Odd behavior of a Dometic AC/Heat unit

[Dr. D]

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?

[Coolerking]

What type of control display do you have?
Does the fan stop running when it reaches setpoint?
Also, it may be that you have the system in auto mode.
It will run in heat or cool relative to the setpoint vs inside temp as read by the sensor.
The sensor....that's where you start.
If you have a remote sensor then check
that it's not touching anything and its in the return air path.
Some units will also have a sensor in the display itself, which the system will default to when the remote sensor is unplugged from the control board.
Think about the sensor you have and how its reacting to the temps.
Consider the system blind, it only knows what IT'S sensor is telling it.

[Dr. D]

Turns out the problem is the hot water tank is a bit over a foot from the sensor. With the hot water heater 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. I turn on the water heater (shore power) and in about 30 minutes the outflow pipe is at 80 F. Enough heat to keep the area around the sensor several degrees warmer than the rest of the cabin. Insulation and turning off the water heater should fix the issue.

[Dr. D]

I thought of another fix which I tried last night and it worked. I placed a small fan on the nav table which is on the other side of the cabin from the air intake. Switched the fan on low (almost noiseless) and pointed it at the air intake. Heat behaved as desired since the unit now was sensing the temperature of the cabin air.