What Is an LGR Dehumidifier and Why Do Pros Use Them?

An LGR dehumidifier is a commercial drying machine that keeps removing water from the air even after the air is already quite dry. LGR stands for low grain refrigerant. Restoration companies use them because getting water out of wood and drywall requires air much drier than a household or standard commercial dehumidifier can produce.

How a regular dehumidifier works

Every refrigerant dehumidifier works the same basic way. A fan pulls room air across a cold coil. The air is chilled below its dew point, water condenses on the coil and drips into a bucket or out a hose. The air is then passed over a warm coil and blown back into the room, drier and a bit warmer than it came in.

The weakness is that the coil can only get so cold before it ices up. Once the room air is fairly dry, its dew point drops below what the coil can reach, and very little water condenses. The machine keeps running but the bucket stops filling. In a damp basement that is fine. On a drying job it means the unit quits right when the hard part begins.

What makes an LGR dehumidifier different

An LGR unit adds a pre-cooling step. Before the incoming air reaches the cold coil, it passes through a heat exchanger that takes some of the heat out of it, using the cold air that is leaving the coil. The air arrives at the coil already cooled, so the coil can spend its effort condensing water instead of lowering the temperature. The result is that the machine keeps pulling water at humidity levels where a conventional unit has all but stopped.

The name comes from how restorers measure moisture in air. They use grains of water per pound of dry air, a measure of the actual amount of water present, which unlike relative humidity does not change when the temperature does. A low grain refrigerant machine is one that keeps working when the grain count is low.

Why very dry air matters for drying a house

Water moves from wet to dry. A wet stud gives up moisture to the air around it only as long as that air is drier than the wood. As the wood dries, the gap narrows and the drying slows. The last few percent of moisture in framing and subfloor, the part that decides whether mold grows later, only comes out if the surrounding air is kept very dry for days.

LGR vs a home dehumidifier vs a desiccant

• Home dehumidifier: built to hold a damp room at a comfortable humidity. Small coil, small fan, a bucket to empty. Good for prevention, too small and too limited for drying a structure.
• Conventional commercial refrigerant: a bigger version of the same design. Removes a lot of water in very humid air, then fades as the air dries.
• LGR dehumidifier: the standard for most house drying. Works well at normal indoor temperatures and keeps working as the air gets dry. Pumps its water out through a hose, so nobody empties a bucket.
• Desiccant dehumidifier: uses a moisture-absorbing wheel instead of a cold coil. It works in cold spaces where refrigerant units struggle, such as an unheated building in winter, and can produce extremely dry air. It is larger, needs ducting and is less common on ordinary house jobs.

One caution when comparing machines. The big number on a dehumidifier, pints or litres per day, depends on the test conditions. Some ratings are taken in hot, very humid air where any machine looks strong. The fair comparison is the rating at standard conditions and, better still, how the unit performs in dry air. A home unit and an LGR can show numbers that look closer than they really are.

What to expect with one in your house

An LGR is about the size of a large suitcase on wheels. It plugs into a regular outlet. A thin hose runs from it to a sink, tub, floor drain or sump pit, and the built-in pump pushes the collected water out every so often with a short hum. It blows warm, dry air, so the room will run a few degrees warmer than the rest of the house. That warmth helps drying.

A few things keep it working properly:

• Keep the windows and outside doors of the drying area closed. An open window means the machine is trying to dry the neighbourhood.
• Leave it running day and night. Do not unplug it to use the outlet.
• Do not move the drain hose or kink it. If the pump cannot empty, the machine shuts off.
• Leave the furnace or air conditioning on its normal setting unless told otherwise.

How many units a job needs depends on the volume of the space and how wet it is. One LGR handles many single-room leaks. A soaked finished basement may need two or three, working alongside air movers, which have their own job to do. We size the equipment from the readings on the first visit and adjust as the numbers come down.

If someone is quoting you a drying job, ask what kind of dehumidifier they are bringing. It is a fair question, and the answer tells you whether the plan is to dry the structure or just the air.

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