Mitsubishi Lossnay+GUG aircon (not actually working)

We have a bit of a mystery here - the brand new, expensive, ducted air-con in the house does not work!

So what does not work exactly?

Not actually able to cool a room!

The biggest issue is it cannot cool a room - not even close. (It can't heat one much either)

This is a temperature plot for my study, 3m x 3m, you don't get a lot smaller than that in terms of a room to air-con. A=Portable unit on, B=Portable unit off, C=Start two hour cool only full fan test.

The test was with the other room vent closed so the unit was only cooling one room, in to a closed empty room, no heating on anywhere in the house, and outside temp of 19C. Not a challenge for any air-con really, and as you can see, the cheap portable unit managed with no problem (even with the door open most of the day). And yes, the fan speed control does work, and was on full.

I expected at point C the room to cool at least as fast as the cheap portable unit. What happened is it just about managed 0.8C in two hours! It was blowing cold air (measured at 9C) but not as lot...

The fix - well, the unit only has a 100mm vent in to the room. This seems small I must admit, and at full fan like this it is noisy. So they are changing to a 150mm vents today...

Did it work? In short, no. The bigger vents are quieter, which is nice, but the same tests showed maybe 1.5C drop and bottoming out just below 21C after more than an hour. Compare these big 3.5kW systems to a much cheaper 1.5kW portable unit which is able to drop my study (and hall way as door was open) from 23C to 18C in 15 minutes. With the door open the new air-con cannot cool my room at all!

This is now waiting on Mitsubishi to explain and fix. I won't say who the installers yet as they have been pretty good. Let's give them the chance to get Mitsubishi to fix it all.

But how is this even possible?

The issue for me here is that they sell this system, even with the smaller 2x 100mm outflow vents. So forgetting, for a moment, whether the suppliers got it wrong for the rooms we are trying to cool (two rooms each unit, one of which is much bigger than this 3m x 3m study), we have the issue that this whole unit running at full cooling and full fan cannot cool a 3m x 3m room.

So what the hell is the this unit for - what size room are they expecting it to be used with, ever?

The change to 150mm vents actually meant them cutting out the larger holes as the cowl as it is only supplied with the 100mm outlets.

This leads me to think something else is wrong, but we have no idea what. How could this ever work anywhere?

Cannot set temperature!

The controller has options - either "return air" temperature measured at the lossnay unit, or the room temperature at the controller. The controller has a sensor in it. Given that we have two rooms per unit, we are using the temperature at the controller (which is not in the room with the return in it). This is all as per the manual.

Only it does not work! Cooling the room down to 19C (using the portable unit, obviously), it showed 24C. Indeed, it even went up from 23C while we did this. Using freezer spray actually on its sensor does nothing, still 24C. No clue where it is measuring but it is not in the room - we even tried putting hot air in the room that has the return air flow but that did not make it go up at all.

The fix: Well, again, waiting on Mitsubishi.

Watch this space.


  1. Lossnay and GUG are ventilation units, not air conditioning units. The GUG is just cooling air for ventilation, ie give you fresh air.

    You don't seem to have any aircon at all, as that would need a ceiling concealed ducted unit in each room (the inverter that normally is put in each room) that's also driven by the compressor outside as part of the multisplit system.

    Ventilation cooling is literally a fresh air device.

    1. This has an outside unit, and a 3.5kW GUG inside unit, more than enough power to cool the room. It has a controller that lets you set cool, heat, auto, or fan, and lets you set a target cooling down to 19C (oddly high, but OK).

    2. If you have a reference that says this is not intended to cool or heat a room, please do let me know, as that was my specification - aircon with a fresh air feed as well.

  2. I was under the impression that the lossnay + gug was intended as a solution used along side one of their traditional indoor units to provide conditioned outdoor air to an otherwise sealed space? Certainly in large installations I've seen of lg kit; they have both conditioned outdoor air via an energy recovery device AND traditional ceiling or wall mount units. I would expect it to be difficult to cool a room with just outdoor air...

    1. Well the requirement was a vented rather than wall mount internal unit, and fresh air. This is what the suppliers spec'd up for it. Reading the manual, it does look like this should, in principle, work. It has the power to cool and heat air, and claims to have the airflow to do it. The target temp should be possible to set at the controller. So should just do what we want. Ultimately if they cannot make it work it would perhaps mean a lossnay on its own just to circulate and provide fresh air, and separately a ducted air conditioning inside unit in the loft as well. Hopefully now, as means two more vents in each room.

  3. What's the cooling KW on the GUG. Sounds like two issues here:

    1. The GUG is undersized for the areas you are trying to cool.

    2. The system you look to have is more aimed at commercial installation. Mitsubishi have a residential version of that setup. I suspect that's why the controls you have are crap - in a commercial setting they would be linked to a building wide controller of BIMS

    1. If they have a residential set up, that would be great. The unit says 3.5kW I believe. That should have no trouble cooling a small 3m x 3m closed room in the middle of winter!

  4. Do you know what model of GUG you have ? I'm not sure if the domestic Lossnay's are designed to have a DX unit to provide heating and cooling

    1. I can probably find out - but right now trying to rest and it would mean going in to the loft. Everything on hold for 10 days quarantine. But we'll find out what the installers are planning I am sure soon.

    2. Oh not - not COVID :-(

      Will keep and eye on here for updates


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