Air Contitioner Freezing Up

[Johnb]

I may be all wrong here but I got to wondering why low refrigerant would cause the evaporator to ice up. From some internet DuckDuckgoing I seem to find that low refrigerant will cause icing in the pipe from compressor to evaporator, not in the evaporator itself. Trying to understand this I am guessing that it is because evaporation is occurring in the pipe rather than in the evaporator due to not enough liquid to fill the space between compressor and evaporator. Low refrigerant would not therefore cause the evaporator to freeze up

Appreciate comment from someone who knows

 

[jamieFL]

From experience... I just had to have my homes a/c replaced. The first symptom something was wrong was the evaporator coils freezing up. I defrosted and restarted, and a few hours later, it was froze up again. We had the serviceman out and he found a small leak. We bought a new system, but it was going to be a week before he could install. I live in Florida and we don't go without a/c here. He charged up our old system and it functioned flawlessly until he came back to install our new system the following week.

My neighbor's son just went through school to become an a/c technician. He explained it to me in detail. I am not qualified to explain it.

Yes...adding freon indescriminately is not a good thing.
Yess... Adding freon when a system is low, if the technician does not find a leak is.

 

[Jim26m]

Due to the properties of the refrigerant, as you lower the evaporator (suction) pressure, the refrigerant boils at a lower temperature (also true of water - which can be used as refrigerant if you're up to a challenge). At any rate, as refrigerant leaks out of a system, the pressures in the system go down. When enough refrigerant leaks out, you flash it all right where it is introduced into the coil. This creates a small zone of very cold metal which freezes the condensate from the air. As this frosts over, it lowers the heat transfer in that section, allowing the frost boundary to continue to progress across the coil until the capacity of the charge won't go any further. Depending on the conditions, it might just be one section, or might turn the whole thing into a block of ice.
The irony is, the system isn't cooling as well due to the low charge, but parts of it are freezing.

You can catch the problem if you constantly monitor your supply air temperature while the system is operating in steady state. The overall supply air temperature will go up as your charge decays. If your typical discharge temp at the closest register is 53 deg; then one day you notice it doesn't get below 62 deg, your charge is on its way down. This observation needs to be made when the conditioned space is at roughly consistent temperatures. In the southeast, or other humid areas, the indoor humidity will start to rise, also; making the house less comfortable even if its holding temperature. You are cooling only in one section of the coil. The air that's not running through that section gets very little cooling effect. And yes, I actually am so much of a geek that I monitor my discharge air supply temp.

While one of my certifications is refrigeration mechanic, I don't have experience with this particular unit. So, when you guys that own one say they all freeze, or have defrost cycles, I'm not arguing with you. I'm just saying that in most of the systems I'm familiar with; if the coil is freezing, it's either airflow or refrigerant charge.

There is one other possibility that we should consider. How cold was the condenser water?

I assumed that the unit ran fine in the past and this is something new. True?

 

[JamesG161]

How cold was the condenser water?

Now you are getting closer to the likely answer!
_____

One last suggestion is to add a 1" Chlorine tablet to seawater suction strainer to clean that heat exchanger.

This is the "Condenser" when the AC is in cooling mode.

You can foul the Condenser with sea life crud and even barnacles!
You must maintain that clean, or little to no heat transfer.
Jim...

 

[Jim26m]

Rich Stidger was spot on.

Been studying various units out of curiosity... Found the following.
U15 DE-ICING CYCLE: Under certain extreme conditions ice may build up on the evaporator coil. Running the a/c at a low set point with the hatches and doors open on a hot humid day would be an example of this. If the a/c runs for an hour in cool mode and during that time the ambient temperature does not change by more than one degree, the de-icing cycle will activate turning the unit into reverse cycle heat for one to three minutes depending on the setting. The programmable parameter may be set at zero (0), one (1), two (2), or three (3) minutes. If the evaporator coil is still iced up after three minutes, see the trouble shooting section of this manual. The factory default is zero (0), which is off.

So, apparently these units are designed, such that if run continuously in hot/humid conditions without providing measurable cooling in the space, the control system assumes it is iced up. Last time I checked, hot/humid conditions are the reason we bought AC units to start with.

If the control system is designed that way, we would naturally assume that the previous version had icing issues right out of the box - even when properly charged. I can't even find words to comment on this.

I stand by the original issues of airflow and charge. In fact the manual I pulled that excerpt from specifically mentions airflow as a potential icing issue. Now, I would add "sometimes it might just freeze up" to that list. Never too old to learn something new.

Moral of the story; if you have a unit with these controls, review your de-icing settings BEFORE calling the mechanic.