Mini-split ductless unit is short cycling.....help!

wanttolearn

I recently had a top of the line ductless mini-split installed in a room in my house that didn't have HVAC. The unit provides both cooling and heat.

The issue I am facing is that in both cooling and heating modes the unit short cyles. It turns on, blows for less than 30 seconds, then cycles off again, only to turn back on a minute or two later. this repeats itself almost all day long.

the unit has no problem producing hot or cold air. when i hold a temperature gun to it, in cooling mode the air is around 45 degrees, and in heating mode around 100 degrees. the problem is that the cycle doesnt stay on long enough to make the room truly comfortable. never mind that the short cycling is also annoying and very inefficient.

It does this no matter what the load on the room is, i.e. on mild days as well as extremely hot or cold days, so i dont think it has anything to do with the load on the room.

i also dont think the unit is over or under sized. its 12,000 BTU unit for a 500 square foot insulated room, so sizing shouldnt be an issue.

i have brought this to the attention of the professional that installed it and he doesnt have a good answer for me as to what is going on. I also called Mitsubishi and talked to their tech support and they confirmed that the cycles should NOT be this short, but couldnt give me any helpful pointers beyond that.

can any of you out there come up with any suggestions as to what might be going on? any help is greatly appreciated.

nicholas bonham-carter

Has the installer double checked his work?:
Enough, and right type of gas, etc?
Adequate electrical service, with good ground?
Fan running properly in the outdoor module?—NBC

wanttolearn

yes he has.....the outdoor fan is working and everything seems to be installed fine

like i said the unit is producing hot and cold air...its just the cycles that are too short

SuperJ

Assuming it's installed and commissioned properly, Most mini splits control off of return air temperature. Some times in a small room, or if it is installed too close to the ceiling, the discharge air recircs quickly back into the mini split making it appear to be satisfied when the room is actually not at setpoint yet.

If this is the case, you can different positions for the discharge louvers to try and minimize recirculating the air.

One option is a an external thermostat that allows the unit to run off a remote sensor rather than the internal one. You need to be careful to configure it properly or it may still run off the internal sensor and just use the thermostat for settings adjustments if not setup properly. Sometimes the external thermostat is a wireless (Mitsubishi is).

Can you provide some more information? make/model, photos of install, Fan speed and direction settings. Mini-Splits typically work best with the fans on auto or high (in heat or cool mode).

The guys on greenbuildingadvisor.com qa section are pretty knowledgeable about mini splits, you might want to post there as well.

How big is the room and what is the heat loss/ heat gain calc? Many contractors grossly oversize thinking its better to error on too big, this can make short cycling and recirculating air problems even worse.

SuperJ

if you crank the setpoint way way up or down/open a window will it keep running? How long is the lineset? anything longer than 25ft typically needs some extra refrigerant added beyond what it ships with.

You could try rigging up some card board air deflectors to help avoid mixing discharge into the return, this might help troubleshooting.

wanttolearn

you might be on to something....due to where the window frame below it is, we had to install the unit very close to the ceiling. there is only an inch or two clearance. the installer said that even though the instructions calls for more clearance than that, it shouldnt be a problem and he went ahead and installed it this way.

i think i need to look into the external thermosatat solution?

wanttolearn

yes, if i crank way up or down, it does keep running....another indication that is the thermostat and the low ceiling clearance that is at issue

SuperJ

If you want smooth control then a remote sensor is probably your only option short of relocating the unit. You probably need the MHK-1. I do know that on LG mini splits they have a remote mount RAT sensor. You unplug the built in one and install one on cable that can be located optimally, I'm not sure if Mitsubishi offers this. On LG it is just a 10k type 3 thermistor.

SuperJ

You might be able to crank it up or down and get temporary comfort, but you will always be tweaking it for comfort. In the meantime, I would suggest playing with the discharge air direction, you might be able to direct the air in such a way that it doesn't recirc as quickly. Maybe point it at the bottom of the far corner of the room, and turn off the auto movement function.

wanttolearn

thanks you have been very helpful and i will definitely look into this more.

Jack

How many sq ft is the space? How high the ceilings? What is the min/max output of the unit? In other words, is it sized to the space?