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Old 03-09-2011, 06:29 AM   #1
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Erratic Duotherm AC performance

Delay of 3 minutes between time unit is turned on (via wall control panel) and start of AC. Runs and cools for 30-60 minutes and then turns off completely. After 15 minutes (or so) unit comes on again on its own.

What's happening?

Thanks in advance.

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Old 03-09-2011, 06:46 AM   #2
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Sounds normal to me. Unit will turn off after selected temp is reached and then start again when the temp warms, just like your home unit. Also mine takes time to getgoing on initial turn on. zz

1984 Avion 30p 9.1 meter. 2006 Dodge 3500 cummins srw short bed crew cab.
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Old 03-09-2011, 07:15 AM   #3
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"Just like my home unit"? This is NOT the way it was operating when I first used it during warm weather in the fall--the fan kept running all the time the unit was turned on and the AC came "on" as soon as I turned it on and I could hear the compressor switch on and off as it operated

My only experience w/TT ACs is w/ my previous SOB in which the AC fan ran all the time, but this was a '94 model.

Sure hope you are right and thanks!
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Old 03-09-2011, 11:16 AM   #4
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HCR I have run into this as well. It's frustrating. The firmware in the Duotherm controller is badly written and appears to have been designed by someone who has never been in an RV.

There are some workarounds for its problems but it's still frustrating.

Problem 1. Bogus three-minute delay.

When you switch the thermostat from "off" to "on" there is a three minute delay while you sweat and the A/C just sits there. This misfeature is supposed to protect the compressor when there's a power outage, but since it runs off the 12 volt supply, it doesn't provide the intended effect. In fact if the 120 goes out for a few seconds and back on the A/C will try to start until it trips the overload.

The workaround is to leave the slider switch on all the time and just turn the temperature up to 95 or something if you don't want to run the A/C. Or you can turn the air conditioning on and off with the circuit breaker instead.

Problem 2. Anticipator over-anticipates.

The controller starts to cycle the air conditioning off once the temperature drops to within 7 or 8 degrees of the setpoint. The idea is to prevent overcooling. Well in practice if it's hot and sunny out the result of this is that the trailer will never cool down to the setpoint and so you end up with a hot trailer and the air off.

The workaround is, on a hot day, you just have to turn the thermostat down 8 degrees cooler than what you really want. So if you want the trailer at 75 you have to set it at 67 or so. Then at night you have to turn it back up because it will eventually get down to 67 if its fairly cool outside.

Problem 3. Reversing valve releases too soon when air conditioning shuts off.

So there's a directional valve that switches between heat and cool (at least on heat pump models, if you have the cool-only one you're spared this particular indignity). It's run by a solenoid that has to be energized for the valve to be in "cool." Well the controller releases the valve right away when it shuts off the compressor in cool mode. Who knows why. So there's this loud klunk and great hissing of refrigerant that takes place. With a 30 second delay this wouldn't happen but no.

Nothing you can do about this.

Problem 4. Auto fan mode is brain dead

Well you can choose from low, high, or auto. The thing is that if it's hot out the auto mode will pretty much stay in high all the time and cycle the entire system -- fan, compressor, and all -- on and off to maintain temperature, with a couple of speed changes thrown in here and there for good measure. Unless you and yours sleep like the dead this should be enough to wake you up and unless you have the speaking voice of a 1890s politician it's enough to interrupt conversations.

A well designed controller would keep the compressor running and cycle between high and low fan speeds when running continuously in low fan doesn't provide enough cooling. Otherwise if low fan provides sufficient cooling then the whole unit could be cycled with the fan never switching to high.

There's no way around this other than to avoid using "auto" and just run the fan continuously on low or high as required by the weather.

Problem 5. No full auto mode

Especially on sunny sites sometimes you have to use heat and cool both in the same day. This is especially true for those of us who leave climate sensitive stuff (musical instruments) in the trailer while gone for a day or two.

Duotherm could have given this to us but no.

The workaround for me is to leave the air conditioning set at 77, and set the thermostat for my electric heat (which has its own thermostat) at 70.

Lewster is a big fan of these controls and will probably show up to provide another point of view.

I have at various times written controller firmware for a living and have been tempted to write something of my own and reflash the chips, but it's a big project and I have paying work to get done.

A more practical alternative would be to modify the air conditioners for mechanical controls and use those instead. I'm tempted to do that, especially since I only use the system in "cool" mode or "furnace" mode and that simplifies the wiring.
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Old 03-09-2011, 01:28 PM   #5
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Ours doesn't turn off on hot days, but does do some of the strange things Jammer outlines. It sounds like it's trying to destroy itself when it turns off as Jammer mentions in his #3.

The unit isn't big enough on very hot days and if I had known better, I would opted for the larger one.

When we've used the heat pump I don't think it always turns on the furnace at 28˚ or so like it's supposed to. I'll usually wake up in the middle of the night and check the outside temp and turn on the furnace myself if it seems warranted. The furnace is loud too. The heat pump saves propane since you are using a campground's electricity, but it also sounds like it's trying to commit suicide.

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