LITTLE A/C for size of space being cooled

OK. Good to know.

Originally Posted by Old-Biscuit

Increasing the drip pan drain holes might help by increasing the flow of condensation.....whether that is possible would depend on size of drain hose

I assume what you are saying is that by increasing the number/size of the drain holes it would allow more water to drain out, thus preventing the drip pan from overflowing. Is that what you are saying? Otherwise I don't understand as I want to prevent the condensation from dripping out of the cowling.

The question is whether or not installing a smaller AC unit would prevent the leak.

[dvspl]

Sounds like the evaporator coil is icing up. This could be due to low airflow through the evaporator during high humidity/temperature cooling. Here's a good article that explains the issue and some potential causes: https://www.thervgeeks.com/rv-ac-freezing-up/

[bentjm]

Ice probe - aka thermistor.

You need to see where or if it is connected to the coil. Moving it to a different place may help. The situation.

Also, make sure the coil is clean.

[NXR]

Dometic makes like a bazillion air conditioners. Which exact one do you have?

Ours is an Atwood, also made by Dometic. There are four bolts on the inside that secure the roof unit to the roof. If the mounting bolts on ours work loose the foam gasket between the roof unit and the roof itself loosens up (it needs to be partially compressed) and the roof unit will start leaking water into the RV. A few people have posted videos of it literally raining inside.

Our bolts use 4.4 ft. lbs torque (four point four, not forty four). That is literally just snugged up. I check them each year and usually find one that has come a bit loose.

If you have the same foam gasket type of seal you might want to check those bolts.

Ray

[AJMike]

Originally Posted by NXR

Dometic makes like a bazillion air conditioners. Which exact one do you have?

This one.

Originally Posted by NXR

Ours is an Atwood, also made by Dometic. There are four bolts on the inside that secure the roof unit to the roof. If the mounting bolts on ours work loose the foam gasket between the roof unit and the roof itself loosens up (it needs to be partially compressed) and the roof unit will start leaking water into the RV. A few people have posted videos of it literally raining inside.

Our bolts use 4.4 ft. lbs torque (four point four, not forty four). That is literally just snugged up. I check them each year and usually find one that has come a bit loose.

If you have the same foam gasket type of seal you might want to check those bolts.

Ray

Thanks for the information. The repair guy said that the bolts were not tight, and he tightened them up, but I will get a torque wrench and take a look. We have another trip coming up and it would be good to find an easy and inexpensive fix to this problem.

I guess I need to get hold of a manual with the technical information so I can try to avoid ruining the entire unit ...

[DancinCampers]

My thoughts:
A/C's are designed to condense water out of the air, which is why they are called air conditioners instead of air coolers. I would check the drain pan under the indoor coil to make sure it is clean, not filled with slime. Also, at the end of the pan where the water is supposed to drain, there is usually a piece of porous filter media designed to keep out bugs. I have to wash mine every couple of years in the humid south where we spend a lot of time in.
If your A/C was way oversized, it would quick cycle, not properly removing water from the air, becoming clammy inside.
I would also look at the area on the cold side of the indoor coil, an area of low air pressure. It could be this area is leaking outside air in, condensing on the interior walls when it meets the cold air, dripping down the inside walls of the cavity. You should be able to observe water marks if this is occurring.

[NXR]

Originally Posted by AJMike

Thanks for the information. The repair guy said that the bolts were not tight, and he tightened them up, but I will get a torque wrench and take a look. We have another trip coming up and it would be good to find an easy and inexpensive fix to this problem.

I guess I need to get hold of a manual with the technical information so I can try to avoid ruining the entire unit ...

The install manual from that link says 40 to 50 inch pounds so about 3.5 to 4 ft lbs, a touch less than ours.

If he cranked them down tight don't use him again. Too tight can leak as much as too loose because it smashes the foam gasket flat.

Snugged up should be good.

Ray

[Doc007]

Is this true: No matter where water condenses or freezes and thaws it should drip into the pan... all else being equal...like clean and unobstructed.

[Toyman01]

Too large of an AC unit will cause short cycling and suboptimal cooling. It will not cause leaking of condensate water into the RV. That is an installation problem or a stopped-up drain issue. All ACs condensate, doubly true in the humid South. Mine will literally have water running off the roof. The only time I would have an issue with water dripping from the cowl is if the drains are stopped up. I would pull the cowl and make sure the drains at the bottom of the evaporator coil aren't stopped up and causing the water to flow in instead of out.

[shepardmike]

In my Dometic Penguin II, if the evaporator coil freezes up, usually because of a failed freeze sensor (thermistor). When I shut it off and it's really hot out, due to the amount of frozen water on the coils, it can overwhelm the drain and overflow into the ducts.

[DancinCampers]

Yes, under normal conditions, it will run down the indoor coil fins into the drain pan, like the condensation on the outside of a glass of ice water. Ice on the coil would also drain into the pan when thawing.
If for some reason you are getting condensation on the walls of the cavity surrounding the indoor fan blower wheel it would not drain into the pan.

I guess if the coil was really dirty some water might be pulled off by the indoor fan suction, but that would be in the air stream coming into the room.

[NXR]

Originally Posted by DancinCampers

Yes, under normal conditions, it will run down the indoor coil fins into the drain pan, like the condensation on the outside of a glass of ice water. Ice on the coil would also drain into the pan when thawing.

Our Atwood (now Dometic) roof units do not have a drain pan under the inside evap coil so once again, it depends.

If the evap coil ices up, which should never happen, it's melting to the floor inside.

Ray