dometic air conditioner recharge guidance

[marcz]

I have a 11 year old VCD18k-hv-410a air conditioner that cools poorly.
The temperature differential between the return and supply is about
11 degrees F and the water flow is normal. Loss of performance
seems to be slow over time. All filters are clean
I plan on adding some 410A refrigerant to the low pressure line.
Dometic recommends vacuuming out all of the coolant, then fully recharging the system by weight (18oz). I don't have a vacuum pump, and thought
I would simply add very small amounts of coolant and monitor the return and supply temperatures, but I'm not sure what to use as a target temperature differential and don't want to risk overcharging the system. Any guidance on this? Is it possible to recharge a unit like this using a superheat or subcooling method?

[Bowdrie]

Originally Posted by marcz

Is it possible to recharge a unit like this using a superheat or subcooling method?

Not sure what you mean by a "subcooling" method of charging.
I believe that your unit is a "cap tube" system, as such there really isn't a "superheat" adjustment, that's for TXV valves.
Units that use cap-tubes are charge critical, and Dometic is correct.
For ~200 bucks or so, you can get a vacuum pump/gages/leak detector at Harbor Freight.
You'll want a pressure/temperature chart for the 410A refrigerant.

[Bowdrie]

Addendum; hook-up your gage set to both the high and low sides and confirm that there is pressure on both sides.
All the components in that system are silver brazed together, as such, unless there has been physical damage the chance of leaks are very small.
The device most likely to suffer a refrigerant leak is the condenser.
While the CuNi condenser is highly corrosion resistant it can develop a tiny leak between the water side and the refrigerant side, especially if any stray current is present.
With pressure in the system disconnect the water and drain the condenser, then with the detector you can find out if refrigerant is leaking thru the condenser.
Don't just automatically hook-up a vacuum pump at first, if there is a condenser leak the pump will try to suck water thru the leak and that is disaster for the system.
Of course you'll check everything else for any leaks also.

[SailingHarmonie]

Originally Posted by Bowdrie

Not sure what you mean by a "subcooling" method of charging.
I believe that your unit is a "cap tube" system, as such there really isn't a "superheat" adjustment, that's for TXV valves.
Units that use cap-tubes are charge critical, and Dometic is correct.
For ~200 bucks or so, you can get a vacuum pump/gages/leak detector at Harbor Freight.
You'll want a pressure/temperature chart for the 410A refrigerant.

I would be really surprised if an 18K air conditioner was a capillary tube unit. But it is possible, I guess... But IF it is, trying to add refrigerant to a cap unit by pressure rarely is successful.

In an AC unit one of the possible symptoms of low charge is icing on the evaporator. The lower pressure from the reduced gas quantity results in lower evaporator pressure, which leads to icing on the bottom of the evaporator, and no cooling from the top.

Arbitrarily adding refrigerant to a machine is not likely to end well. You can get a vacuum pump on Amazon for <$100. I would strongly suggest you go that route, or else hire someone who actually knows what they are doing.

[marcz]

Originally Posted by SailingHarmonie

I would be really surprised if an 18K air conditioner was a capillary tube unit. But it is possible, I guess... But IF it is, trying to add refrigerant to a cap unit by pressure rarely is successful.

In an AC unit one of the possible symptoms of low charge is icing on the evaporator. The lower pressure from the reduced gas quantity results in lower evaporator pressure, which leads to icing on the bottom of the evaporator, and no cooling from the top.

Arbitrarily adding refrigerant to a machine is not likely to end well. You can get a vacuum pump on Amazon for <$100. I would strongly suggest you go that route, or else hire someone who actually knows what they are doing.

SailingHarmonie and Bowdrie, thanks!
I know I'm over my head in this matter, but the service guy can't check out the boat for 2 months and it will be a moot point by then. I didn't think a trial of slowly adding a few ounces of refrigerant would likely cause a problem.
Yes, the evaporator does freeze, but it has done so for years and opening the cover
to the settee that the unit is installed in fixed that until recently. I had come to the
conclusion that the original installation was suboptimal and that airflow restriction was responsible for the evaporator icing. Likely it was. Until last year, I would measure a much greater differential between the return and supply air temperatures (Wish I had
recorded that data, but I did not).


With respect to leak testing, I'm not sure I understand the suggested testing method. Is it to operate the A/C for a period of time, turn it off, remove the water supply hose from the thruhull, drain the water, and then check for leaks? I did not think the compressor would run if the system does not have water flow....