Refrigerant Sight Glass?
Flashing And Clearing?
Over And Over And....?

Is the refrigerant sight glass flashing and clearing?

What do I mean?

Does the glass become clear, only to resume bubbling later?

Then after a few minutes, clear again?

Repeating this pattern over and over?

Here's a few suggestions.

The pattern could be the result of the expansion valve metering the refrigerant into the evaporator.

As the valve opens it increases the flow, out of the liquid line, causing the glass to flash.

When the valve closes, the flow is restricted and the refrigerant "backs-up" in the liquid line. The glass becomes clear.

Normally this should not happen, if the refrigerant lines are properly sized and installed.

But if it does, the installation of a receiver is recommended.

The liquid receiver allows additional refrigerant to be added to the system. It stores any excess and releases it when the expansion valve opens.

Problem solved!

But what if the refrigeration equipment has a receiver, but still repeatedly flashes and clears?

Are you using fan cycle controls to maintain high side pressure?

Many many times I have watched a clear glass begin flashing after the condenser fan motors starts, only to clear again after the motor stops.


You have to visualize what's going on inside the condenser coil and/or receiver.

After the motor starts, the head pressure drops. This dropping pressure causes the refrigerant in the coil and/or receiver, to fall out of liquid form and evaporate.

The glass flashes.

When the motor stops, the pressure increases. This additional pressure, forces the gas back into liquid form.

The refrigerant sight glass clears.

What can be done?

Sometimes, adding more refrigerant helps.

You can also try adjusting the fan cycle control to cycle the fans more quickly, with less pressure difference between cut-in and cut-out settings.

But, when all else fails, I just live with it!

It's really not a big deal and doesn't affect the system performance.

Why sweat the little things?

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