Air Conditioner Freezing Up

K

kgaffney22

New Member
Mar 5, 2011
7
New Jersey
Boat Info
1991 310 Express Cruiser
Engines
Twin 454 Mercruiser Inboard
I have a 1991 Express Cruiser 310 that has a Mermaid Marine Air / 11,000 BTU air conditioner in it. Whenever I run it, the coils freeze up after about 30-40 minutes. I’ve done the research here and lack of airflow seems to be the main culprit. The unit is mounted under the bed in the bow. There is a 14X20 hole cut in front of it with a filter and grate. If I hold a piece of paper in front of the coils (when they are not frozen), it pulls it to the coils telling me air is being sucked in. The outlet is teed off to 3 different spots and they all blow cold (when it’s not frozen). As soon as it freezes, I have to turn the A/C off manually at the thermostat and leave the fan on to de-ice it. Then manually turn the A/C back on when the ice is gone. Not fun when you are trying to sleep. The unit will freeze every time, even when the grate and filter are removed.

The unit in its home:
IMG_0188(480x360).jpg

Frozen:
IMG_0191 -(450x338).jpg

I’ve even tried putting an electric fan right in front of the coils and that seems to give me a couple of minutes more before it freezes but not much.

Any ideas? How long should it be able to run before I have freezing issues?

Thanks.
 
Do you have water flowing throughout the system? I there water coming out the side of your boat from your A/C pump? Is your strainer clean? Is the through-hull seacock that feeds the strainer open? You have already eliminated air flow, now check water flow before you get into any further diagnostics.
 
No Water flow would cause high head pressure, just like if you lost your condenser fan motor on the a/c unit at your house. With high head you will not get any cooling effect.
 
Air conditioner flow is limited by both the intake and the outlets. If it's having to force too much air through too few/small hoses, that will limit the airflow. In this case, you don't have a problem with the intake, since it is 280 square inches (14*20). Your outlet hoses are a total of 37.68 square inches, but it looks (in the picture) like they are fed from a single 4" trunk. It really looks like your outlet is very constricted. Ideally you would have a plenum (box) that feeds the outlets individually, rather than branching them off of one small trunk.

Also, remember that every bend in the hoses further restricts the airflow.

My best guess is that if you were able to open up the outflow side, you would have many fewer problems with it freezing up.
 
WOW - Quick reply's. Thanks. I usually set the thermostat at around 75 and it does cool the berth off nicely if I shut the condenser down manually and don't let the unit freeze. I'd like to set it lower but cant even get it to cool to 75 on a hot day without freezing. Water is flowing through the system. It pumps out on the port side at the front whenever the unit is turned on and I cleaned the strainers again this year when it was put back in the water.

Thanks again for the help.
 
src said:
Air conditioner flow is limited by both the intake and the outlets. If it's having to force too much air through too few/small hoses, that will limit the airflow. In this case, you don't have a problem with the intake, since it is 280 square inches (14*20). Your outlet hoses are a total of 37.68 square inches, but it looks (in the picture) like they are fed from a single 4" trunk. It really looks like your outlet is very constricted. Ideally you would have a plenum (box) that feeds the outlets individually, rather than branching them off of one small trunk.

Also, remember that every bend in the hoses further restricts the airflow.

My best guess is that if you were able to open up the outflow side, you would have many fewer problems with it freezing up.
Click to expand...

Agreed.
 
No water flow should not even allow the compressor to start. Just trying to ensure that the basics are covered off before moving on.

Agreed on the restriction issue. After installing a plenum box, perhaps he could also take off another 4" run to somewhere else to help ease the restriction situation.
 
Would a good test to see if it's the exit airflow be to disconnect all of the existing duct work and connect one vent out to the berth? (No splits or tee's) How long should it be able to run without freezing up? What temps do most people set the thermostat at? It looks like the unit was installed by a previous owner and the exit duct has what looks like dryer vent line (metal) as well as the white flexible vinyl hose seen in the picture patched together. Thanks again for all the help. I suffered through a hot cabin all last year and really want to fix this before it gets too hot.
 
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If you get the airflow correct you could be able to run the stat down to 68 or so at night and not see a icing condition. Now if you can actually achieve that temperature depends on the unit capacity versus space heat load.
 
Removing ALL of the hoses, including whatever fitting that is on the outlet (get back to whatever came with the unit, not what was bolted on during installation), and letting it run with the storage area open would be a decent, if not fantastic, test. It would still be trying to cool the whole cabin since the thermostat is mounted remotely. It would be recirculating colder air sooner than normal (less efficient at cooling the room), but that would also tend to promote freeze-up. If it could cool the cabin in that state without freezing, I think that would be a pretty good indicator.
 
Being an hvac company owner the two major things that cause evaporator freeze is 1 clogged return filter which causes lack of air flow across the coil and low refrigerant charge low ambient temp cause frozen compressor I can run my ac to 57 degrees no freezing so I would check return and for crimped feeds or obstructed feeds or covered return
 
src said:
Air conditioner flow is limited by both the intake and the outlets. If it's having to force too much air through too few/small hoses, that will limit the airflow. In this case, you don't have a problem with the intake, since it is 280 square inches (14*20). Your outlet hoses are a total of 37.68 square inches, but it looks (in the picture) like they are fed from a single 4" trunk. It really looks like your outlet is very constricted. Ideally you would have a plenum (box) that feeds the outlets individually, rather than branching them off of one small trunk.

Also, remember that every bend in the hoses further restricts the airflow.

My best guess is that if you were able to open up the outflow side, you would have many fewer problems with it freezing up.
Click to expand...

I bet the above is correct!
First look at the install the supply lines jumped out as way too small and without a plenem box.
Lucky because it looks like there is room to make it right.
 
kgaffney22 said:
Would a good test to see if it's the exit airflow be to disconnect all of the existing duct work and connect one vent out to the berth? (No splits or tee's) How long should it be able to run without freezing up? What temps do most people set the thermostat at? It looks like the unit was installed by a previous owner and the exit duct has what looks like dryer vent line (metal) as well as the white flexible vinyl hose seen in the picture patched together. Thanks again for all the help. I suffered through a hot cabin all last year and really want to fix this before it gets too hot.
Click to expand...
You have enough feed back and suggestions to keep you busy for a while. If you are capiable of preforming the tasks that have been suggested thus far and plan to roll up your sleeves and get dirty,I can give you some tips for doing it "systematicly" so as to miniumize cost and time. For now here's motivation to get it corrected asap wherther you plan to diy or call in a pro. The freeze thaw cycles are stretching the coils and will soon split them pretty much making total system replacement more cost effective than repair.
 
The freezing of the coil is ice forming over the coil it is the condensation freezing it will not split the coil if the raw water part of your marine ac froze if would split air flow is the most common problem
 
The problem lies in the freon coolant level. Needs a recharge. Unit will freeze every time when low. Had the same issue.
 
There is defiantly plenty of advice here but before you go adding any refrigerant check air flow if you over charge a unit with refrigerant you will case compressor faliure also if you do not hold a 608 certification and you vent r22 into atmospher and you get caught which i know almost never happens it carry a $27500 fine and anyone that turns you in gets half of fine just to let you know the EPA regulations on Cfc And HCFC refrigerant just my $.02
 
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My A/C did the same thing and all I did was was put a gate value on the inlet hose just before the unit and choked it off a touch. Once the seawater warms up you can reopen the value.
 
So this isn't something new that has just recently surfaced....it goes back to last year. Was wondering about your weather and how humid it gets where you are. High humidity can also be a contributing factor. But indeed.....proper air flow and air distribution is very important to the proper operation of an a/c system.

Just a little example for you....using hypothetical numbers.....

If your fan is capable of....or rated for say 300 CFM (specs for your system should give the real number to you).....and your total CFM of air being put out through your ducts where they discharge totals only say 150 or 200.....you have an imbalance in your system. There are meters for measuring air flow. Don't know if you'd be able to find one and check your discharge air flow or not. Try to figure ways to get closer to a balance.....maybe try to increase duct and discharge grill sizes if you can.

But if at all possible....before spending any money....I'd try to figure a way to measure your air flow "in and out" to determine how out of whack it may be.

Now having said that....there are other reasons a coil can freeze up. I'm also wondering about who actually did the install of that system? An improper refrigerant charge in an a/c system can also cause freezing of a coil. That coupled with perhaps high humidity....and perhaps a poorly balanced air distribution system....could all be causing your problem

Try everything you can....maybe you'll get lucky....but you may also eventually have to bite the bullet and get an a/c tech in to have a look.

Good luck.
 
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