Our new GE 6600 btu unit is short cycling. It is definitely undersized. Our apartment is 800 sqft and we have units in both bedrooms. One is a window unit 8k btu and then this new through wall 6600btu. They are undersized because our circuit cannot support anymore, as they are not on dedicated circuits.

Anyway, back to the 6600 btu through wall... With door closed, unit cycles fine. Will do 3 or 4 an hour. However, with door open, and exposed to the warm air from the hallway, kitchen and living room, the compressor runs 5 minutes, shuts off, fan remains on for 3 minutes then compressor kicks back on and repeat. Does this all day until we shut the door. From what I read, these are all symptoms of oversized units. What could cause this?

We have the thermostat at 77. Air is blowing cold and my thermometer I have near the unit will usually read 76-79. Humidity is around 55-58 near the unit.

I'm wondering if I should just lower it further to see if it will just run constantly during day, and then at night with door shut it will get some rest when it has normal off times...

 

Is it possible the compressor is short cycling as some sort of protection mode, such as something overheating or freezing?

Also, forgot to mention I usually keep fan on low. What's best fan speed in undersized situations?

 

An undersized unit does not short cycle.
If it's undersized..... the room temperature rises slowly... if at all.

Warm air hitting the unit should cause the compressor to remain running.
If the cold air gets back into the front of the unit.... that will cause short cycling.

When the door is open... try running the fan on high and see what happens.

 

Okay and you may be onto something. During the day, I am usually sitting right next to it lol. Perhaps I am causing cold air to remain in front of it..... I will run with door open, fan on high, and with myself out of the way.... Does it matter if vent pointed up or down? Unit is near the floor, about 12 inches from it.

Question, if this fixes it, and my compressor runs most of the day, what is the risk of damage or coil freezing? Also, how can one figure out the thermostat differential (manual didn't indicate).

 

If this is a typical unit it has the discharge grille just above the intake. In this case a certain amount of the cooled discharge air will immediately drop and be pulled into the intake, cooling the intake air quite a bit below the nominal room air temperature. Since the sensing bulb for the thermostat is almost always mounted directly in front of the evaporator (cooling) coil the unit will often short cycle.

What I did with my window A/C was to make a short duct that fastens to the discharge grille and directs the discharge air up and out on a 45 degree angle. I used a standard 4X10 inch 90 degree floor boot and an adjustable 6 inch diameter elbow to accomplish this. I have also used a piece of 1/2 inch foam insulation board the width of the inlet grille and about 10 inches long to separate the inlet and outlet grilles.

It is NOT pretty but it DOES work. Moving the thermostat sensing bulb to immediately outside the case rather than right next to the evaporator can also help but you then have to be careful of the evaporator freezing.

For testing you can use some cardboard bent and held in place with tape. Anything to keep the inlet and discharge air streams apart.

 

That's good to know and I'd love to see a pic. However, if this explains my short cycle during the day, why would it run normally at night with door closed?

 

I wish I had an answer for you. All I know is that without the duct or baffle both of my window units were short cycling and doing little but eating up electricity. I have a 6k BTU unit in the bedroom (about 150 square feet) and I had a 10k BTU unit in my "project" room where I used the piece of foam insulation board. I thought the results were nothing short of amazing, especially with the larger unit.

Right now I am in the bedroom and the digital indicator on the unit reads 68. A digital thermometer across the room, by the entrance door, reads 71 and the thermo-hygrometer on my nightstand reads 70 degrees and 50% RH. Out in the main part of the house it is 75. If I close the door I can easily bring the room temperature down to 66 degrees, maybe even 65.

One downside to the metal duct is that it condenses the room humidity quite readily. It never gets so bad that it drips but it would be better if I could insulate it on the outside. Another detriment is that if the thermostat shuts down the unit the condensate on the duct re-evaporates rapidly and raises the room RH several points. Plus, without the fan running the unit's thermostat is VERY slow to respond to rising room temperature so I try to keep it running as much as possible.

As for a picture...I used to have one but I must have deleted it. Right now it is too junked up to take another but maybe I can do some cleaning in the next few days.

 

Yesterday and today as long as my fan has been on high and vent pointed up, I have not been short cycling but I will say it's so eratic as to how long the cycles are. Sometimes it will run for an hour straight and sometimes just 10 minutes. I think there are too many variables to try and narrow it down, such as the bedroom doors being open and closed often, and the morning sun that is directly hitting the unit until mid day.

However, as long as I'm not cycling on and off every few minutes like before, I am happy. I'm glad we are able to live comfortably in 800sqft apartment with only 2 ac units totaling 14k btu. Pleasantly surprised.