Replace both air cons or attempt repair?

[Nani Kai]

We have two Dometic 16,000BTU air conditioners on our 2012 Leopard 39. One of them runs but doesn’t cool the air. It’s been problematic for a while and finally gave up even pretending to work.

The other one works pretty good but during a particularly HOT week in Brunswick, Georgia where she is docked, the cooling and heating coils froze up completely, caked in frost. Turning it off for a bit until the coils thawed out worked for a while but then it froze up again. It’s working okay now, again in oppressive heat but it could freeze up at any moment.

We left her in early 2020 at the beginning of the pandemic, visiting every 4 months or so to check on her and do necessary repairs. I left both air cons on 24/7 in de-humidifier mode where they have pretty much remained for the last 2 1/2 years.

With the non-stop use, albeit in de-hum mode not full cool mode, their age, the failed starboard unit and the port unit freezing over I figure it probably makes sense to replace them both.

We are at the boat now, finally fully retired, and are prepping for the upcoming cruising season.

What thinks the Hive Mind? Good idea to make a clean sweep of it, start fresh with new equipment and sail cooly South or try to save a few bucks and have both units overhauled?

[Rumrace]

Mine is under the Dinette only 6 years old but failed. Replaced 450$ in parts it’s like brand new used.

[Sailmonkey]

The problem with working on these units is that the labor costs to even access them quickly approaches replacement costs.

A unit that freezes is typically low refrigerant. Probably an easy fix to add some gas. But depending on where the leak is may be an uneconomic repair.

The dead one is also a crap shoot.

And if repaired they’re still 10 years old.

[Rumrace]

Honestly it appears we broke ours by stuffing luggage on top of it. It’s a huge loss in stowage. Access is easy and the water system is right behind it in the wall. A new unit was $5800.00. I had a fella recharge it and close the fillings I bought. The lack of activity for winter is apparently hard on them. Dunno

[Nani Kai]

I promised an update when the air con problem was resolved and here it is: Both Dometic units were toast. The unit to starboard had multiple refrigerant leaks due to corrosion of various components. Didn't make financial sense to replace all the parts plus labor. The one on the port side had a short in the compressor. The tech found the short while trouble shooting erratic operation of the unit. The outcome was replacement of both units. Since then it's been great; nice cool air, no icing of the coils, both units cooling equally.

Plus an unexpected BONUS. I had been chasing an electrical short for over a year that had been impossible to find as it was seemingly random and couldn't be connected to any particular event. Every now and then one of the main breakers downstream of the shore power cable would trip. I would reset it and things would be fine until weeks or months later it happened again. Turns out it was the port A/C.

Wound up putting in CitiMarine air cons as Dometic was way behind on production and couldn't promise any units until January 2023 at the earliest. The CM's are specifically designed to replace the Dometics (16,000 btu's) and were essentially plug and play. The control boxes communicated directly with the Cruisair remote controllers and with the exception of one temp sensor needing replacement it went off smoothly.

Thanks for the support.

[ItDepends]

Originally Posted by Sailmonkey

T
A unit that freezes is typically low refrigerant. Probably an easy fix to add some gas. But depending on where the leak is may be an uneconomic repair.

Can someone explain this to me? It seems counter intuitive. I could understand how reduced air flow across the evaporator could cause it to ice up, but how can low refrigerate charge? I understand in detail how the refrigeration cycle works, but this cause/effect pair confuses me?

[fourlyons]

Originally Posted by ItDepends

Can someone explain this to me? It seems counter intuitive. I could understand how reduced air flow across the evaporator could cause it to ice up, but how can low refrigerate charge? I understand in detail how the refrigeration cycle works, but this cause/effect pair confuses me?

To put it simply (probably too simply) all other things equal, a low charge equals low pressure, low pressure equals low temperature. For the same reason, too much refrigerant raises system pressure, leading high evap temperature, not cooling enough. I know you're familiar with the pressure/temperature relationship. I think this is why fixed orifice units are especially sensitive to charge level, low side pressure is a function of the charge level.

That is my non-expert understanding, hopefully correct. I also found it counterintuitive at first.

[Sailmonkey]

Originally Posted by fourlyons

To put it simply (probably too simply) all other things equal, a low charge equals low pressure, low pressure equals low temperature. For the same reason, too much refrigerant raises system pressure, leading high evap temperature, not cooling enough. I know you're familiar with the pressure/temperature relationship. I think this is why fixed orifice units are especially sensitive to charge level, low side pressure is a function of the charge level.

That is my non-expert understanding, hopefully correct. I also found it counterintuitive at first.

This is correct.

The pressures inside the system don’t really care what atmospheric pressure is. When you operate with a gas at a vapor pressure that gas will want to achieve a temperature that corresponds with the vapor pressure. Lots of times that temperature is below the freezing point of water.