R21EC A/C cools but quits after 20 Minutes when underway

[DMcD]

We've got a 2013 R21EC with factory A/C (Dometic 5,000 BTU 12 v) and Yanmar engine. The previous owner never used the A/C and did not even have the 2 batteries for the A/C system installed. Upon sale to us they installed new batteries.

The A/C works while underway, but quits blowing after about 20 minutes. Upon return to slip and plugging in to shore power it resumes operating.

Marina mechanic checked the batteries (which are relatively new) and said they check out.

All this points to the alternator either not providing enough power to keep the system charged, or, is it possible that somehow the alternator is only charging the house and starter batteries but not the A/C batteries?

Any suggestions would be appreciated.

 

[Capt’nKarl]

Good morning. Is the AC getting it’s current from an inverter ? If so I suspect the batteries are being drained to a point where they can’t supply enough current to the AC system so it shuts down. If the system works ok on shore power then it’s likely a voltage issue. Does the control panel show a code when the AC quits?
Karl

 

[DMcD]

Good morning. Is the AC getting it’s current from an inverter ? If so I suspect the batteries are being drained to a point where they can’t supply enough current to the AC system so it shuts down. If the system works ok on shore power then it’s likely a voltage issue. Does the control panel show a code when the AC quits?
Karl

Thanks for the reply, Karl.

I believe I have the unit that is true 12v. I don't believe the control panel has any information displays - just 2 knobs.

Some older threads here say that some people that have the 12v unit are able to use it underway - which is why I wondered if somehow my a/c batteries are not connected to the alternator - perhaps because the previous owner did't have them installed and the technician that put them in prior to the sale wasn't familiar with the wiring.

 

[iggy]

Good morning. Is the AC getting it’s current from an inverter ? If so I suspect the batteries are being drained to a point where they can’t supply enough current to the AC system so it shuts down. If the system works ok on shore power then it’s likely a voltage issue. Does the control panel show a code when the AC quits?
Karl

Thanks for the reply, Karl.

I believe I have the unit that is true 12v. I don't believe the control panel has any information displays - just 2 knobs.

Some older threads here say that some people that have the 12v unit are able to use it underway - which is why I wondered if somehow my a/c batteries are not connected to the alternator - perhaps because the previous owner did't have them installed and the technician that put them in prior to the sale wasn't familiar with the wiring.

I mean this in the best possible way. You need to know what powers the AC unit. I am thinking to that its being powered from a inverter too. BUT know matter which way you need to do the math. How many amps in D.C. is the unit pulling. How many amp hours are the two batteries? Once the math is done, that will tell you how long the batteries will last. What type of batteries are you using would be helpful too. Flooded should only be brought down to 50%, AGMs can go as low as 40% which changes your run time.

The mechanic says the batteries are good. But I bet he comes hours after your back and on shore power with the charger on.

 

[lproulx]

It is important to know for sure if your unit is a true 12V, or 110V run from an inverter. An easy way to tell is to look at the cooling water pump under the left side of the V berth. It will be clearly marked with a voltage rating.

If your system is 12V, connect a volt meter (cheap digital multi meter will do) to the AC batteries and watch the voltage as you go along. If it drops rapidly down to the 11 V range, they are not being charged properly from the engine. They will not run the AC much longer than 30 min without being charged. The engine alternator on my Volvo engine is 115 amps and can easily keep up with the AC load and anything else on the boat. If you have the Yanmar, it must be similar.

If you find there is a charging problem, check out the charging setup. It may be a problem with the so called automatic charging relay, discussed at length in other threads. In any event, you must fix things so the AC batteries charge properly from the engine.

If you have a 110V unit, most of the above holds true. The difference is that the system run by a dedicated inverter mounted on the area under the sink. If the battery voltage holds up OK, it implies a problem with the inverter. The inverter may be defective. It has a cooling fan. If the fan doesn't work, it may be overheating after 30 minutes or so and shutting down. Basically, good 12V at batteries and no 110V at unit = defective inverter.

To put all the above in perspective, the system draws about 400 watts of AC power, which will translate to probably 40 amps of 12V, factoring in the efficiency of the inverter.

I have messed with my 21EC AC extensively. Feel free to PM to talk further.

Larry

 

[iggy]

I forgot about the Alt. But why does the AC unit have its own bank? Why is it not connected to the house bank?

A 6000 btu unit from defender will pull 4.6 amps at 120V AC. Which means that it will use roughly 46 amps DC. Now add the inverter loss and its higher. https://www.defender.com/product.jsp?na ... id=1848266

If you want to quickly see if the AC bank is charging. Take a voltage reading of the batteries with everything off. You should read about 12.8 to 13.2 volts. Start the engine than turn the AC on. Than do a voltage check between the house back and the AC bank. Both should be reading about 13.8 volts. That higher voltage is the charging voltage from the ALT. If the AC bank is lower check to see if there is a ARC and that it is closing.

But pulling over 50 amps on one unit and what ever else (VHF, GPS, lights, sounder, AIS, fridge) is on is a lot of work for a 115 amp alt.

 

[lproulx]

They put in the two AC batteries with the somewhat optimistic idea that you could actually run the AC in a meaningful way off the batteries alone. You can't. They really just provide a little buffer while the engine may be idling or off for very brief periods.

Re: Load on 115 amp alternator - The other stuff mentioned draws only a few amps per item. The fridg is remarkably efficient - Nova Kool says only 40 watts. If you turned on every single DC load on the boat I doubt it would exceed 60 or 70 amps, so you should be all set with the engine running at cruise RPMs.

Another consideration in all of this that we haven't talked about it that the 21EC has only a 20 amp battery charger. When you hook up shore power, you aren't really providing power directly to the AC. You are just charging the batteries. On shore power with the AC on you will be drawing at least 40 amps out and putting only 20 back in. It can go on for a while, but the batteries will eventually get discharged to the point where the inverter will shut off.

Larry

 

[ohioan55]

You should have 2 charging relays, one for the house battery and one for the AC battery. You need to make sure you are charging the AC batteries underway. My Dometic unit has a Prowatt 1000 inverter for a pump that needs to run to circulate cooling water for the AC unit. Yours may be 115V or 12V, not sure on this year of boat, but it must run or the AC will shut down when it gets hot. You should get a continuous discharge on the port side while the cooling pump is running. The Dometic 3500 draws 29.2 amps.

 

[iggy]

The whole concept on how this is set up to me is just nuts and please don't be upset with me . Why not just have a 120V unit with a genset.

A genset would (if not underway) would use less fuel and power other devices. The A/C batteries would not be needed and when back on your slip just use shore power. A simpler setup. A genset can be used for so much more.

Your on the hook, run the A/C, make coffee, recharge the batteries and make hot water.

 

[Capt’nKarl]

That’s exactly why I replaced the original 12 volt DC air conditioner unit on our 21 EC with a 115 volt AC air conditioner and use a Honda 2200 ie genset to run the air conditioner anytime, anywhere and I’ve done the same thing on our R-23.

 

[DMcD]

That’s exactly why I replaced the original 12 volt DC air conditioner unit on our 21 EC with a 115 volt AC air conditioner and use a Honda 2200 ie genset to run the air conditioner anytime, anywhere and I’ve done the same thing on our R-23.

On the R21, where did you put the Honda 2200?

 

[CaptZach]

We place the generator on the swim deck, tied securely to the platform/railing. We have a generator form Harbor Freight, runs our AC while we are underway. Used it about 6 hours straight and barely used half gallon of gas.

 

[Capt’nKarl]

DMcD. You might not be ready to swap out your 12 volt AC unit for a 115volt unit at this time but I did the whole job on my 21 many years ago and it’s probably the best upgrade I ever did. The hard part has been done, you already have the thru-hull. There’s an old thread on this forum where I listed step by step instructions and I’d be glad to answer any questions you may have.
Stay cool,
Karl

 

[DMcD]

I want thank everyone posting here for all the great information and suggestions.

I gathered some data today - but I'm not sure it points to a solution yet.

First, the hardware ID: I was wrong about the 12v (in my defense, the bundle of manuals and paperwork included Dometic 5000 BTU booklet). Turns out it IS the Dometic 115/60 3500 BTU unit that everyone thought it was.

I started out by cleaning out the sea strainer and just running the AC while still plugged in to shore power. I get a good stream of water from the discharge and the fan blows and the AC cools - for about 10 minutes. Then the Dometic shuts down.

I measured 13.2v on all 4 batteries before turning on the AC. Upon turning on the AC I measured again (Still plugged in to shore power) and got 13.2v on start and house and 12.3v on both AC batteries.

Next I unplugged and started up and went for a ride. While underway I turned the AC on to see what would happen and it came on. Same deal - ran for 10 minutes and shut down. Checked the AC batteries and they were at 12.9v.

This last thing makes me think it's not a charging issue - surely 12.9v is plenty to run the AC?

Finally, back in the slip again I ran the AC briefly just on batteries so I could hear better. After manually turning it off I could hear a fan running under the sink for a brief time after I switched to off - presumably the cooling fan on the inverter.

So, does all this lead anyone to figure out whats going on?

 

[lproulx]

You seem to have covered most of the bases. This sounds a lot like an inverter problem. It's a little work,k but a project you can do is remove the inverter (take stove out and work through opening) disconnect battery wires and unplug from AC unit.

Remove the inverter. You will find five wires coming out of the inverter, white, green, and black, just like in the house, and two blue ones. Cut the wires between the inverter case and the connector. Extend the cut white, black and green wires on the system side with a little three conductor cable and put a plug like you would use to make an extension cord. Ignore the blue wires.

Now, if you plug the new power plug into an extension cord to shore power you will be running the AC directly off shore power without the inverter. If it runs properly, you have a bad inverter. You can replace the inverter with one that has a built in outlet and just plug your unit into it for under way operation, or directly into a shore power cord. An inverter with the built in outlet can also be used to run other stuff on the boat.

The only operational change is that you have to remember to turn the new inverter on and off as needed. That's what the blue wires used to do with the original inverter.

Larry

 

[DMcD]

I took some more data today...

AC batteries started at 13.7 volts with the charger running. I made a long cord with alligator clips at both ends so I could monitor the battery voltage from the cockpit.
• Unplugged and started motor and turned on AC – AC battery reads 12.48 under load
• Drove boat and AC quit after 10 minutes – voltage had dropped a little to 12.45
• With AC off voltage climbed back to 13.8 at cruise
• On return trip with AC on, voltage dropped from 13.8 to 12.9 before it quit, left knob in on position
• Shut off motor and was tying up when it came back on by itself with voltage still at 12.8

I wonder about the Automatic Charging Relays – which are supposed to put the batteries in parallel for charging but separate for discharge. Could they starve the AC for current if their switching threshold is too low?

Or is the inverter malfunctioning or overheating? I can hear the inverter fan running for several minutes after the AC shuts itself off.

 

[lproulx]

I think the inverter is shutting off, not the AC per se. If the AC shuts itself off for some reason, the cooling water pump will continue to run. If everything shuts off, it really points to the inverter. The fact that the fan continues to run means it may have overheated and is trying to cool itself off before re-starting.

 

[DMcD]

I think the inverter is shutting off, not the AC per se. If the AC shuts itself off for some reason, the cooling water pump will continue to run. If everything shuts off, it really points to the inverter. The fact that the fan continues to run means it may have overheated and is trying to cool itself off before re-starting.

So, if I do the inverter removal experiment you described a couple of posts ago and the AC runs fine directly on shore power - is there a particular inverter you would recommend to replace it (assuming i just want to power the existing AC from the 2 battery bank with alternator backfill)?

 

[lproulx]

There are a gadzillion of them on the market. I chose a Gandel PM-1600KAR, presently available on e-bay for a little over $100. It is 1600 watts - I figured I would allow for a lot of extra capacity so as to not over stress it. It has a couple of useful features - there is a digital display that alternates between showing the input dc voltage and the output power in watts. It also has a remote on-off control with a long cable, so you can turn it on and off from a remote location. Part of my big AC project was to actually move he AC unit over under the radio. The inverter fits perfectly, mounted against the left side of the space under the sink, leaving lots of storage space where the AC and inverter used to be. That probably won't matter to you.

 

[DMcD]

followup post:

A sincere thank you to Larry for sharing his experience and advice. I successfully followed his advice in 2 steps:

1. Cutting the wires between the Dometic dedicated power supply (inverter) and splicing in a standard high amperage 3 prong plug. Connect the new plug to shore power and test the A/C to find that it will run for as long as I had patience to run it in the slip.

2. Pulling out the old inverter and replacing it with a new 1600W inverter. Plugged in A/C to new inverter and ran while underway for as long as it took to return to home port.

It remains to be seen if the Dometic A/C has enough capacity to cool the pilothouse during an Alabama summer, but at least it is now reliably functional and providing cooling during an Alabama autumn.

Thank you for all your help!
DMcD