Cruisair/Dometic SXT 16 BTU with SMX Panel switching from cool to heat

[zboss]

Folks,

Our reverse cycle AC had been working fine for the summer but recently its been automatically switching over to heat even though its manually set to cool.

The compressor will run fine for a while pumping out cold air... the compressor will turn off and on cold a few times like its working normally but then for some reason during an on cycles, it'll quit and the system switches over to heat. The only indication is that the panel quickly flashes "dash dash" and then goes back to the temp reading. No error code.

We have a second system that is running fine and they share a common saltwater pump and groco filter, which I just cleaned.

[Sailmonkey]

If I remember correctly the cruises or systems shuttle the reversing valve prior to starting to unload the system and lessen the startup surge.

I suppose it’s possible that the reversing valve is getting sticky and sticking on the heat side of the spool occasionally.

You may be able to electrically disconnect the valve and see if that help/hinders anything.

[zboss]

I think I traced it to poor waterflow. Either the water pump is getting old (its 12 years old I think) or the lines need to be cleaned. I turned off the 2nd AC unit and now the problematic AC unit seems to be functioning fine.

I guess I will put in a new pump (who knows when that pump will give up the ghost anyway) and I'll clean the lines with barnacle buster.

[Coolerking]

Pump relay trigger?
Poor water flow/ fouled condenser will usually show HI-PS fault on SMXII control.
Does this happen when you run the genset?

[zboss]

Apparently the double dash means "lost gas". Its the very last thing at the bottom of the directions and in teeny tiny print. It worked all day yesterday and kept it at 68 degrees while it was in the 90's and sunny outside.

I don't know why it just doesn't shut off and decides to switch over to heat. Then the sun went down and it started doing the switch to heat thing again.

We haven't run the genset on this boat yet... its brand new to us two weeks ago. Only ran it at the seatrial.

[Brewgyver]

Originally Posted by Sailmonkey

If I remember correctly the cruises or systems shuttle the reversing valve prior to starting to unload the system and lessen the startup surge.

I suppose it’s possible that the reversing valve is getting sticky and sticking on the heat side of the spool occasionally.

You may be able to electrically disconnect the valve and see if that help/hinders anything.

Generally, heat pump units default to heat, and must have the reversing valve solenoid energized to switch to cooling. When the call for cooling ends, the solenoid is de-energized, and the valve switches back to heating mode. This is what produces the "whoosh" sound of the circuit unloading when the compressor cycles off. Cooling only units don't make that sound, and the circuit equalizes slowly through the capillary or TXV.

[Cadence]

I'd have to think sailmonkey may have hit in something. Reverse cycle solenoid valves are notoriously prone to hanging up.

[.Paul.]

Originally Posted by zboss

Folks,

Our reverse cycle AC had been working fine for the summer but recently its been automatically switching over to heat even though its manually set to cool.

The compressor will run fine for a while pumping out cold air... the compressor will turn off and on cold a few times like its working normally but then for some reason during an on cycles, it'll quit and the system switches over to heat. The only indication is that the panel quickly flashes "dash dash" and then goes back to the temp reading. No error code.

We have a second system that is running fine and they share a common saltwater pump and groco filter, which I just cleaned.

Some of the control panels for Dometic-Cruisair air conditioners have a De-Icing Cycle. This can be enabled (or not). Here is a description of the feature that came with the units on my boat.

P-8: De-Icing Cycle
The de-icing cycle prevents ice build up on the evaporator coil during extended periods of cooling operation. Installation
variables such as grille sizes, length of ducting, insulation, and ambient temperatures determine the run time required to
achieve set point. Factors that substantially increase run time include operating the system with hatches and doors open, and
programming an unrealistic set point (e.g. 65°F/18.3°C). Such situations can cause the evaporator to form ice on warm humid
days.

• For software revision A13 and older, de-icing is accomplished by switching the reversing valve into Heat Mode while
the system is cooling. The valve remains energized for the programmed cycle time. The cycle is programmable to
“OFF” or to a period of 1, 2, or 3 minutes.

• For software revision A21 and newer, de-icing is accomplished by closely monitoring the room air temperature in 10-
minute intervals during a cooling cycle. Depending on the parameter value and the change in room temperature during
these monitoring intervals, the control performs various actions to prevent ice from forming or to melt ice that has
already formed. This is accomplished by short compressor shutdown periods combined with a one-speed increase in
fan speed, and by periodic Heat Mode cycles with the fan turned off.

I tried the de-icing, but I turned it off because I kept the hatches closed while air conditioning is in use.

You should be able to download the programming instructions that apply to your system from Dometic.


Paul

[zboss]

I know this is odd but it occured to me that maybe the IR receiver was getting stray IR either from other remotes or from maybe another boat. I covered the panel with tape and its been going strong for 24 hours without any hiccups.

[Brewgyver]

I think you have something else going on. Check your heating and cooling setpoints, you should have a deadband of at least 4 degrees, 5 or 6 would be better. A good controller won't allow overlapping setpoints anyway.

If you have this deadband, and the compressor was running in cooling mode, even IF the control received an errant signal to switch from cooilng to heating, the sensed temperature should be well above the heating setpoint, and the unit would simply shut off, awaiting a call for heating.

It would be helpful to know what kind of controls you have. The next time it switches, look at the thermostat, does it actually show that it's in heating mode?

[zboss]

Originally Posted by Brewgyver

I think you have something else going on. Check your heating and cooling setpoints, you should have a deadband of at least 4 degrees, 5 or 6 would be better. A good controller won't allow overlapping setpoints anyway.

If you have this deadband, and the compressor was running in cooling mode, even IF the control received an errant signal to switch from cooilng to heating, the sensed temperature should be well above the heating setpoint, and the unit would simply shut off, awaiting a call for heating.

It would be helpful to know what kind of controls you have. The next time it switches, look at the thermostat, does it actually show that it's in heating mode?

Yes, it will show either HEAT or show COOL and HEAT simultaneously. I've not turned on the automatic switching and I have tried a factory reset. Its an SMXIR

Yesterday I turned off the TV, which is right next to control panel, and the AC turned off also.

[Brewgyver]

Sounds like you have a bad controller, it should never show Cool and Heat simultaneously.

[Coolerking]

FYI pressing Cool and Heat on SMXII is dehumidify mode.
Sounds like the IR in your SMXIR is reading signal from the remote.
The sensor is just below the first digit in the display, a small piece of black electrical tape will prevent stray IR signals from messing with it.
Do you have the DX manual?

[zboss]

I ended up covering the panel with tin foil and only take it off to change the temp. It's been a week and no problems. It's definitely interacting with the TV remote.