The refrigerant sight glass is flashing! Quick! Add some more refrigerant!
So I thought when I first entered this trade.
How quickly I learned, all is not as it appears in the field of refrigeration.
Just because the glass is flashing, doesn't necessarily indicate a lack of refrigerant.
What's the number one reason for getting it wrong?
With a properly installed glass, a partially restricted filter drier will give the impression of a low refrigerant charge.
Always check the filter drier before adding more.
Another possible problem arises when the condensing unit is using fan cycle controls.
They can cause the glass to flash.
OK, now that I have addressed a few mistakes that can happen.
What about a truly low system?
What are some indications that the glass is not misleading?
First and foremost, check the high and low side pressures. A low system will of course have lower than normal pressures.
Touch the liquid line. If it's cooler than normal, chances are the system is low.
Try adding some refrigerant.
As you add, pay attention to the glass. If the system is really low, you'll see a steady change.
What kind of change?
The refrigerant will slowly fill the glass and begin to slow down, then clear.
Also, the liquid line will gradually begin to warm up.
Now you are on the right track.
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