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Old 08-03-2020, 07:30 PM   #61
Fireduck
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Originally Posted by Steveo57 View Post
OP here. Still have not resolved the issue. I gave up on fixing it for a while but have dug back in to it recently. I've been going back and forth with Dometic for weeks now with nothing new to report.
@Steveo57 please keep me posted, if you find out anything.
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Old 08-03-2020, 07:40 PM   #62
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Originally Posted by Steveo57 View Post
OP here. Still have not resolved the issue. I gave up on fixing it for a while but have dug back in to it recently. I've been going back and forth with Dometic for weeks now with nothing new to report.
@Steveo57 please keep me posted, if you find out anything.
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Old 09-30-2021, 06:45 PM   #63
Bagger56
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Dometic fan cycles with no compressor

I have had the same problem since we bought our Freedom Express in 2018. I have 8 years of residential ac experience, I had two different mobile tech's and of course the dealer. Just like everyone else, the t-stat, control board, the whole upper unit has been changed. I even slaved in new wiring with no luck. We even used a stand alone battery because Dometic said I had dirty 12v power coming to the unit. Guess what, I still have the problem. It runs fine during the day but in the evening till in the morning the fan cycles. It's not a true short cycle in that the compressor isn't cycling, just the fan.
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Old 09-30-2021, 08:56 PM   #64
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Originally Posted by Bagger56 View Post
I have had the same problem since we bought our Freedom Express in 2018. I have 8 years of residential ac experience, I had two different mobile tech's and of course the dealer. Just like everyone else, the t-stat, control board, the whole upper unit has been changed. I even slaved in new wiring with no luck. We even used a stand alone battery because Dometic said I had dirty 12v power coming to the unit. Guess what, I still have the problem. It runs fine during the day but in the evening till in the morning the fan cycles. It's not a true short cycle in that the compressor isn't cycling, just the fan.
Not to make light of your residential AC experience, but the Dometic system is designed to "adjust fan speed from "low to high and back" based on the temperature differential between thermostat setting and thermostat sensed temperature.

Typically, during the day, when the temperature remains above the thermostat setting, the compressor will run with the fan on high speed. It seldom gets close enough to the thermostat setting for the "speed cycle" system to function.

What is "supposed to happen" is this:

With the fan set to HI or LO, the fan runs continuously in that speed with no regard to compressor operation. With the fan set to AUTO, the following will occur:

Thermostat set to desired temperature (lets use 70F)
If the room temperature is above 70F, the fan will start to spin up and after a short delay, the compressor will start.

If the temperature is greater than 4 degrees higher than the thermostat setting, the fan will switch to HI. If the temp is less than 4 degrees higher, the fan will run on LO.

Once the room temperature reaches 4F of the thermostat setting, the compressor will remain running, the fan will stop, switch from HI to LO and resume running. Once the temperature reaches thermostat setting, the compressor will stop, fan will continue to run on LO for about 30 seconds (to push all the cold air out of the ductwork so it won't cause condensation) and then the fan will also stop.

Here's the part that I think you're experiencing:

If, when the fan switches to LO and resumes running, if the temperature rises above the 4F mark, the fan will stop, switch to HI and resume running. If, the temperature falls below the 4F mark, it will again stop, switch to LO and resume. It will "cycle" between HI and LO indefinitely unless the temperature reaches the thermostat setting.

If the heat gain in the trailer is such that HI speed allows the trailer to cool down but LO speed does not move enough air to keep the temperature near the thermostat "below that magic 4F differential" then the cycling will continue.... Usually, at night, the LO speed can continue to keep the temperature decreasing, but during the day, the heat gain in the "uninsulated space between the roof and the top of the ductwork in the ceiling" is not capable of keeping the air cold enough on LO speed to keep the interior air temperature on the "downward trend" and as soon as it starts rising, the the fan speed goes back to HI....

I think, if what I read from your "failed repair attempts" that if you sit down when the trailer AC is "short cycling" and listen carefully, you'll hear the compressor continue to run and only the fan change from HI to OFF to LO to OFF to HI to OFF to LO to OFF to <and the cycles continue> My Cougar has been doing this since it was new. The dealership changed out everything in the trailer and NOTHING helped.... Then, I sat down with the thermostat owner's manual and "actually read the entire manual and theory of operation" and then paid attention to what my system was really doing and, well, it's working exactly like it's supposed to work.....

Maybe yours is too ????
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Old 10-02-2021, 09:10 AM   #65
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I agree with your understanding of how the ac is suppose to work but my sys and the gentleman that started this thread is not working the correct way. As stated before, the sys works correctly during the day, cycling as you stated it should but at night it's does not. As the temp reaches the set point, the fan comes on but the compressor may or may not start. It acts as if the compressor is not getting a signal to turn on. I have verified that it is fan cycle by checking the voltage and amp readings. The factory has been involved with the troubleshooting and have no answer to the problem.
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Old 11-23-2021, 05:27 AM   #66
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@Steveo57 please keep me posted, if you find out anything.
I gave up trying to get anything out of Dometic. They stopped responding to my emails.

I redid all of the wiring connections and ended up switching to the MicroAir EasyTouch thermostat. So far it's working perfectly.
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Old 05-10-2022, 11:33 AM   #67
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Originally Posted by Steveo57 View Post
I gave up trying to get anything out of Dometic. They stopped responding to my emails.

I redid all of the wiring connections and ended up switching to the MicroAir EasyTouch thermostat. So far it's working perfectly.
Did you just replace the dometic thermostat with the MicroAir EasyTouch? Is it still working and not short cycling the fan? What model did you purchase and what modifications if any did you have to do?

Sorry for all the questions but I'm fed up with Dometic too. They just keep telling me it's the normal operation and don't listen to the fact that the compressor never comes on when the fan is short cycling.
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Old 05-10-2022, 12:57 PM   #68
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Originally Posted by JRTJH View Post
Not to make light of your residential AC experience, but the Dometic system is designed to "adjust fan speed from "low to high and back" based on the temperature differential between thermostat setting and thermostat sensed temperature.

Typically, during the day, when the temperature remains above the thermostat setting, the compressor will run with the fan on high speed. It seldom gets close enough to the thermostat setting for the "speed cycle" system to function.

What is "supposed to happen" is this:

With the fan set to HI or LO, the fan runs continuously in that speed with no regard to compressor operation. With the fan set to AUTO, the following will occur:

Thermostat set to desired temperature (lets use 70F)
If the room temperature is above 70F, the fan will start to spin up and after a short delay, the compressor will start.

If the temperature is greater than 4 degrees higher than the thermostat setting, the fan will switch to HI. If the temp is less than 4 degrees higher, the fan will run on LO.

Once the room temperature reaches 4F of the thermostat setting, the compressor will remain running, the fan will stop, switch from HI to LO and resume running. Once the temperature reaches thermostat setting, the compressor will stop, fan will continue to run on LO for about 30 seconds (to push all the cold air out of the ductwork so it won't cause condensation) and then the fan will also stop.

Here's the part that I think you're experiencing:

If, when the fan switches to LO and resumes running, if the temperature rises above the 4F mark, the fan will stop, switch to HI and resume running. If, the temperature falls below the 4F mark, it will again stop, switch to LO and resume. It will "cycle" between HI and LO indefinitely unless the temperature reaches the thermostat setting.

If the heat gain in the trailer is such that HI speed allows the trailer to cool down but LO speed does not move enough air to keep the temperature near the thermostat "below that magic 4F differential" then the cycling will continue.... Usually, at night, the LO speed can continue to keep the temperature decreasing, but during the day, the heat gain in the "uninsulated space between the roof and the top of the ductwork in the ceiling" is not capable of keeping the air cold enough on LO speed to keep the interior air temperature on the "downward trend" and as soon as it starts rising, the the fan speed goes back to HI....

I think, if what I read from your "failed repair attempts" that if you sit down when the trailer AC is "short cycling" and listen carefully, you'll hear the compressor continue to run and only the fan change from HI to OFF to LO to OFF to HI to OFF to LO to OFF to <and the cycles continue> My Cougar has been doing this since it was new. The dealership changed out everything in the trailer and NOTHING helped.... Then, I sat down with the thermostat owner's manual and "actually read the entire manual and theory of operation" and then paid attention to what my system was really doing and, well, it's working exactly like it's supposed to work.....

Maybe yours is too ????
I appreciate your knowledge but I think you're missing the point of this thread. At no point in the fan cycling people are discussing is the compressor running. The fan is not cycling between HI and LO. The system is off after successfully cooling the RV and has shut down as you described. Once this has happened and the RV temp creeps up slightly past the set point, the following happens....


The fan comes on on HI and runs for 30 seconds, the compressor does not start. After 30 seconds, the fan shuts off for 60 seconds. Then the cycle repeats (fan on 30 sec, no compressor, fan off 60 sec). This repeats until you get fed up and turn the unit off, change the set temp several degrees colder or the RV temp climbs several degrees above the set temp
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Old 05-10-2022, 01:46 PM   #69
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The MicroAir is head and shoulders better than the Dometic CT thermostat....head and shoulders. A bit pricey but far and away better. Yes, it stopped the short cycling on mine (Dometic 15k Whisper Quiet?) along with giving you remote access and lots of other features. Simply remove the 3 wires from the Dometic and push them in the proper slots on the MA, hang it on the wall, set it and forget it.
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Old 05-10-2022, 02:17 PM   #70
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Thank you!
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Old 05-10-2022, 07:29 PM   #71
Steveo57
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Originally Posted by ScottB58 View Post
Did you just replace the dometic thermostat with the MicroAir EasyTouch? Is it still working and not short cycling the fan? What model did you purchase and what modifications if any did you have to do?

Sorry for all the questions but I'm fed up with Dometic too. They just keep telling me it's the normal operation and don't listen to the fact that the compressor never comes on when the fan is short cycling.
I have the RV351 with my Dometic 15k BTU AC. It's working flawlessly, no issues since I replaced it. And it's a simple installation, no mods to do.
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Old 05-11-2022, 05:16 AM   #72
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I have the RV351 with my Dometic 15k BTU AC. It's working flawlessly, no issues since I replaced it. And it's a simple installation, no mods to do.
Awesome, 351 has been ordered. Thanks a ton!!
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Old 05-24-2022, 01:56 PM   #73
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After reading this thread my AC fan started short cycling in the evening. I then adjusted the ceiling AC vents so that they would not blow toward the 3 button thermostat and the problem ceased.
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Old 07-26-2022, 07:55 PM   #74
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Awesome, 351 has been ordered. Thanks a ton!!
I installed the MicroAir 351 and it has worked flawlessly. It was a VERY simple installation by the way.

We have had some fairly warm weather and I've been residing in the RV for over a week. I have not had a single episode of 'short cycling'. The AC unit works like its supposed to work. I can sleep without being woken up to the fan running for 30 seconds every minute.

Dometic needs to read this forum and realize their 3 button digital T-stat is the culprit.
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Old 07-26-2022, 08:02 PM   #75
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Thank you!

Quote:
Originally Posted by Steveo57 View Post
I have the RV351 with my Dometic 15k BTU AC. It's working flawlessly, no issues since I replaced it. And it's a simple installation, no mods to do.
Thank you for your suggestion of the MicroAir T-stat. We can actually enjoy our RV again.
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Old 03-24-2024, 10:38 PM   #76
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Not sure if my ac unit is short cycling or what its doing. It will run for a while then half way turn off then turn right back on and run for a while, once it finally shuts off, it won't come back on unless you turn it off at the control, let it set for a minute or so then turn it back on. Has anyone had an issue like that?
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Old 03-25-2024, 05:44 AM   #77
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It will run for a while then half way turn off then turn right back on and run for a while
This part of your description sounds like the unit switching "auto" fan speeds from high to low as it approaches target temperature, which is entirely normal. The rest isn't.
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Old 03-25-2024, 08:49 AM   #78
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Not sure if my ac unit is short cycling or what its doing. It will run for a while then half way turn off then turn right back on and run for a while, once it finally shuts off, it won't come back on unless you turn it off at the control, let it set for a minute or so then turn it back on. Has anyone had an issue like that?
The first statement you make is "normal operation of the air conditioner when operating in the AUTO mode. There is a "temperature differential" of 5 degrees built into the system. The A/C operates on HI when the cabin air temperature is greater than 5F from the thermostat setting. When the cabin air temperature cools to less than that 5F difference, the compressor continues to run and the fan stops, switches from HI to LO and then resumes operation until the set temperature is reached or until the cabin temperature rises above 5F again. If the temp rises, the compressor continues to run and the fan shuts down, switches to HI and cooling resumes. If the cabin temperature reaches the thermostat set point, the compressor shuts down, fan continues for 30 seconds on LO, in order to clear the chilled air in the ducting to prevent condensation and potential mold growth.

So, that part is "normal for the A/C operation and isn't a problem"

I have a question about the "bolded part of your post" . You say that once it shuts down, it won't restart until you shut down the thermostat, wait a minute and restart. There is a "temperature differential setting" in the thermostat. If it is set on 2 or 3 degrees differential, depending on the thermostat mount location in the trailer, you may notice a significant ambient air increase before the thermostat reaches that 2 or 3 degree rise and call for more cooling. That may be what you're experiencing. What you need to do is change the "temperature differential setting" to 1 degree, to make the thermostat more sensitive to the rising cabin temperature. Also take a look at the thermostat location to see if there is a window allowing sunlight to hit the location, maybe it's next to a television producing heat or some other "outside source of hot or cold air" that affects the rooftop air conditioner operation.
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