LGR vs. Conventional Dehumidifiers: Which to Use When

Choosing the right dehumidifier isn't just about grabbing the biggest unit off the truck. LGR (Low Grain Refrigerant) and conventional refrigerant dehumidifiers serve different purposes, and the S500 standard expects you to know when to deploy each type. This is also a frequent topic on the WRT exam.

How Conventional Dehumidifiers Work

Conventional refrigerant dehumidifiers pull humid air across a cold coil, condensing moisture which drains into a collection system. They work well when GPP levels are high — typically above 65-70 GPP. In early-stage drying when the air is heavily moisture-laden, conventional units are efficient and cost-effective.

Their limitation: as the air dries and GPP drops below 60, conventional units lose efficiency rapidly. The coils can't get cold enough to condense the lower moisture content, and the unit essentially recirculates air without removing much water.

How LGR Dehumidifiers Work

LGR units pre-cool incoming air before it reaches the primary cooling coil, allowing them to extract moisture at much lower grain levels — down to 30-40 GPP. This makes them essential for the later stages of drying when you're trying to reach your final drying goals. They're more expensive to purchase and operate, but they finish the job.

When to Use Each Type

Here's the practical decision framework:

• GPP above 65: Conventional dehumidifiers work well and are more cost-effective
• GPP between 40-65: LGR units are significantly more efficient
• GPP below 40: LGR is essential — conventional units are nearly useless at these levels
• Cold environments (below 65°F): LGR units perform better; conventional units may ice up

Many experienced technicians start with conventional units during the first 24 hours of heavy extraction, then swap to LGR units once initial moisture levels drop. Use our GPP calculator to monitor when to make the switch.

Placement and Airflow

Dehumidifier placement matters as much as selection. Position units centrally in the drying area, away from walls. Ensure air movers are directing evaporated moisture toward the dehumidifier intake. The S500 specifies equipment ratios based on water damage class — higher classes require more dehumidification capacity per square foot.

When is a desiccant the right call?

Reach for a desiccant when the air must go drier or colder than a refrigerant unit can usefully manage. Instead of condensing moisture on a cold coil, a desiccant passes air over a moisture-attracting medium and exhausts the collected water as wet air, so its performance does not collapse as the space gets cold.

That makes desiccants the usual answer for unheated structures in winter, for Class 4 losses where bound water in plaster, hardwood or concrete needs a very low vapour pressure to move, and for large open commercial spaces where one ducted unit replaces a row of refrigerant machines. The trade-offs are real: you need to duct process and reactivation air, you need somewhere to exhaust wet air outside the drying chamber, and running cost and setup time are higher.

Practically, a desiccant is a decision rather than a default. If refrigerant units are holding the chamber at a workable grain depression and materials are still losing moisture day on day, stay as you are. If the grain depression has flattened, materials have stalled and the space is cold, that is the point to change technology rather than add more of the same. Check the numbers first with the dehumidifier sizing tab of the GPP calculator and log the change in your drying log, then read Class 4 water damage for the specialty-drying context.

What shows up on the exam?

Expect questions about dehumidifier selection. Know the grain thresholds, understand why an LGR outperforms a conventional unit at low grain levels, know the basic operating principle of each type, and know when a desiccant is preferred. Review the exam preparation guide for more strategy.