Mitsubishi Mr Slim cool air problem

[RGS]

I've had a Mitsubishi Mr Slim MXZ-FE18NA heat pump for several years now as my main heating in Nova Scotia and in general I'm quite happy with it even though I live in an old farmhouse without the best insulation and sealing, but it does have one problem that is annoying and appears to be a programming mistake. With the vertical vent on auto and moving up and down from time to time as it blows heat into the room it will not infrequently blow what feels like cool air on people sitting below it. I've been told this air is still warmer than the temperature of the room so it's still heating the room, but it's cooler than the human body (anything between 70 and 98) and so feels decidedly cool and uncomfortable to anyone sitting below it. Of course, one can manually adjust the vent to the horizontal position but this is no solution because it would require adjusting the vent numerous times every hour depending on the temperature of the air being delivered. I think it should never blow air downward when it's below human body temperature. It should be possible to change this with a programming update, but I don't know if this is available. I've written them.

But this is one think to keep in mind if you're in the market for a heat pump. I don't know if the other Japanese makes have this problem.

 

[Dana]

During defrost it will be blowing at colder than room air temps, since like all mini-splits, the FE18 reverses the cycle into air conditioning mode (at the lowest blower speed) to heat up the exterior coils to clear them of frost.

Defrost cycles happen most frequently when the outdoor air is 0-10C and has much higher moisture capacity. They become much less frequent when it's cooler than -5C outside.

Generally speaking it's better to place the ductless head where it won't be blowing directly on people, if possible. In heating dominated climates (I'd say NS qualifies) it's sometime even better to place the head closer to the floor, where it pulls in cooler air, which improves both capacity & efficiency, and blows on your feet, not your face.

Since they were initially designed with air conditioning in mind they usually get mounted near the ceiling, which is great for air conditioning efficiency since it's pulling in the hottest air in the room, for the biggest delta-T on the coils. Most installers put them up high, even in heating dominated climates, which isn't best-practice. Most manufacturers also make floor-mount heads which work pretty well in heating dominated climates, less well in air-conditioning mode.

 

[Sponsor]

 

[RGS]

Thanks for the reply, Dana. I'm familiar with the defrost cycle and I'm not referring to that. The Mitsubishi closes the vents when it's doing that so it doesn't blow any cold air in. Your comments on the placing of the indoor unit are quite interesting. I always wondered why they would be placed so high when they were primarily for heating but I went along with the installer's (and manufacturer's) recommendation and let them place it high in the room. It's quite pleasant when it's blowing warm air on you but less so when blowing cooler air.

Perhaps one reason why they recommend a high placement is that there won't by any obstructions to air flow high on the wall while there will be when it's close to the floor.

But maybe I should look into getting the unit moved closer to the floor. There's room under one window.

 

[Dana]

I'm somewhat familiar with the FE18 (having set up one of my older relatives with one, and seeing them used as the sole heating source for a number of higher-R houses in my area.) If the air is running cool when NOT in defrost mode the refrigerant charge or some sensor might be off. It usually feels like a warm summer breeze, with 40-45C output air. If the tepid-air output is a new behavior it's probably worth getting it tested by a qualified tech before the heating season takes off in earnest.

I confess that the notion of mounting height making a difference was a recent epiphany for me too. One of the specified test conditions for HSPF testing in the US (at which it's efficiency and capacity is specified) is 65 (18C ) intake air, which isn't normally found near the ceiling in a 20C room, but WOULD be found under a window sill(!). The cooler the intake air, the better the lower the temperature difference between the interior & exterior unit coils, resulting in higher capacity & efficiency. The fact that removes wind-chill when the exit air is lower temp would be another benefit.

 

[Reach4]

Can you have the unit direct the air differently?

 

[Reach4]

Of course, one can manually adjust the vent to the horizontal position but this is no solution because it would require adjusting the vent numerous times every hour depending on the temperature of the air being delivered. I think it should never blow air downward when it's below human body temperature.

Horizontal all of the time does not seem so bad if the air in the room mixes enough.

 

[Beads]

Is this a model with I-see? They way I understand that, it is supposed to pick the area needed warming or cooling the most and point the air at that. If it is I-see, turn that feature off and see if that fixes it.

 

[Dana]

The FExxNA series is about a decade old, and does not have that feature (which didn't exist prior to second generation 0f the FHxxNA series.)