Question about Mitsubishi Mr. Slim MXZ30TN Air Conditioner

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We have a MXZ-3A30NA that does not seem to be cooling down the house. Indoor temps are reading 75 to 78 degrees and the units are set at 65. We have gone through the book and have followed the check list but we still get only slightly cooler than indoor temp air out. Any ideas or suggestions on what to do next?

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Hi!!

Check the following:

Make sure the filter and the evaporator coil are clean. Also check condenser coil in the outside unit is clean.

When unit is turned on there should be (on the led display, inside unit) one green light on for the blower and another one for the compressor, both lights should be ON untill the unit reaches the preset temp. If the light for the compressor (usually the light on the right side) does not stay ON, check the outside unit.

On the outside unit, check that when turned on, both the fan and the compressor kick in and stay running. If the fan motor stops, the compressor overheats and the thermal proteccion shuts it down to prevent damage. Also, if the R22 gas is low, the low pressure switch prevents the compressor from running.

Good Luck!!





Posted on Aug 04, 2010

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Check the air filter and if it is dirty, replace..
Turn equipment off and let the ice melt.
After ice melts, place a/c in cooling mode and set the thermostat to 3 degrees below room temp display.

The indoor unit fan should start blowing cold air immediately and then turn off once the room temp drops one degree below the cooling temp setting.
If the indoor unit does cycle off, check the outdoor unit and make sure it turns off at the same time.
If the outdoor unit continues to run and the indoor fan is not running, the indoor unit will ice up.
A defective compressor relay that is stuck in a closed position will cause icing of the indoor unit because it keeps the compressor running when it should turn off with the indoor fan.

Do you have an error code being displayed on the controller?
Is there an led light that is flashing on the indoor unit?

Another possibility causing the indoor unit to ice up is if the freon in the system starts to get low.
Make sure the ice has all melted for this next check.
Pretty easy to check without special tools.
If the air filter is clean and there is no dust/ dirt blocking the fins on the indoor unit, turn a/c back on to cool and check that the indoor unit fan comes on and is blowing air good.
Once everything seems to running good go to the outdoor unit.
There will be two lines with black rubber insulation covering them.
Peel back the insulation three inches on each line and observe the copper lines.
If either line starts to frost up the a/c system is most likely low on refrigerant.


Another cause of indoor unit icing could be that the aluminum fins behind the air filter are clogged with dust or other foreign material.
This would block the air that must be pulled in causing the indoor unit coil to become too cold due to lack of air flow and it will ice up.

Operating the cooling system when the outdoor temperature is below 60 degrees can cause the indoor unit to ice up also.

I'm Robert.
Florida certified a/c contractor. 30 years experience.
Hope this helps.


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it really sounds like it is still undercharged.
Check the temp drop, let me know what it is.
Also tell me what the outdoor ambient temp is.

( for example.... 98 degree outdoors with 85 degree indoor temp could result in pressures something like 325 on the high side and 75 or 80 on the low side.
With the exact same unit, but 80 degree outdoor temp combined with 70 degree indoor temp, your pressures could read 215 low side and 55 low side. Not to mention that more efficient units often have higher pressures on the low side because they have more coil space and remove heat more efficiently)

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