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Old 12-03-2012, 11:22 PM   #1
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Can an old Univolt be the source of my AC problems

We have a 94 Classic Excella. We replaced the original AC with a 15,000 BTU Dometic Penguin with heat pump less than a year ago. At the same time we added a second 13,500 unit to the bedroom. The 15,000 has never worked properly. It kicks on sometimes, and lately hardly ever. We can't have been troubleshooting like crazy, and have replaced every component on the AC possible, but still no success. The bedroom AC has saved us but if we can't figure this out by the summer it will be miserable.

Recently someone suggested that the old univolt may have some DC bleed? Or something like that. And that some new electronics are ultra sensitive, like the AC, and that could be interfering with the way it operates. It was suggested that we by a DC/AC filter. We have also had trouble with our electric ignited water heater. Could this be the same thing? The Univolt always has a bery pronounced hum.
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Old 12-04-2012, 03:52 AM   #2
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Originally Posted by yve79 View Post
We have a 94 Classic Excella. We replaced the original AC with a 15,000 BTU Dometic Penguin with heat pump less than a year ago. At the same time we added a second 13,500 unit to the bedroom. The 15,000 has never worked properly. It kicks on sometimes, and lately hardly ever. We can't have been troubleshooting like crazy, and have replaced every component on the AC possible, but still no success. The bedroom AC has saved us but if we can't figure this out by the summer it will be miserable.

Recently someone suggested that the old univolt may have some DC bleed? Or something like that. And that some new electronics are ultra sensitive, like the AC, and that could be interfering with the way it operates. It was suggested that we by a DC/AC filter. We have also had trouble with our electric ignited water heater. Could this be the same thing? The Univolt always has a bery pronounced hum.
The Univolt has nothing to do with the AC.

But, so that you can do the ultimate test, disconnect the univolt, and then try the AC.

If the AC still has an issue, then I don't think it was properly installed.

Andy
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Old 12-04-2012, 04:09 AM   #3
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I don't know what kind of controls operate your air conditioner and water heater, but usually appliances that are powered by 120 volt AC are controlled by 24 volt ac, derived directly from the 120 volt power, so there usually isn't any need for 12 volt DC to make them operate.

However, your comment regarding your Univolt's loud hum tells me your 12 volt DC system has a problem. You can do a quick check using a voltmeter. Check the DC voltage, which should be approximately 13.4 volts. Then turn the voltmeter to measure AC voltage and check the DC system again. If you have significant AC volts, say something like 1 volt or more, your Univolt is not filtering your DC adequately.

It would be good if the AC voltage was less than 0.2 volts. In older converters, hum can still be present even if the converter is working properly. This is because the plates in the core of the transformer are a little bit loose and magnetic effects cause them to vibrate. Modern converters don't use the old style heavy transformers. If your converter is vintage 1994, it may still use a heavy transformer and could be working fine electrically, even though it's humming.

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Old 12-04-2012, 07:04 AM   #4
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Ok, to clarify, I meant AC bleed from converter, not DC, that we were told can mess up electronics. And yes the Dometic does run on 120v, but the thermostat is 12v. When the Dometic works, it works perfect but, it is like it doesn't get told to kick on all the time. We have replaced the thermostat as well. Still same issue.

Andy, it may turn out to be that it was improperly installed, but the same person also installed the 13,500 BTU in the bedroom works without any issues ever.

Any ideas?
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Old 12-04-2012, 07:38 AM   #5
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I take it both a/c's run off of separate thermostats. Does the 15K BTU use the wall mounted Climate Control Center (CCC) thermostat? Is yes, that does require 12 VDC. Does the bedroom a/c use a different style of thermostat?

Also, the water heater DSI (Electronic ignition) also uses 12 VDC. Does the fridge run ok on propane while you are plugged in and the Univolt is running? I'm making an assumption that you have the fridge set to Auto, so it's running on 120 VAC while you are plugged in, so you may not know or even thought to test it on propane with the Univolt running.

I'm with Andy. Unplug the Univolt and try the main a/c and water heater. The batteries will supply 12 VDC for a while for the test.

Chris
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Old 12-04-2012, 07:48 AM   #6
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Most thermostats are 24 volts DC, common is a transformer inside the A/C unit that steps down 110 A/C volts to 24 volts D/C.
Not aware of a unit that uses the house battery voltage to power the thermostat, but as I understand the newer refers us 12 volt D/C off of the house battery control the circuit board.
SO COULD BE.

I would contact "Lewster" a member here, he is the guru as far as A/C
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Old 12-04-2012, 08:11 AM   #7
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No matter what, if you have a device that has "Univolt" labelled on its case, its time has probably come and gone.

On second thought, I withdraw the "probably", since you tell us that it is humming. The humming can be the transformer's laminations loosening up, but is also usually because the filter capacitors have dried up and aren't functioning (not removing the hum).

Nothing to do with your AC's operation, but life without the hum will be better, and your 12V system will thank you.
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Old 12-04-2012, 08:26 AM   #8
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Yve79

I have written extensively about the Dometic CCC (thermostat) and the many things I don't like about it. I have a 15,000 Penguin in front and 13,500 in back though they were factory installed. Nonetheless, there have been a number of problems.

The first thing to realize is that the CCC is extremely fast acting and if it is in the outlet airflow from the A/C it will tend to shut the A/C off way too soon, and then won't turn it back on for 3 minutes. You can tape up a dry hand towel over the thermostat and see if that makes a difference -- it often does, because it slows the thermostat down just enough that the A/C gets a chance to do its job. The only real permanent fix is to relocate the thermostat so that it's completely out of the airflow.

The second thing to realize is that the "auto" fan mode on the CCC doesn't work well in Airstreams. Maybe it works in big diesel pushers but I doubt it. Don't use it, you'll get quieter operation and more cooling in "low" fan and considerably more cooling in "high." (The problem again is that the CCC chases its tail, it will start the A/C in low fan, "realize" that it needs high fan once the air starts moving around and switch, run for a while, then shut the A/C off completely, then realize it's still hot and want to turn the fan on, but the 3 minute delay isn't up yet so it just starts the fan on low, then by the time the 3 minutes is up it realizes it's still hot and kicks the fan up to high. Rinse, repeat).

Try turning the thermostat down to the lowest setting so that the A/C always runs and see if you get the cooling you expected.

I doubt if the Univolt has anything to do with the problem, the CCC is capable of finding its way straight to hell given the cleanest power in the world, but as others have pointed out upthread just turn it off and run off batteries for a little bit and see if anything changes.
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Old 12-04-2012, 08:33 AM   #9
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Yes, units use separate thermostats and both use the comfort control.

We switched off the converter and the AC still did not kick on.

It feels ridiculous to have to replace the AC completely this soon, but it looks like we may have to, along with the water heater and converter (if only because the him rattles my brain, even though hubby says he doesn't notice. I don't think we will go with the same unit because another forum post showed that several of these units have failed, one guy having his replaced 6 times inside of a year!

In hindsight, we should have taken it in for warranty work, but it is difficult because we are in it full time with a cat and it is too inconvenient to be out of it. Have searched, to no avail for a mobile tech who can do warranty work inAustin.

We appreciate all the suggestions thus far and I have sent a pm to Lewster as suggested. Hopefully, he can help. In any case I will repost to let you know what we figured out.
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Old 12-04-2012, 09:08 AM   #10
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Those Comfort Control units are actually a computer, and get their power from the 12 volt system which is connected at the airconditioner unit. They are powered over a four conductor telephone type cable with the clip-in connectors, and those connectors can give lots of problems. They are actually not designed to carry the current needed for the computer to operate properly. I've had lots of troubles with mine.

If the numbers are dim when you turn it on, I'd almost bet you have troubles with the telephone type connectors
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Old 12-04-2012, 09:15 AM   #11
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Steve H,


I definitely would not be surprised if the phone cable is an issue. Does anyone else find it ludicrous that Dometic doesn't supply a designated cable for this purpose?

We replaced ours twice, but didn't improve. Currently we have a Cat 5 cable from thermostat to unit.
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Old 12-04-2012, 09:24 AM   #12
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I had a connector block go bad on my system up under the inside air conditioner panel where two of the telephone cables came together, and that was giving all kinds of intermittent troubles. Sometimes everything would work fine, and then nothing would work. Then other times only the front A/C unit would work, others only the back A/C unit would work.

It was a burnt connection on the power pair of the cable in the connector block. Went to Radio Shack, bought a connector block for $6, replaced it and it's been good ever since. This, after spending over $200 with two different mobile "techs" that couldn't find the trouble.

A call to Dometic and insistance on talking with the manager of the CS dept got me the info I needed to find and fix the trouble.
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Old 12-04-2012, 09:29 AM   #13
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Additionally, if the univolt is old, producing "humm", like to the point the humm is heard in the radio, it can be a problem to the modern computerized control systems.

It's what's called high percentage of ripple voltage caused by the filter capacitors going bad with age. If you unplug the converter from power, but leave the 12 volt system up and running on battery only, and the A/C still will not work, the univolt is probably not your problem.
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Old 12-04-2012, 09:32 AM   #14
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Where would one buy the ac-dc filter mentioned in post #1
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