Fan speed issue on Mitsubishi

[bemak187]

Hi. Looking for some help. Got a Mitsubishi srk45zsp-w split system. When on cooling with the fan set to low or auto after 20 - 30 mins the fan speed jumps up to medium speed even if still set on the remote as low. Its an instant jump not a ramping up. No way to get it back to low speed other than select a mode other than cooling then switch back or turn the system off and back on. However happens again after another 20 - 30 mins. Also as soon as the fan speed jumps the air flow button on the remote will only allow the highest position or swing it wont go into any of the other positions. System is brand new. Any ideas?

[frank]

Self protection or anti dew mode kicking in?

[bemak187]

Given that it is fairly consistent in the behaviour I am assuming it is doing it for a reason. However it never goes back down even if left for days and the weather changes from hot to just average temperatures outside and the same inside, over a range of temperatures. You have to put the system off or move to another mode and then back to cooling but it will happen again. The issue is it is in a domestic setting so the noise from the fan is very intrusive. There is a srk35zsp-w (less powerful unit) elsewhere in the property and it doesn't do this.

[Brian_UK]

Yes, looks like a dew mode effect.

See page 8 of this manual...
https://www.google.com/url?sa=t&sour...NwuN1wCeq5X5a7

[bemak187]

Thanks Frank and Brian. Yes looks like dew mode, behaviour is identical to what is described. The 45 seems to have a lower threshold for the dew mode if I am reading it right hence why the 35 isn't doing the same. Is there any way to disable this or is it hard coded into the system? As the system is in a residential setting the fan is too loud when this is kicking in and far too strong.

[bemak187]

Any ideas if this can be disabled or is it hard coded? The system is basically like this all the time when on cooling. Or would it be better to replace the system with a 35 similar to what is fitted elsewhere in the property and is working perfectly? That unit going off the specs has a much higher threshold for the dew protection kicking in.

[frank]

I would first check that the problem is not caused by a refrigerant side issue.

As it's a new install, the first call should be to the installing company.

[bemak187]

Thanks Frank. Sorry wasn't clear in my earlier posts. The installer has already been back out and actually replaced this system like for like with another 45 right at the start at though first one was faulty. New unit doing exactly the same and has been since it went in. Installer coming back to look into it further however I am assuming given that this looks like the dew protection mode that this is just how this system works. Going on the specs the 45 needs 60% humidity and that's it where as the 35 needs 68% humidity plus a specific compressor speed for this mode to kick in. The 35 I have elsewhere in the house has worked perfectly since the start. Its not that the 45 doesn't work in that it cools fine its just we have zero control over the fan speeds and its incredibly noisy. 60% humidity isn't that high for the UK in summer so potentially the unit will just always run in dew protection mode when in cooling hence would it be better swapping it for a different model?

[frank]

If the installer is due back to have another look, ask him to take the following readings both before it ramps up and then after the fan speed increase so that these can be compared against the data in the service manual:

Indoor coil temperature
Indoor air temperature
Indoor humidity
Outdoor temperature
Compressor frequency
Suction gas pressure

What position have you set for the swing flap?
Does the same problem occur when you select 'Econo' mode?
How long is the pipeline between indoor/outdoor units?

Sound pressure level	Indoor	Cooling (Hi/Me/Lo)	44/ 39 / 24
Heating (Hi/Me/Lo)	48 / 41 / 30
Outdoor	Cooling/Heating	51/ 51

The fan sound levels do not look high (compared to some)

[bemak187]

Thanks Frank. I think this is what he is coming back to do. Same thing on Econo, when I select it the fan doesn't change. Before this kicks in I can select anything I want. However once this kicks in if I select auto, low or econo nothing changes. I can put the fan up to high or the high power setting but will not go below approx medium speed. What I haven't done is try starting the unit up on econo before the suspected Dew mode kicks in. Once it ramps up the flap will only allow me to pick swing or the highest/1st position. I have it on swing at the moment. Pipeline is fairly short, about 4 feet as its just the other side of an exterior wall. Noise level compared to the other unit I have which doesn't do this I would say is at least 4 or 5 times as loud and just sits in this suspected protection mode continually (ie for days and weeks without resetting).