HVAC Reference Library · Technical Reference

Filter Driers and Sight Glasses

Filter driers remove moisture, acid and debris from refrigerant, and sight glasses with moisture indicators show refrigerant flow and dryness. This sheet covers liquid-line, bi-flow and suction-line driers, how drier part numbers are commonly coded and how to read a sight glass.

Ref HV-0016AHRI 710

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

A filter drier is a shell filled with desiccant, usually molecular sieve with or without activated alumina, plus a filter screen. Liquid-line driers go in the liquid line after the condenser and are installed on almost every system service where the system is opened. Bi-flow driers are built for heat pumps, where liquid flows both ways. Suction-line driers go before the compressor, usually for cleanup after a compressor burnout, and often have pressure taps to read pressure drop. Replaceable core driers have a shell with removable cores for large systems. A sight glass is a window in the liquid line. Most include a moisture indicator element that changes color when moisture is present.

How to identify

Read the drier label. It shows the model, flow direction arrow, connection size and type (flare or sweat), refrigerant compatibility and capacity. Many makers code liquid-line drier models with the desiccant volume in cubic inches followed by the connection size in eighths of an inch. For example, a drier coded 163 is commonly a 16 cubic inch shell with 3/8 in connections, and an S suffix often means sweat connections. Coding varies by maker. Confirm in the catalog.

Drier and indicator types

Item	Where installed	Purpose	Notes
Liquid-line drier, one-way	Liquid line, flow arrow toward metering device	Moisture, acid and debris removal	Replace whenever the system is opened
Bi-flow drier	Liquid line on heat pumps	Same, with flow in both directions	Do not use a one-way drier on a heat pump liquid line
Suction-line drier	Suction line before the compressor	Cleanup after burnout	Watch pressure drop; remove or replace after cleanup per guidance
Replaceable core shell	Larger commercial systems	Cores can be changed without cutting pipe	Order cores by shell model
Sight glass with moisture indicator	Liquid line, after the drier	Shows flow and moisture	Indicator color chart is on the glass or in the maker's data

Key points

Match the drier to the refrigerant and oil. POE and A2L systems may need specific desiccants and listings.
Install with the flow arrow in the direction of flow.
Do not overheat the shell when brazing. Wrap it or use a heat sink.
Bubbles in a sight glass do not always mean undercharge. Flash gas from restrictions or low subcooling also causes bubbles. Charge by the maker's method.
An indicator showing wet means moisture is in the system. Replace the drier and evacuate.

Common mistakes

Reusing the old drier after opening a system.
Installing a one-way drier on a heat pump.
Leaving a suction-line cleanup drier in place indefinitely with high pressure drop.
Charging to a clear sight glass alone.

Governing standards: AHRI 710 (liquid-line driers). Equipment maker charging and service instructions.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

More in Technical Reference

← All HVAC sheets