A/C freezing up - SailNet Community
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post #1 of 11 Old 04-08-2008 Thread Starter
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A/C freezing up

My air conditioner froze up last weekend. It is a Mermaid 16,500 BTU unit with reverse cycle heat. The thermostat was replaced in February by the PO and they used a household type thermostat from Home Depot. They claimed the local A/C repair guy suggested this as a fix. I should mention here that the thermostat was replaced because the heat would not cycle on during the survey so this thermostat was installed and the heat worked. I was on the boat last October and the A/C worked great.

Is it possible that the new thermostat was connected incorrectly and that is the causing the unit to run until it freezes? I don't have the instructions so I am unable to double check the connections that way. Are there some other troubleshooting steps that I can complete before getting the HVAC guy to come out and take a look at it? The closest one is in Austin and charges mileage to come out to our lake.

Again, thanks for your assistance in advance
Bill

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post #2 of 11 Old 04-08-2008
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My recolection is that when an a/c freezes up, the system is low on freon.
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post #3 of 11 Old 04-09-2008
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Believe Pigslo may be correct. Basically, there are two reasons an A/C unit will freeze up. One is low on refrigerant--technically not Freon any more, but what do you expect from a pig. The other is if the air flow over the coils isn't sufficient...which is usually caused by a bad fan. I doubt it is caused by the thermostat installation, unless the thermostat is responsible for controlling the fan in the A/C system (which is very unlikely).

BTW, have you checked the duct work and filters to see if they're clear? Obstructions in the duct work or filters can reduce the airflow enough to cause the freezing problem, especially in a very humid environment of a boat.

Two websites you might want to read LINK LINK2

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post #4 of 11 Old 04-09-2008
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As has been said before, low freon (99% chance) or low fan speed.

Steps:

1) When you turn it on, do you hear the compressor kick on? Note, this is the compressor... not fan. The compressor kicks on and makes a deep hum... If so...

2) Are you making water? If so...

3) Are you moving good air? If so...

4) (assuming it is defrosted), are you making COLD air? Not cool, COLD? The answer to this is most likely no. If it is low on freon it will move cool air and slowly freeze up at the coils, or will not move any cool air, or the compressor will not kick on at all. Most compressors will not run if they are real low on freon, but will run if moderately low on freon.

Honestly, I have only had a unit shut down because of not moving enough air once. If it is a low fan speed issue, you will make very cold air... just not move enough across the coils and it takes a while for it to shut down. Any time I have ever seen or heard of a unit shutting down outside of that it was because of low freon... and have NEVER EVEN HEARD of a unit freezing up because of a bad thermostat. Either you were sold a bag of goods or I learnerd that a thermostat can freeze up your coils??? I don't see how, honestly. Most units have a max 55 degree low setting and that is a long way from 32 (assuming you could ever even hit 55).

Refilling with refrigerant will require a tech that is licensed. Sorry. I am not sure you could do it yourself. I do not think you can even buy refrigerant unless licenesed.

Another point... why are you low on refrigerant? Have a leak (most likely)? As such, you will have to track that down. Most techs will not keep filling refrigerant. The first time assumes it might be a very slow leak and you will probably get it filled. The second time, they will want to track it down, IMO.

- CD

PS My disclaimer: I am NOT a licensesd marine ac mechanic. I have done a lot of my own work. My father in law is licensed (home) so I do a lot of work with him. THis is what I have learned, so take my advice as such.

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post #5 of 11 Old 04-09-2008
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Not knowing a whole lot on this issue, but once at a motel, the window unit was freezing up. We noticed that 80% of the radiator fins had been bent over thus restricting air flow. We spent quite a while straightening these out and were able to improve the cooling ability.
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post #6 of 11 Old 04-09-2008
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Quote:
Originally Posted by Stu01 View Post
Not knowing a whole lot on this issue, but once at a motel, the window unit was freezing up. We noticed that 80% of the radiator fins had been bent over thus restricting air flow. We spent quite a while straightening these out and were able to improve the cooling ability.
These units use water to remove the heat, not air. Without water movement, you will get a high head pressure within moments. THis is often shown as a PS (looks like P5) on the display for Cruiseair. I would assume Mermaid has a similar readout for high head pressure. Different animal than air exchanged units (window units).

- CD

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post #7 of 11 Old 04-09-2008
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CD,
I would PM you to thank you for the clarification/education but I need some more posts. One less now!
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post #8 of 11 Old 04-09-2008
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Let me add my two cents. I have noticed that the systems can become less efficient which can lead to freeze up when the coils get dirty. Living in FL, the high humidity will make your boat damp if you are not running the AC all the time or have a dehumidifier. I clean my coils twice a year with a spray solvent I get from local AC suppliers. After flushing I spray the coils with an anti-fungus spray. I also get that from local AC suppliers. With the growing popularity of people cleaning their home ductwork, these products may be obtainable from Lowe’s or Home Depot.

Another test for Freon is to check the temp across the coil when running. You can get the thermometers at Home Depot or where ever. From what I understand, a home system will pull about 10 degrees difference between the intake and the exhaust side of the coil. I have been told a boat AC will pull between 15 and 20 degrees difference. Going either side can mean either to little or too much Freon. I was also told that Freon will be phased out of marine AC units in the next 10 years. I just replace a 16k BTU Cruise Air and was told that by the tech. I defer to anyone with more knowledge to add or subtract form this.
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post #9 of 11 Old 04-09-2008
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I occasionally have a freeze up problem with my Mermaid A/C. But it's not due to any of the above described problems. I replaced my old unit with a slightly larger one by Mermaid. If you read the owners manual, it describes the amount of vents you need to properly ventilate the area. If you don't have enough vents, then the cold air can't get out and thus backs up causing the unit to freeze. So the answer from Mermaid is to set the thermostat a couple of degrees warmer so the compressor won't run as long and thus get rid of the cold air it needs to function correctly. Sounds corny, but it's in the owners manual how many vents you should have.
Another solution depends on the vents you have. On the unit with the freezing problem, the vent blows straight up. Someday, I'll replace it with a 45 degree vent that blows outward. That should help air flow blow away from the vent and dissipate better.
Scott


Below is what the Mermaid site says. Look at the last section:

Air Conditioners Evaporator is Freezing

While the marine air conditioner is running, the evaporator is freezing. This device looks like a radiator or a car.

Supply air vents closed or insufficient number of vents. Open and/or add additional vents.
Fan not running or intermittent.
Return air blocked. Clean air filter and/or remove any blockage.
Thermostat set too low and is not allowing unit to cycle and remove icing. Set thermostat at a higher setting.
Thermostat improperly located in a position not giving "true" cabin temperature. Examples would be in a sunny spot near glass, or near the hatch. Relocate thermostat.
Low freon due to leak. Unit will require professional service.
NOTE: The most common of this problem is insufficient ventilation coming off the unit. Additional vents are needed or "cleaning" the existing ducting scheme to ensure better airflow off the system. Remember, the more vents the better on ANY unit. Each unit has minumum vent requirements, review your product under Marine Air Conditioners if needed.

Last edited by Schuckerman; 04-09-2008 at 02:52 PM.
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post #10 of 11 Old 04-10-2008
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I just had a similar problem. If you do not have sufficient water flow, and are using the unit for heat, the compressor will ice up because it cannot disperse the cold it is collecting. My unit does not auto shut down in this situation.

It will auto shut down if it is overheating because I am asking it to cool down and it is collecting heat that it can not disperse.

Overheat = shutdown, Overcold = Iced up

My pump had an air block (they are not self priming) and once cleared all was well. Check the unit for adequate water flow ie. pump, thru hull, raw water filter, etc. May save you a service call.

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