My Mini Split is in defrost mode this morning.

[Janderso]

I have a one ton Mr. Cool mini split.
It’s in the 20’s outside this morning.
The manual just says, defrost mode.
Anyone else seen this?

 

[Boswell]

I don't know about a mini-split specifically buy the outside coils on my heat pumps for my house can freeze up when the conditions are right. well below freezing and high humidity. I know they have a Defrost mode that will prevent this most of the time. I assume it heats the outside coils.

 

[Janderso]

Interesting. It makes sense.
When I walked out in the shop this morning and saw, what I thought was an error code, nooooooo.

 

[RJSakowski]

I haven't seen it with my Mr. Cool installation but I don't run mine in heat pump mode unless the temperature is above freezing.

 

[Janderso]

I haven't seen it with my Mr. Cool installation but I don't run mine in heat pump mode unless the temperature is above freezing.

I have it on heat. I don’t know what that means mechanically though.

 

[Tozguy]

In heat mode the system is chilling the outside coil and heating the inside coil. It is normal for the outside coil to frost up especially in the late fall when temps drop and relative humidity is high.

My heat pump regularly goes on a defrost cycle to remove the frost that builds up and impedes air flow through the outside coil. It does not have to be below freezing outdoors for this to happen. The defrost mode reverses the system so it is temporarily heating the outside coil and cooling the inside one. The outside fan stops during this cycle and the outside coil will drip water. An auxiliary heat source (electric element or fuel burner) is often applied inside so that no cooling is felt indoors. The system senses when a defrost cycle is necessary and kicks it in automatically. The defrost cycle on mine lasts around 3 minutes or less.

 

[RJSakowski]

I have it on heat. I don’t know what that means mechanically though.

A heat pump is a refrigerator running in reverse. The compressor does work moving heat from a colder point to a warmer point adding its work as heat. As a result, you actually get more energy out than you put in. So more than 100% efficiency. However the efficiency drops rapidly with an increasing difference between input and output temperature which is why basic heat pumps aren't used in colder climates. Drawing the heat from the ground either with a large reservoir or a deep well can make cold weather heat pumps viable but that is definitely not the case with the mini split.

Add to the mix, there is an additional burden with thermal conductivity bringing heat into the outside unit,now operating as an evaporator so you can get a freeze ip even though the outside temperature is above freezing. This is why they add the defrost cycle.

 

[Janderso]

How did you guys get so smart?
Very interesting.

 

[pdentrem]

We found that our 1.5 ton MRCOOL would become inefficient right about 20F. We only use it from above that, otherwise works great.
Pierre

 

[Tozguy]

My 3 ton York is the same. It does not heat well when outside temps are below freezing. It also makes more noise when straining near its maximum capacity. We simply turn it off and turn on the oil furnace.