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Old 07-22-2020, 09:57 AM   #1
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A/C issues

Ok all, here I go again;

My ac Dometic 13500 acts weird mostly in the evening, differently weird during sunny days.

To catch you up, I cleaned the heat exchanger, looks brand new up there. I reset the thermostat and checked the on/off differential set to d2. Beautiful installation by South Dakota Discount RV. All taped and smooth air flow/separation for inlet/output. Clean foam filter regularly.

Daytime highs mid 80’s, nighttime lows upper 60’s After sunset the unit completely shuts off then back on it a few minute intervals, until everything seems to cool off around midnight. Then the intervals lengthen to 10 or 15 minutes till morning. During sunny hot days, the fan will stop with the compressor still on for 10 or 15 seconds then resume cooling. Gas furnace seems to run fine when requested.

So what’s left? Replace thermostat or control board on a budget.

Thanks Samtummo
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Old 07-22-2020, 10:16 AM   #2
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These newer units are crazy. On some of them they run the fan every few minutes to move air across the temp sensor (thermostat in most cases) to sample the temp to see if they should turn on the A/C.

If you have the fan in Auto, they also stop the fan and switch speeds whenever the temp either gets closer to setpoint or is drifting away from setpoint. So sounds like what you're experiencing is normal.
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Old 07-22-2020, 11:33 AM   #3
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Originally Posted by Bama Rambler View Post
These newer units are crazy. On some of them they run the fan every few minutes to move air across the temp sensor (thermostat in most cases) to sample the temp to see if they should turn on the A/C.

If you have the fan in Auto, they also stop the fan and switch speeds whenever the temp either gets closer to setpoint or is drifting away from setpoint. So sounds like what you're experiencing is normal.
Thank you for your reply, Bama Rambler.

You are correct about stopping to change speeds, I tried it by switching at the thermostat fan settings. It has a delay start and shutdown between the thermostat and unit when making adjustments. I can hear the compressor continuing to run during the day when the fan shuts down to change speed. That must be the daytime issue.

However, During the evening that doesn’t explain the entire unit shutting down for a minute or two long intervals. The fan speed doesn’t change then. It shuts down both the fan and compressor then restart both motors moments later.

I’ll try setting the fan on low after sundown and see how often the compressor engages. I will report the results.

Thank y’all for helping,

Samtummo
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Old 07-22-2020, 11:49 AM   #4
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the fan shutting off with compressor running make no sense whatsoever, the opposite of the fan running with no compressor is now the new normal on all sticks/bricks systems, it's a energy thing, gets the last of the cooling effect out of the cold coil. As for short cycling at night if you can open up the 2d dead bank that might help, sounds like it's satisfying and cycling. Also try the an on "high" and see if that helps from awakening you, I know I have to do that as I'm a light sleeper and I wake up every time it cycles
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Old 07-22-2020, 12:13 PM   #5
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the fan shutting off with compressor running make no sense whatsoever, the opposite of the fan running with no compressor is now the new normal on all sticks/bricks systems, it's a energy thing, gets the last of the cooling effect out of the cold coil. As for short cycling at night if you can open up the 2d dead bank that might help, sounds like it's satisfying and cycling. Also try the an on "high" and see if that helps from awakening you, I know I have to do that as I'm a light sleeper and I wake up every time it cycles
Thanks nayther,

The fan shuts off momentarily to change speeds. I read in the manual when the set temperature is >5° from actual temperature the fan will run on high. If <4° it swaps to low.

“As for short cycling at night if you can open up the 2d dead bank that might help”

It is set to d2. I only have two choices d1 and d2. According to the manual that is 1° or 2° difference between set and actual temperature for on / off. Should I try d1? Seems like a greater spread would slow restarts.

Samtummo
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Old 07-22-2020, 12:23 PM   #6
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Quote:
Originally Posted by Bama Rambler View Post
These newer units are crazy. On some of them they run the fan every few minutes to move air across the temp sensor (thermostat in most cases) to sample the temp to see if they should turn on the A/C.

If you have the fan in Auto, they also stop the fan and switch speeds whenever the temp either gets closer to setpoint or is drifting away from setpoint. So sounds like what you're experiencing is normal.
Quote:
Originally Posted by samTummo View Post
Thank you for your reply, Bama Rambler.

You are correct about stopping to change speeds, I tried it by switching at the thermostat fan settings. It has a delay start and shutdown between the thermostat and unit when making adjustments. I can hear the compressor continuing to run during the day when the fan shuts down to change speed. That must be the daytime issue.

However, During the evening that doesn’t explain the entire unit shutting down for a minute or two long intervals. The fan speed doesn’t change then. It shuts down both the fan and compressor then restart both motors moments later.

I’ll try setting the fan on low after sundown and see how often the compressor engages. I will report the results.

Thank y’all for helping,

Samtummo
I seriously doubt it RESTARTS BOTH the fan AND the compressor moments later. The locked rotor amps of the compressor wouldn't allow that to happen. I suspect just as Bama Rambler mentioned, the unit is shutting off and then restarting the fan ONLY to run air past the sensor for sampling.

I've witnessed R/Vs with this set-up doing this every few minutes for 5-15 minutes before it then actually kicks in the compressor to begin the cooling cycle again. All perfectly normal operation.

I'm glad I don't have one of these types systems. It would drive me nuts.
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Old 07-22-2020, 12:43 PM   #7
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Yep...unfortunately(?) I'd say it's "normal". My system with the 13,500 operates similarly. Attempting to understand the thermostat "program" with my pea-sized brain is a challenge...but, best I can (and, now, I've actually read the instructions...wife is amazed!), the "on-off'edness" described by the OP is...normal. Not "good" and certainly not user-friendly, but "normal"(?).
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Old 07-22-2020, 01:01 PM   #8
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I don't think it is running air across the sensor for sampling. I think it is circulating the air in the RV. The sensor is in the thermostat. Do you have sunlight shining directly on the thermostat? Some RVs have separate sensors from the thermostat especially if they have multiple zones.
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Old 07-22-2020, 01:08 PM   #9
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I don't think it is running air across the sensor for sampling. I think it is circulating the air in the RV. The

sensor is in the thermostat. Do you have sunlight shining directly on the thermostat? Some RVs have separate sensors from the thermostat especially if they have multiple zones.
Thanks everyone

No sunshine after dark when it seems to be recycling on off every few minutes. Only one zone.

So I’m going to try turning the fan low on and listen for the compressor after sunset.

💤samtummo
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Old 07-22-2020, 01:10 PM   #10
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mine used to cycle like that in auto. Now I just leave in on low. Atleast then the noise change doesn't wake me up.
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Old 07-22-2020, 01:28 PM   #11
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Quote:
Originally Posted by mwdilday View Post
I don't think it is running air across the sensor for sampling. I think it is circulating the air in the RV. The sensor is in the thermostat. Do you have sunlight shining directly on the thermostat? Some RVs have separate sensors from the thermostat especially if they have multiple zones.
Not on all models. (some are, some aren't)
They have a separate remote sensor on another wall.





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Old 07-22-2020, 02:05 PM   #12
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I don't understand why the fan has to stop when changing speeds, it' not a high amp draw motor and should work fine when changing speeds on the fly. In fact, the fan will use less running amps when it changes speed on the fly instead of stopping then having to start again. Fans don't see high amp draw when starting. Fans only see full load amps (FLA) when starting from a dead stop or on high speed. The only time a fan will see locked rotor amps (LRA) is if something physically stops the fan blades from turning.
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Old 07-23-2020, 12:31 AM   #13
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Y’all were right

After dark, about every 15 minutes the fan comes on for about a minute, then off for two. After about 3 or four cycles the compressor comes on and cools the room. Ten or 15 minutes later it does it again. Fan on low now it cools once in a while.

Strangest thing I’ve ever seen. Thanks for telling me that is “NORMAL”! I would have never guessed!

Samtummo
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Old 07-23-2020, 07:30 AM   #14
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Yeah, it's crazy, but some designer somewhere thought that's how they should work.
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