Mitsubishi mini split frost on outside coils

soapgirl13

New Member
Dec 18, 2019
1
Pennsylvania
I hope I am being overly concerned. We had freezing rain, cloudy , damp, cold day. I have heat as normal, no flashing indicator lights, but because
I had an issue last season with the polar vortex and Icing up of the outside unit, I had to check ...it had a bit of clear ice on top and on the coils.
I turned it off, went to work. I got home, ran the AC and watered it with warm tap water to defrost it as it didn't melt fully during the day. I started it back up and it runs fine, as far as I can tell ( I have heat). I checked this morning, temp outside just at 32, it was frosted up again. Temp in room is as usual. Should I be concerned?? No flashing on the indicator light. I left it running. How come the defrost mechanism is not totally defrosting the coils?? The unit is 16 years old, but has always worked without issues. Check up by hvac was good in early November. Is this normal and I should stop worrying???
To be honest up until last year I really never took notice of the outside unit. I have only gone by the indicator light and if I had heat.
 

Z33

Burning Hunk
Apr 14, 2014
218
Atlanta, Georgia
It is normal to have some icing. The unit should go through a defrost cycle to melt the ice off the coils.

If it's not doing this it could be an issue with the defrost control board in the unit.
 

snaple4

Burning Hunk
Dec 18, 2017
217
AR
If excess ice buildup is happening on a 16 year old unit then a few things could be going on.
1: malfunctioning defrost control board and sensors.
2: malfunctioning reversing valve (most default in heat)
3: dirty coil
4: low on refrigerant

do you have a picture?
 

Highbeam

Minister of Fire
Dec 28, 2006
16,861
Mt. Rainier Foothills, WA
Kinda surprised you got 16 years out of it. Don't spend too much money fixing it. The new ones surely have improved.
 

rwhite

Minister of Fire
Nov 8, 2011
1,610
North Central Idaho
Have a similar problem with minis that we have on a new office building. We had to install them for our IT/electrical room to keep the servers at 70 degrees all year. Which means they are usually running in ac mode or are off. Rarely do they have to heat. The problem is that the units are sized properly to heat and cool the room but are to small to over come below zero outside temps and defrost properly. Thus the unit that is exposed to the prevailing wind will often freeze up and quit cooling. Not only is cold wind blowing over the coils its also spinning the fan which is pulling all the defrost heat off the unit. Never happens to the other unit which is blocked from the wind by a building. I'm told that the proper solution is to get a bigger unit. So far our make shift wind break has helped.
 

peakbagger

Minister of Fire
Jul 11, 2008
5,031
Northern NH
My unit is mounted on the downwind side of a house near a corner. It normally doesnt ice up but if it fine blowing snow it will build up ice on the coil on the side nearest the corner. The rest of the coil is still exposed. If its real cold it will take a couple of days to melt off. The unit is still putting out heat but usually that is the time I am running the wood boiler.
 

snaple4

Burning Hunk
Dec 18, 2017
217
AR
Have a similar problem with minis that we have on a new office building. We had to install them for our IT/electrical room to keep the servers at 70 degrees all year. Which means they are usually running in ac mode or are off. Rarely do they have to heat. The problem is that the units are sized properly to heat and cool the room but are to small to over come below zero outside temps and defrost properly. Thus the unit that is exposed to the prevailing wind will often freeze up and quit cooling. Not only is cold wind blowing over the coils its also spinning the fan which is pulling all the defrost heat off the unit. Never happens to the other unit which is blocked from the wind by a building. I'm told that the proper solution is to get a bigger unit. So far our make shift wind break has helped.
for low temp cooling like a server room you should have a wind guard installed. Surprised it is even running that low. Most have a factory default settings that cuts them off cooling when the outside temp gets below 47ish. The dealer should be able to order it. Could be called a “low ambient kit” as well.