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Old 02-04-2019, 09:45 AM   #1
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Dometic roof A/C icing up - HELP!

I have a Dometic roof mounted A/C unit on my Forester. It doesn't have controls on the ceiling, rather if has a thermostat mounted on an interior wall.

We didn't use the RV for the last year as we were building a new house and didn't have time to get away.

We finally packed up and headed south to get out of the snow. I had my first opportunity to turn on the roof A/C yesterday and after running a couple hours I had an unpleasant surprise. It sounded like a stick got jammed in the blades of the fan and water started dripping from the inside panel under the A/C unit. I immediately shut it down and noticed there were small pieces of ice in the interior bypass shutter of the A/C unit. We cleaned up the dripping water and I went on to the roof and took the cover off the A/C. Everything looked fine. No leaks or broken wires that I could see. I pulled the entire interior cover off, not just the return air filter and everything looked fine there too. I noticed that the filter was slightly dirty so I washed it out and reinstalled everything. I checked the internal "radiator" and it didn't appear dirty or blocked in any way.

What would cause the unit to ice up like that? What do I need to do to avoid it again? I'm in Florida right now headed west to Texas, Arizona, Utah, Nevada and California so I don't want the A/C giving up the ghost on me when I hit the hot weather areas. As I said, the filter was only slightly dirty so I cleaned it.

Thanks
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Old 02-04-2019, 10:00 AM   #2
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Quote:
Originally Posted by Forester Fred View Post
I have a Dometic roof mounted A/C unit on my Forester. It doesn't have controls on the ceiling, rather if has a thermostat mounted on an interior wall.

We didn't use the RV for the last year as we were building a new house and didn't have time to get away.

We finally packed up and headed south to get out of the snow. I had my first opportunity to turn on the roof A/C yesterday and after running a couple hours I had an unpleasant surprise. It sounded like a stick got jammed in the blades of the fan and water started dripping from the inside panel under the A/C unit. I immediately shut it down and noticed there were small pieces of ice in the interior bypass shutter of the A/C unit. We cleaned up the dripping water and I went on to the roof and took the cover off the A/C. Everything looked fine. No leaks or broken wires that I could see. I pulled the entire interior cover off, not just the return air filter and everything looked fine there too. I noticed that the filter was slightly dirty so I washed it out and reinstalled everything. I checked the internal "radiator" and it didn't appear dirty or blocked in any way.

What would cause the unit to ice up like that? What do I need to do to avoid it again? I'm in Florida right now headed west to Texas, Arizona, Utah, Nevada and California so I don't want the A/C giving up the ghost on me when I hit the hot weather areas. As I said, the filter was only slightly dirty so I cleaned it.

Thanks

Three things will cause an AC to ice up, low/lack of air flow, low refrigerant charge, turning thermostat too low. The first and last item are user issues, check air flow and that something has not fallen down into the ducts, did you turn off any supply diffusers? You said filter was clean and coil not plugged so that's not it. Low charge is unlikely as it is a sealed system and if it's "low" it's going to be "empty" real fast. If there is a leak you should see oil somewhere around the compressor or piping when you have the cover off. You can try to run it with the cover off too and see if ice forms or not.



And if you're running it when it's not very warm it's possible there's a freeze thermostat that is not cycling the compressor, look for a round silver diode looking thing with two wires attached, it should be attached to the coil.
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Old 02-04-2019, 10:05 AM   #3
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When you start the AC for the first time in a very humid environment, it is always a good idea to open the dump vent to allow for maximum air flow over the coils to prevent freeze up. Once the temp and humidity start to decrease, you can slowly close the dump until it is completely closed.
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Old 02-05-2019, 09:46 PM   #4
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X2 on nayther's post. But also have to ask, did the unit seem to be putting out cold air? What fan speed were you running the unit on low or high?
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Old 02-07-2019, 01:32 PM   #5
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Thanks for the responses. The plot thickens a bit though.

Since I've owned the rig our "hot weather" camping has been in Las Vegas, Death Valley, Arizona and Maine so they really weren't high humidity areas. As mentioned I cleaned the filter and put everything back together but the weather was actually a bit cool for the past several days so I didn't use the A/C again.

Yesterday morning we were camping in Ocala NF in Florida and it was a chilly morning. To make my wife happy I turned on the furnace to warm the rig up a bit before she got into the shower. Except...the A/C turned on about 2 seconds after the furnace. WTH????
I tried it a few more times and every time I turned the furnace on (yes, I am sure it went to the furnace setting on the thermostat) the A/C would kick on.

As we're headed out west to AZ, NV, UT and southern CA I wanted to make sure the A/C was functioning properly...I tend to get cranky when I get hot. We were only about 20 minutes from the Camping World in Ocala so I called them - figuring that they'd say they'd see me next week - and the service rep told me to come right in. They got right on it too as soon as I pulled in, I was impressed. If you haven't been to the CW in Ocala it's huge. It's a combination CW, Overton's, Gander. My wife had to keep pulling me away from the gun department....

Anyway after a bit the service rep comes over and asked if anyone had done any service on the A/C unit. Nope, just me washing the filter. Apparently when FR built the rig and mounted the A/C on the roof nobody bothered to mount the freeze sensor on the coil. The service tech found it coiled up and tucked away behind some wires. That's why it was turning into an ice maker. The other day was first time I've used it in a really high humidity environment. They let the A/C run for about 3 hours (while my wife continued to shop and spend money in the store!) and it worked perfectly. They also added some metal duct tape to the interior of the unit to make sure the intake air wasn't bypassing the coils and going directly to the output.

They also thought that the wall thermostat was bad and was causing both the A/C and furnace to come on at the same time. I told them to go ahead and put a new thermostat in as I didn't want to have issues down the road. The service rep came back out a few minutes later and asked me to go out to the rig. The service tech told me he didn'st want to put on a new thermostat as the new Dometics are pretty much garbage and will fail. He prefers the type I had, the square one with the buttons on the side. Because he didn't like the new ones he decided to troubleshoot mine. Apparently the Dometic wall thermostat isn't just Fan/Cool/Heat. When you're on Fan mode you can also switch it from Auto to Hi to Lo. Nobody told me about that when they did the PDI at the dealership. Turns out that if the thermostat is set to Hi or Lo the fan - not the A/C compressor, just the fan in the A/C roof unit will also come on. They switched it back to Auto and it works perfectly.

So, bottom line is that FR cost me $170 to have the freeze sensor properly installed in the A/C coil because someone at the factory didn't bother. Unfortunately I'm off warranty so I know all I'll get out of FR is crickets if I asked for reimbursement. The A/C - heat at the same time issue was operator error and fortunately it didn't cost me anything other than embarrassment.

Everything is running properly now but for those of you who are having a problem with icing, make sure the factory installed the freeze sensor. And, if you've got one of the older (2015) model Dometic wall thermostats and your A/C fan and heat come on at the same time make sure that it's not set to Hi or Lo for the fan setting.

And to Camping World in Ocala, particularly the service rep Kevin Murray and the service tech Chris, thank you for the quick service and your honesty. They could have installed a new thermostat and charged me when I really didn't need it. It's nice to know that there are still honest people in this world.
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Old 02-08-2019, 11:50 AM   #6
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Quote:
Originally Posted by Forester Fred View Post
Thanks for the responses. The plot thickens a bit though.

Since I've owned the rig our "hot weather" camping has been in Las Vegas, Death Valley, Arizona and Maine so they really weren't high humidity areas. As mentioned I cleaned the filter and put everything back together but the weather was actually a bit cool for the past several days so I didn't use the A/C again.

Yesterday morning we were camping in Ocala NF in Florida and it was a chilly morning. To make my wife happy I turned on the furnace to warm the rig up a bit before she got into the shower. Except...the A/C turned on about 2 seconds after the furnace. WTH????
I tried it a few more times and every time I turned the furnace on (yes, I am sure it went to the furnace setting on the thermostat) the A/C would kick on.

As we're headed out west to AZ, NV, UT and southern CA I wanted to make sure the A/C was functioning properly...I tend to get cranky when I get hot. We were only about 20 minutes from the Camping World in Ocala so I called them - figuring that they'd say they'd see me next week - and the service rep told me to come right in. They got right on it too as soon as I pulled in, I was impressed. If you haven't been to the CW in Ocala it's huge. It's a combination CW, Overton's, Gander. My wife had to keep pulling me away from the gun department....

Anyway after a bit the service rep comes over and asked if anyone had done any service on the A/C unit. Nope, just me washing the filter. Apparently when FR built the rig and mounted the A/C on the roof nobody bothered to mount the freeze sensor on the coil. The service tech found it coiled up and tucked away behind some wires. That's why it was turning into an ice maker. The other day was first time I've used it in a really high humidity environment. They let the A/C run for about 3 hours (while my wife continued to shop and spend money in the store!) and it worked perfectly. They also added some metal duct tape to the interior of the unit to make sure the intake air wasn't bypassing the coils and going directly to the output.

They also thought that the wall thermostat was bad and was causing both the A/C and furnace to come on at the same time. I told them to go ahead and put a new thermostat in as I didn't want to have issues down the road. The service rep came back out a few minutes later and asked me to go out to the rig. The service tech told me he didn'st want to put on a new thermostat as the new Dometics are pretty much garbage and will fail. He prefers the type I had, the square one with the buttons on the side. Because he didn't like the new ones he decided to troubleshoot mine. Apparently the Dometic wall thermostat isn't just Fan/Cool/Heat. When you're on Fan mode you can also switch it from Auto to Hi to Lo. Nobody told me about that when they did the PDI at the dealership. Turns out that if the thermostat is set to Hi or Lo the fan - not the A/C compressor, just the fan in the A/C roof unit will also come on. They switched it back to Auto and it works perfectly.

So, bottom line is that FR cost me $170 to have the freeze sensor properly installed in the A/C coil because someone at the factory didn't bother. Unfortunately I'm off warranty so I know all I'll get out of FR is crickets if I asked for reimbursement. The A/C - heat at the same time issue was operator error and fortunately it didn't cost me anything other than embarrassment.

Everything is running properly now but for those of you who are having a problem with icing, make sure the factory installed the freeze sensor. And, if you've got one of the older (2015) model Dometic wall thermostats and your A/C fan and heat come on at the same time make sure that it's not set to Hi or Lo for the fan setting.

And to Camping World in Ocala, particularly the service rep Kevin Murray and the service tech Chris, thank you for the quick service and your honesty. They could have installed a new thermostat and charged me when I really didn't need it. It's nice to know that there are still honest people in this world.
It's your choice but I would contact Forest River for reimbursement if it was never right from the beginning. Hopefully your invoice noted the reason for the "repair". There is a 50/50 chance they will cover it for the price of a phone call or email.
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Old 02-08-2019, 12:13 PM   #7
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The freeze sensor falling out/not being installed correctly is a somewhat common report here. While that really isn't FR's fault (they don't build the A/C units, they just install them) they might help. Have you spoken with your selling dealer?

I would contact FR or your dealer and ask for reimbursement assistance.
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Old 02-08-2019, 06:05 PM   #8
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Not sure how FR does their AC installs but when I was a service tech the Coleman units we installed were delivered complete with the exception of the inside panel which we had to wire. Just a guess but the issue probably would go back to Dometic instead of FR.
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Old 02-08-2019, 06:30 PM   #9
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First i would check to see if the freeze sensor fell out from the coils . while in there you can see the divider between intake and output air flows makes sure that sealed off . but my guess is your freeze sensor fell out
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Old 02-08-2019, 07:07 PM   #10
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First i would check to see if the freeze sensor fell out from the coils . while in there you can see the divider between intake and output air flows makes sure that sealed off . but my guess is your freeze sensor fell out

And coiled itself up and crawled behind wires because it was scared?

OP's quote, "The service tech found it coiled up and tucked away behind some wires."
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