Odd behavior of Dometic AC/Heat unit

Dr. D

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Nov 3, 2018
272
Beneteau Oceanis 35.1 Herrington Harbour North
My Oceanis 35.1 has two Dometic DTU12 AC, reverse cycle heat units, one in the main cabin and one in the aft cabin. The one on the aft cabin works fine. The one in the main cabin is quirky (rest of post is about that unit). In the main cabin there is a small weather station with thermometer and that is at shoulder level. The thermal probe for the unit is at ankle level.

The unit is in heat mode as it is getting chilly here on the Chesapeake Bay. What happens is the control unit or thermal probe get hung. For the last two nights I set the unit to 66 F. About 3:00 AM I wake up as I am cold. The Dometic control unit has the temperature at 74 F while the weather station reads 60 F. It feels like 60 F. I increase the setting on the control unit to 78 F and the unit starts up. Within less than a minute the temperature reading on the control unit starts dropping until it reaches 64 F.

This morning I fiddled with the control unit and I could reproduce the behavior, with it getting hung on a temperature and not reflecting the actual temperature until the unit turned on.

Anybody see this behavior on other units? Recommendations?
 
May 17, 2004
5,028
Beneteau Oceanis 37 Havre de Grace
Does the fan continue to run when the compressor is off? If not, it may be that the location of the temp probe needs airflow to get a good reading.
 

Dr. D

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Nov 3, 2018
272
Beneteau Oceanis 35.1 Herrington Harbour North
The fan only runs with the compressor. The probe is 3" from the intake grill, so it should be getting ambient air OK, but that is a consideration.
 
Mar 20, 2016
594
Beneteau 351 WYC Whitby
If you have the stat in heat mode and the temperature is dropping ,I would guess it is still in A/C as the reverse solenoid is stuck and is common .My manual clearly states to switch it into heat mode every so often to prevent this from happening . With the unit on give a tap on the solenoid with the handle of a screw driver and see if that frees it up
 

Dr. D

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Nov 3, 2018
272
Beneteau Oceanis 35.1 Herrington Harbour North
Mechone: Thanks, but I have been putting the units into heat mode every few weeks.

So the unit in the aft cabin works fine and the one in the main cabin doesn't. Before I left the boat today I examined both installations. The big difference between the two is the unit in the main cabin is under the port settee and about one foot in front of the unit (but also under the settee) is the hot water tank. I turn on the electric, shore power, heating of the water. Now I wonder if at night the water in the tank cools, the electric heater turns on. The heating element is inside the tank which is insulated, but perhaps the connection point to the tank heats up. The space is small enough it wouldn't take much heat to add a few degrees to the local air.

Next time I am there I will leave the electric water heater off and see if that makes a difference.
 

Dr. D

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Nov 3, 2018
272
Beneteau Oceanis 35.1 Herrington Harbour North
Yeah, the hot water tank is the culprit. With the heater turned off and the control unit set to 58 F, everything is about the expected temperature. The hot water tank is about 57 F on all surfaces. Turn the water heater on. About 30 minutes later the pipe for the outflow of the hot water is 80 F. That would be enough heat to warm the area around the thermal probe.
 

SG

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Feb 11, 2017
1,670
J/Boat J/160 Annapolis
Unless your heat pump, i.e., reverse cycle AC unit doesn't care about the hotwater heater's temperature. It used the water flow of "raw water" to exchange the heat. (i.e., you have have water source heat pump).

The reversing valve might be an issue, but usually that says in the mode it was in when the compressor turns off -- until the controls instruct to switch from heating to cooling.

I'm really not sure what a "Weather Station" thermostat is? If that's solely for information (and has nothing to do with anything), then I'd focus on the one above. It's possible, in cold ambient outside temperatures, your nicely ventilated Beneteau just needs more heat. (Our main cabin's AC unit has to be turned up higher to maintain balance air. When we run the Espar, the discharges for the heaters are closer to the cabin floor, and it is more comfortable. The AC discharges tend to be higher (as does the thermostatic control) because "cold air settles" and the issue is usually cooling -- not heating.

QUESTION: When you awaken and notice the temp is not warm enough, if you turn up the thermostat, does HOT AIR COME OUT? If that's the case it may just be you need to set it higher to be comfortable. If not, and the "heating mode" light is on and the thermostat is calling for hot air but none comes out, then you have either a control issue (e.g., bad thermostat or board; or, you have a bad reversing valve.).

So what is going on?
 

Dr. D

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Nov 3, 2018
272
Beneteau Oceanis 35.1 Herrington Harbour North
The AC/Heat unit is in a small space along with the hot water tank. When the hot water tank is heated (by either shore power or the motor) the pipe which the outlet for the hot water gets hot; finally reached 95 F. That heats the space and the temperature sensor which is in the space inside of the air intake (as the owners manual says it should be). So the cabin cools off but that space stays warm. I need to put in some insulation to stop the heat transfer.
 
May 17, 2004
5,028
Beneteau Oceanis 37 Havre de Grace
There may be a setting on the thermostat to just keep the fan on all the time. In the summer that’s a good way to keep things feeling cooler even when the compressor isn’t running. In the winter it wouldn’t feel great to have a draft but it would help with that problem.
 

SG

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Feb 11, 2017
1,670
J/Boat J/160 Annapolis
On our MarineAire (now Dometic) the fam is always running when the units are on.
 

Dr. D

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Nov 3, 2018
272
Beneteau Oceanis 35.1 Herrington Harbour North
There may be a setting on the thermostat to just keep the fan on all the time. In the summer that’s a good way to keep things feeling cooler even when the compressor isn’t running. In the winter it wouldn’t feel great to have a draft but it would help with that problem.
Yes, that is a fix, too.

I thought of another fix which worked last night. On the nav table on the starboard side I placed a small fan switched on low (nearly noiseless) and pointed it at the air intake on the port side where the sensor is. The heat behaved as desired since it was sensing the real temperature of the cabin.
 
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