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Old 07-10-2014, 10:25 AM   #1
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AC Freezes up

My apologies if this has already been posted but my AC unit continues to freeze up. Ive got through and cleaned the coils (inside and out), cleaned the blower, and cleaned the filter. The unit is an older Dometic that does not have ducts that run through the RV. I took the cover off to make sure there was nothing blocking air flow on the inside and all the vents are opened. The only thing that makes a difference is that on the coldest setting the unit will freeze up a lot quicker compared to being on a middle temp setting.

Normally this means there is an air flow issue but I have cleaned everything. Are there any other possibilities?

Matt
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Old 07-10-2014, 11:44 AM   #2
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An A/C that freezes is usually low on refrigerant (freon).
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Old 07-10-2014, 11:54 AM   #3
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The other two things I can think of are fan motor speed and refrigerant charge.

The motor speed is hard to judge, but if slower than specified, the air flow would not be as much as needed and freeze up would happen.

If the refrigerant charge is too low, what happens is that the amount which is metered into the evaporator coil flashes immediately into a gas, sub cooling the area close to the input, rather than semi flooding the coil and evaporating more slowly over a larger area. So, the area close to the input gets really cold and ice forms there, then as less heat can be picked up in that area, the next section gets the flash gas and freezes too, until the problem progresses slowly across the entire coil. Sometimes you can measure the input air temp and output air temp and you should generally have a difference of about 20 degrees in normal use. If you are not getting that high a differential, and yet the coils are freeing, it may indicate a low charge of refrigerant, with some parts of the coil very cold, and other parts warm.

Since there are no service ports on most RV AC units, it is very hard to check the refrigerant level and pressure in the normal way.

I hate to say it, but if it is an original unit from 1976, it is 38 years old and it may be time for replacement. Of course most anything can be repaired, but there gets to be a time when it is better to just replace, considering the cost of repairs vs. the cost of replacement.
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Old 07-10-2014, 11:56 AM   #4
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Other distinct possibilities are insufficient air flow across the evaporator and a mixing of return air with exhaust air.


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Old 07-10-2014, 12:04 PM   #5
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Low on gas shouldnt be an issue, my father worked on them for years. There should be a way to add a service port to the unit and then pull a vacuum once in order to put the gas back in. I talked to him and he should be able to do it. I guess for now I will leave it on a warmer setting so the compressor can cycle a little more often.
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Old 07-10-2014, 01:52 PM   #6
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In high humidity even a properly operating A/C may freeze up. Even a slight loss of airflow makes this worse.

Comb the fins if they need it

Clean the fins if they need it

Run the fan on Hi if you have more than one speed

Still problems ? Replace the fan motor and cap.
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Old 07-10-2014, 03:07 PM   #7
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One check for low coolant is to look at the suction line, The larger of the 2 pipes to the compressor. If there is any indication of frost on the suction line you are low on gas. Normally that line will have condensation on it but not frost.
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Old 07-10-2014, 07:12 PM   #8
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You can also check the amp draw on the compressor after it has been operating for at least 10 minutes. The amperage listed on the data plate os for 'standard test conditions' which for RV roof A/C units is 95ºF. Lower ambient temps will have less amp draw, but you should be seeing at least 10-11 +/- amps in mid 80º weather. If you have lost some refrigerant, these values will be more in the 6-8 amp range.
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Old 07-10-2014, 07:28 PM   #9
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Common problem, run the fan on a higher speed. Happens in Fl all the time. Jim
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Old 07-10-2014, 07:40 PM   #10
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As a retired auto tech I'm surprised there is no thermostat on the evaporator coil to shut down the compressor when low on refrigerant , low refrigerant means low pressure ,low pressure means low temperature , or a low pressure cut out switch, if you run the a.c. With no or low pressure you can damage the compressor as the oil for the compressor is mixed with the refrigerant .this is the case for automotive but I never worked on r.v. A.c. Units
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Old 07-10-2014, 07:53 PM   #11
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When an RV roof A/C leaks or has a bad compressor, the most cost effective thing to do (we're NOT talking about Armstrong units here) is replace the unit. This is due to the lack of process valves on ANY RV unit, and the complexity of using piercing valves to drain the system, solder in Schrader valves, pressurize to find the leak, repair the leak and/or replace the compressor (if you can even get a replacement), purge the system with nitrogen and check again for leaks and the integrity of your solder joints and then properly re-charge the unit to spec.

And if you slightly over heated he solder joints, you might have scale present in the inside of the tubing which will flake off and eventually clog a capillary tube.

Unless you can find an A/C tech who works for $10/hour, it's now simpler, quicker and most important....CHEAPER to replace the unit and get the new unit warranty.
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Old 07-10-2014, 09:47 PM   #12
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If it is humidity related turn it off for 30 minutes every 4-5 hours to melt, worked wonders for us


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Old 07-10-2014, 10:19 PM   #13
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Quote:
Originally Posted by scamp View Post
As a retired auto tech I'm surprised there is no thermostat on the evaporator coil to shut down the compressor when low on refrigerant , low refrigerant means low pressure ,low pressure means low temperature , or a low pressure cut out switch, if you run the a.c. With no or low pressure you can damage the compressor as the oil for the compressor is mixed with the refrigerant .this is the case for automotive but I never worked on r.v. A.c. Units
Sealed units like home window AC's and RV's have, along with home refrigerators, dehumidifiers and such, are never expected to leak. They have hermetically sealed compressors and all piping is factory brazed. Thus they have no low pressure switches or low temp switches, as units which are open, like automotive AC systems. Auto systems are expected to leak some over their life, and crash repairs are also needed. Not so with fixed home units.

So, they also don't have any service ports, as they are never expected to need service, and in general never do.

If they fail or leak, it is generally not economical to have someone repair them, as Lewster mentions and explains in his post above.
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Old 07-10-2014, 10:42 PM   #14
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Quote:
Originally Posted by mprokop87 View Post
My apologies if this has already been posted but my AC unit continues to freeze up. Ive got through and cleaned the coils (inside and out), cleaned the blower, and cleaned the filter. The unit is an older Dometic that does not have ducts that run through the RV. I took the cover off to make sure there was nothing blocking air flow on the inside and all the vents are opened. The only thing that makes a difference is that on the coldest setting the unit will freeze up a lot quicker compared to being on a middle temp setting.

Normally this means there is an air flow issue but I have cleaned everything. Are there any other possibilities?

Matt
Just had same problem in florida while on vacation. Both units shut down after the front unit froze up Water leaked from unit and cable connection got wet. THERMO went crazy and shut everything down. THERMO was showing inside temp at zero degrees. Took unit apart and dried cable. Used electrical tape to seal cable connection and ran reset procedure for THERMO. Didn't work until after I killed all electrical and 12 volt power and ran reset again. Got units back in operation. TURN THERMO UP TO 72 DURING DAY AND 70 AT NIGHT AND RUN AT HIGHER FAN SPEED. We've done so and have had no further issues.
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