Filter Dryer and Sight Glass

[pehanlon]

Is there any periodic time that a Filter Dryer should be changed (i.e. every five years...)?

Also if a piston system does not have a sight glass installed should one be installed as an additional means to diagnose a system (verify no bubbles in liquid line)?

[dorrmann]

The only time a filter drier should be changed (unless it is causing a problem) is when the system is opened for repairs.

As for the sight glass, it's better that you don't have one. They are no good for diagnosis on a fixed orifice system.

[docholiday]

A drier, IMHO is a one time deal. It is used to eliminate any moisture in the system. If moisture is continuing to enter the system then you have much larger problems. Anytime the system is opened (meaning a leak or line repair is made) the drier should be replaced.

Sight glasses in residential applications are for the most part useless. The idea of a sight glass is to verify a solid column of liquid is getting to the TXV. A sight glass when used needs to be used at the TXV not outside. The pressure drop in the liquid line can cause flash gas by the time it reaches the indoor coil. You cannot see the indoor coil or sight glass when outside charging. Subcooling overcomes the pressure drop and you cannot read subcooling with a sight glass. The moisture indicators are not always accurate so I wouldnt bank on them either. Save the $ and dont waste your time with sight glasses in residential applications.

[billygoat22]

In the case of filter dryers if you see a greter than 3 degree drop across the dryer, it is dirty. Haven't seen one like that yet, however, since any cleanup filter/dryers I take out of the system in a matter of days.

[docholiday]

Good point billy goat. You bring up a valid argument. Especially in suction line driers, they are most often used in a clean up application and should be removed and the liquid line drier replaced after a few hours or days of service.