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Old 12-02-2018, 10:45 AM   #1
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reverse cycle AC

I have two Dometic marine AC unit on my boat.They are both set up the same way.I'm using it for heat now the water is still warm enough.One unit fan is only running at a slow speed again they are both set up the same way on there program any idea what is causing the problem and can I corrected or do I need a mechanic to fix it. Tom
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Old 12-02-2018, 11:02 AM   #2
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Make sure it's not on the "Humidity" setting. Mine (Webasto) only runs at slow speed when set that way..
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Old 12-02-2018, 01:36 PM   #3
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Hi Tomsboat,


Go through all of the setup parameters on both units...one of the parameters is to 'reverse' the fan speeds when in heat mode. If I recall correctly, it is P13. Make sure it is set at the default. Check the other setup parameters as well, they should be the same for both units.



On my Mainship 390, I have two (new) Dometic Marine Air units. The way I have them set up for heat mode (factory defaults), leaves the fan running slower when actual temp is more than 5 degrees lower than your 'set' temp. As the boat warms up inside, the fan speed >>increases<< as the actual temp approaches the 'set' temp. When the difference between actual and set is less than two degrees, the fan should speed up to maximum.


I know this seems counter-intuitive, but there's a good reason for it, which I believe has something to do with the Second Law of Thermodynamics ;-).

So in heat mode (only), when P13 is set for 'reverse' (as it should be), fan speed is inversely proportional to the difference between the temperature you want and the temperature you are at. My guess is that this is what your are seeing.

Hope this makes sense...
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Old 12-02-2018, 01:51 PM   #4
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Oh...and one more thing. If your MS390 is like mine, then the main A/C is 16K btu and the forward unit is 10K or 12K btu, and the different sized units behave differently in reverse cycle mode. For some reason the smaller unit takes longer to start cranking out heat even though they are both sharing the same (temperature) water supply. I'm not sure why this is, but I've seen the same thing on several boats.


I'd sum this up by saying that if your units are different sizes, but both units are making heat (eventually), then they are probably both OK and you can ignore the fan speed difference.
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Old 12-02-2018, 03:37 PM   #5
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AC

both my AC units are set up on p-13 rEF reverse cycle also I have a 16000 btu in the salon which is working OK the cabin is a 10000 btu which is set up the same as the 16000 btu but the fan speed will not go on any speed but slow.It did work OK at one time. Tom
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Old 12-04-2018, 02:10 PM   #6
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What happens with your 10K unit if you let it run for awhile, then bring the set-point down to about 2 degrees above cabin temp?

In heat mode (with P13 at defaults) the fan should >>always<< be running slow, when cabin temp is more than 2-3 degrees below set temp. This allows head-pressure to build up faster and makes for more efficient operation. The fan should then speed up >>only<< when cabin temp gets to within ~2 degrees of set point.


If this is not happening, try manually controlling the fan speed up and down. If this works, then the triac is good. Next, check that your max fan speed parameter is at 95. Lastly, set P13 to 'nor' (normal) and try again. If you still can't get the fan spinning fast in reverse-cycle heat mode (but it will spin fast in cooling mode), then time to call Dometic tech support.


Another thought. If both units have p13 at defaults, it may be that the unit with the fast-spinning fan (your 16K unit) is the defective one. A failed triac can cause the fan to run on high-speed all the time.
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Old 12-04-2018, 03:26 PM   #7
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AC unit

Thanks all I will have to wait to spring they winterized the boat yesterday but I think your right the fan should run at slow speed till it gets up heat. My 16000 btu might might not be right. Tom
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