Quick answer
A dehumidifier is a machine that removes water vapor from the air, lowering humidity so that wet building materials can give up their moisture to drier air.
Household units collect water in a tank. Restoration units are larger and drain through a hose. Both are rated in pints of water removed per day, but the rating only means something at stated temperature and humidity.
Quick facts
The key facts about Dehumidifier, with sources:
- What it does
- Removes water vapor from the air; used to lower humidity in affected areas to aid drying 1
- How common units work
- Refrigeration coils, a compressor and a fan, with a humidistat that cycles the unit 2
- Capacity rating
- Pints of water removed per 24 hours; efficiency is liters of water per kilowatt-hour 2
- Rating depends on conditions
- One restoration model lists 70 pints a day at 80°F and 60% RH, 135 at 90°F and 90%, and 8 at 80°F and 20% 3
- Operating range
- The same model lists 33°F to 100°F and shuts off if the inlet air drops below 45°F 3
- Cold-weather limit
- ENERGY STAR notes frost can form on the coils of a household unit in spaces that fall below 65°F 2
Key takeaways
- A dehumidifier pulls water vapor out of the air. It does not dry a wet wall directly; it keeps the air dry enough that the wall can give water up 1.
- The pints-per-day number is a rating at set conditions. The same machine can remove many times more at 90°F and 90% humidity than at 80°F and 20% 3.
- Air movers and dehumidifiers work as a pair. A manufacturer’s manual says to increase air movement if the room is not drying 3.
- A dehumidifier does not stop a leak, remove mold or make a contaminated space safe.
- Ask which machine is running, what it is rated for, where it drains, and what the inlet and outlet readings are.
What a dehumidifier is, in plain English
Air always holds some water vapor. When a building gets wet, the wet materials give water to the air, and the air gets more humid. A dehumidifier pulls that vapor back out.
NIH’s water-intrusion procedure describes dehumidifiers as devices designed to remove water vapor from the air, used to lower humidity levels in affected areas to aid in drying 1.
The reason this matters is a simple gap. Water moves from a wet material into the air only while the air is drier than the material. A dehumidifier holds that gap open. Our page on vapor pressure differential explains the physics, and restoration psychrometrics explains the readings.
A household dehumidifier is built around refrigeration coils, a compressor and a fan, with a built-in humidistat that cycles the unit on and off to hold a set humidity, according to ENERGY STAR 2.
Restoration dehumidifiers follow the same idea in a larger, tougher form. One manufacturer’s manual for a restoration model describes a refrigerant system with R32 refrigerant, coils, a defrost cycle and a built-in pump for a long drain hose 3.
A second family, desiccant dehumidifiers, removes vapor with a drying material instead of a cold coil. That design suits cold or very dry conditions and has its own page: refrigerant vs desiccant dehumidifiers.
How a refrigerant dehumidifier removes water, step by step
The dehumidifier steps describe the cycle in a refrigerant unit, as ENERGY STAR and a manufacturer’s manual describe it 23.
- Room air is drawn in. A fan pulls it through a filter. The manual says to inspect the filter before each use and replace it if dust and debris are visible 3.
- The air passes a cold coil. A cooling coil works on the latent load when its surface is below the dew point of the air 5. Vapor condenses on it as liquid water.
- Water drains away. A household unit fills a tank. A restoration unit pumps water out through a drain hose, and the manual lists a 40-foot hose 3.
- The air leaves drier and slightly warmer. ENERGY STAR notes that a unit slightly warms nearby air as it removes moisture 2.
- The coil is defrosted when needed. The manual shows a defrost cycle that clears ice from the coils, and the unit completes it even if turned off 3.
- Sensors report the result. The display shows inlet and outlet temperature and relative humidity, and a grain depression value 3.
- Air inFan pulls room air through the filter
- Cold coilVapor condenses on the coil
- DrainWater leaves by pump and hose
- Air outDrier air returns to the room
- DefrostIce cleared from the coil when needed
- Back to step 1
A restoration unit repeats this loop continuously while the readings are above the goal. Source: [2] [3] [5]
Types of dehumidifier, and how a household one is sized
| Type | How it works | Notes |
|---|---|---|
| Portable household unit | Refrigeration coils, compressor and fan with a humidistat 2 | Collects water in a tank; capacity in pints per 24 hours 2 |
| Whole-home unit | Installed to use the home’s air ducts and can be permanent 2 | Suited to relatively well sealed homes with central air that read above 55% humidity 2 |
| Restoration refrigerant unit | Refrigerant system with coils, defrost and a pump for a long drain hose 3 | Operating range of 33°F to 100°F for one listed model 3 |
| Restoration desiccant unit | Separate process and regeneration airstreams, per one manufacturer’s manual 4 | Described as having the wide operating range and ultra-low grains of a desiccant 4 |
Household sizing from ENERGY STAR
For a small to medium room under 2,000 square feet, ENERGY STAR lists 20 to 30 pints per day for slightly to moderately damp conditions (50% to 75% humidity), 25 to 40 for very damp (75% to 90%) and 30 to 50 for wet (90% to 100%).
This is household guidance, not a restoration sizing rule 2.
Example: why the same machine has three different ratings
The dehumidifier ratings table uses the ratings printed in the owner’s manual of one restoration model, the Dri-Eaz LGR 130, as an example of how a rating depends on conditions.
It is not a recommendation of that model and no price is given 3. The grain figures are computed with National Weather Service formulas at sea-level pressure from the inlet conditions each rating names 89.
| Rating condition printed in the manual | Water removal | Computed water content of that inlet air |
|---|---|---|
| 80°F and 60% relative humidity (AHAM) | 70 pints per day | about 92 grains per pound |
| 90°F and 90% relative humidity (saturation) | 135 pints per day | about 195 grains per pound |
| 80°F and 20% relative humidity (low grain) | 8 pints per day | about 30 grains per pound |
The pattern is clear. A dehumidifier removes far more water from warm, wet air than from cool or dry air, because there is more to remove.
At the low-grain point the manual lists 8 pints a day, a small fraction of the 135 pints at saturation 3. A bigger number on a spec sheet therefore says nothing until you know the conditions behind it.
Ask what the rating was measured at, and compare machines only at the same conditions.
Where you see a dehumidifier on your job
In the house. Restoration units are boxy machines with a drain hose, a display and a power cord. The manual requires an upright position, an uncoiled drain hose, a GFCI-protected 115-volt outlet rated for at least 15 amps and no extension cord 3.
On the log and the invoice. A drying log records daily temperature and relative humidity, indoors and out, along with moisture readings and the drying goal 1. See the moisture log page. Equipment appears on the invoice by machine and by day. The cost guide explains how equipment lines are built.
On the display. The manual lists inlet and outlet readings and a grain depression menu item. The unit shows grains per pound when set to Fahrenheit and grams per kilogram in Celsius 3.
In the plan. A good plan names how many dehumidifiers and air movers will run and why. See structural drying for what a measured plan looks like.
Ask for the rating and the readings
Ask the company which dehumidifier model is on your job, what conditions its rating was measured at, and what the inlet and outlet readings were on the last visit.
What a dehumidifier does well, and what it does not
What it does
- Lowers the humidity of the air so wet materials can release water 1.
- Helps keep indoor humidity at the 30% to 50% range EPA and ENERGY STAR recommend 62.
- Reduces secondary damage. NIH says moisture evaporating into the air should be exchanged with less humid air or removed through dehumidification 1.
- Provides readings. Some units display inlet and outlet conditions and a grain depression 3.
What it does not do
- Stop a leak. The source must be fixed 6.
- Remove water from inside a closed wall cavity on its own. Cavities need airflow and access.
- Clean contamination or remove mold. Neither is a job for a dehumidifier.
- Dry the room alone. The manual says to increase air movement with air movers when a room is not drying 3.
Limits of a dehumidifier
Capacity depends on conditions. The same unit lists 135 pints per day at 90°F and 90% humidity and 8 at 80°F and 20% 3. A rating of 70 pints per day does not mean 70 pints in your house.
Temperature matters. ENERGY STAR notes frost can form on the coils of a household unit if the space falls below 65°F, and some units have an anti-frost sensor 2. The restoration model in the manual shuts off if the inlet air falls below 45°F and restarts at 50°F 3.
Air must reach the unit. The manual’s troubleshooting list names poor air movement and excessive moist-air infiltration as reasons a room may not dry, and says to increase air movement and seal off the area 3.
Maintenance affects output. The manual says the unit collects too little water when the filter is full or the coils are clogged 3.
Finally, it dries air, not materials. Drying ends when materials reach the goal set from a dry standard, not when the air is dry 1.
What drives the cost of dehumidifier use, and how long units last
We publish no prices, and no source we loaded gives a fair national figure for rental or purchase. What we can describe is what moves the cost.
On a drying job, the equipment line comes down to how many units, for how many days. The count is driven by the volume of the space, the amount of wet material and the conditions.
The days are driven by when readings reach the goal 1.
For household use, running cost depends on efficiency. ENERGY STAR says a certified unit uses refrigeration coils, compressors and fans to remove the same amount of moisture while using 20% less energy, and rates efficiency in liters of water removed per kilowatt-hour, with a higher number meaning a more efficient unit 2.
Life and warranty come from the maker. The manual asks for a filter check before each use, coil checks monthly and a pump inspection annually, and says rough handling can void the warranty 3. Use the drying time tool to see what affects duration, and the estimate checker to review an equipment line.
How a restoration company sets up and runs one
Professionals set up the equipment. These steps follow the manufacturer’s operating instructions so you can recognize good practice 3. No permit is normally needed.
- Place the unit upright. If it was laid down, stand it for at least 30 minutes before turning it on 3.
- Uncoil and straighten the whole drain hose, with no kinks or coils on the unit, and route it to a drain 3.
- Plug into a GFCI-protected 115-volt outlet rated for at least 15 amps. Do not use an adapter or an extension cord 3.
- Close the space so the unit works on a defined volume of air. The manual names moist-air infiltration as a cause of a room not drying 3.
- Set humidistat mode OFF for maximum dehumidification, per the manual 3.
- Read inlet and outlet daily and log them next to the material readings 1. At the end, purge the pump and let the unit finish any defrost before moving it 3.
Keeping a dehumidifier working
Dehumidifier upkeep points come from one manufacturer’s maintenance schedule; a household unit’s manual may differ 3.
- Before each use. Inspect the cord and the filter, and replace the filter if dust and debris are visible.
- Monthly. Check the coils and clean them when visibly dirty. Dirty coils can cause the unit to overheat.
- Check the drain. Look for obstructions in the drain hose and clear any debris.
- Annually. Have the pump system inspected by a qualified service center.
- After a contaminated job. The manual says to replace the filter if it was used on a mold remediation job or exposed to potentially dangerous contaminants.
Warning signs a dehumidifier is not doing its job
- Little or no water in the hose or tank with damp air. The manual links this to a full filter, clogged coils or a cold room 3.
- A room that stays humid. Poor air movement, too little time or humid air getting in are listed causes 3.
- A unit that shuts off or shows a defrost message. In cold rooms the unit may be cycling through defrost or shutting off at low inlet temperature 3.
- Water dripping when moved. The manual says this happens when a unit is unplugged before purging is complete.
- Dust or particles in the air around it. The manual warns that dust can overheat the unit and not to run it during sanding or spray painting 3.
Electrical and refrigerant hazards
The manufacturer warns of electric shock and of explosion or combustion from flammable refrigerant.
Do not drill into or modify the unit, do not service the refrigeration system yourself, unplug it before cleaning and keep it away from open flames and heaters 3.
Never enter a flooded or fire-damaged building before it has been judged safe.
Standards, listings and guidance
Efficiency program. ENERGY STAR is a voluntary program that rates household dehumidifiers by integrated energy factor in liters per kilowatt-hour and capacity in pints per 24 hours 2.
Safety listing. The LGR 130 manual states it is certified to UL 60335-1 and 60335-2-40 and the Canadian equivalent 3. A certification covers the machine’s safety design, not its drying results.
Restoration standard. ANSI/IICRC S500 covers psychrometry and drying technology. The IICRC lists the fifth edition as the 2021 edition and says a revision is under way 10. It is voluntary and purchased, and this page does not reproduce it.
Humidity guidance. EPA’s guideline is below 60% indoor humidity, ideally 30% to 50%, and drying water-damaged areas within 24 to 48 hours 6.
Your contract. Equipment terms and daily rates vary by company; read the work authorization.
Dehumidifier compared with related equipment
| Equipment | What it does | How it differs from a dehumidifier |
|---|---|---|
| Dehumidifier | Removes water vapor from the air 1 | Lowers humidity; does not move air across surfaces |
| Air mover | Forces air along floor and wall surfaces 1 | Moves air; does not remove vapor from the room |
| Air conditioner | Cools air and removes some moisture; EPA suggests using it and dehumidifiers when needed 6 | Meant for comfort; a dehumidifier is built for moisture |
| Air scrubber | Filters particles from the air | Cleans particles; does not dry |
| Hygrometer | Reads humidity in the air 6 | A meter, not a machine that changes humidity |
Household unit
- Tank with a humidistat
- Rated in pints per 24 hours
- Best for steady humidity control
- Frost below about 65°F in the space
Restoration unit
- Drain hose and pump
- Rated at several named conditions
- Built for continuous drying jobs
- Listed range 33°F to 100°F for one model
Common myths about dehumidifiers
- Myth A higher pints-per-day number always means a better machine.
- Reality The number depends on test conditions. One model lists 135, 70 and 8 pints per day at three different conditions 3.
- Myth A dehumidifier alone will dry a flooded room.
- Reality The manufacturer’s manual says to increase air movement with air movers if a room is not drying 3.
- Myth It will remove mold.
- Reality It lowers humidity. NIH says controlling moisture is key to minimizing mold growth, but cleanup is a separate job 1.
- Myth The bigger the unit, the faster the job.
- Reality Output also depends on temperature, humidity, airflow, the filter and the coils 3.
Climate and altitude change what a dehumidifier can do
Two regional variables matter. The first is temperature. Cold basements and unheated spaces cut a refrigerant unit’s output, as ENERGY STAR’s frost warning for spaces below 65°F shows, and one restoration manual lists a shutoff below 45°F inlet air 23.
The second is outdoor humidity. EPA describes humid climates where outdoor air has a dew point above 70°F through most of a typical year, which makes opening windows work against drying 7.
One manufacturer lists a maximum use altitude of 10,000 feet for its restoration model 3.
When a dehumidifier matters, and what to do next
A dehumidifier matters on any job where wet materials are dried in place, in humid seasons, and in basements and crawl spaces after a leak. It matters less after a small spill dried quickly.
The next step is to ask which machines are running, what conditions their ratings were measured at and what the daily inlet and outlet readings are. If you are choosing a company, the selection guide lists more questions, and if water just arrived, the first 24 hours guide covers the first moves.
- If The room stays humid
- Then Check the filter, coils, drain and airflow; ask whether more units are needed
- If The space is cold
- Then Ask about heat or a desiccant unit; refrigerant units lose output in the cold
- If A rating is quoted without conditions
- Then Ask what temperature and humidity it was measured at
- If Water source not fixed
- Then Fix the source first; a dehumidifier cannot keep up with a leak
Questions about Dehumidifier
What does a dehumidifier do after water damage?
It removes water vapor from the air so that wet materials can keep releasing moisture into drier air.
NIH describes dehumidifiers as devices designed to remove water vapor from the air to lower humidity in affected areas to aid drying.
It works together with air movers, which push air across wet surfaces, and it does not fix the water source.
What is an LGR dehumidifier?
LGR is a label restoration equipment makers use for a class of dehumidifier.
One manufacturer’s manual for an LGR model describes a refrigerant system with coils, a defrost cycle, a drain pump and a rating at a low-grain condition of 80°F and 20% relative humidity.
The sources we loaded do not give a general definition, so ask the company what its machine is rated at.
How many pints per day does a dehumidifier remove?
It depends on the conditions. One restoration model lists 70 pints per day at 80°F and 60% relative humidity, 135 at 90°F and 90%, and 8 at 80°F and 20%. ENERGY STAR gives household sizing, such as 20 to 30 pints per day for a small to medium room that is slightly to moderately damp.
What humidity should I aim for?
EPA advises keeping indoor humidity below 60%, ideally between 30% and 50%. ENERGY STAR calls 30% to 50% the optimum range for a building and suggests 30% to 40% in colder climates during the heating season to prevent window condensation. A hygrometer lets you check the number yourself.
Why is my dehumidifier not collecting water?
A manufacturer’s manual lists common causes: the room air is dry, the room is too cold, the filter is full, or the coils are clogged.
In a cold space, frost can form on the coils, and some units shut off at low inlet temperature. Check the filter and coils first, and confirm the humidity with a hygrometer.
Can I use a household dehumidifier after a flood?
For minor damp, a household unit can help keep humidity in range. For structural drying after a real loss, restoration equipment is built for continuous work and larger loads. Do not enter a flooded building until it is judged safe, and if the water was sewage or floodwater, a professional should decide what stays.
Do I need air movers with a dehumidifier?
Yes, when the room is not drying fast enough. One manufacturer’s manual says to increase air movement with air movers if the room is not drying and to combine the unit with air movers for the best drying.
NIH also describes blowers as directing air toward wet materials. Air movers help water leave surfaces and the dehumidifier removes it from the air.
How long should a dehumidifier run?
No source gives a general number of days, and none should be promised. Drying ends when readings reach the goal set from a dry standard. The unit should run until then, and the company should show daily readings. Use the drying time tool to see what affects duration.
Is a dehumidifier the same as an air purifier?
No. A dehumidifier removes water vapor from the air. An air purifier or air scrubber filters particles. NIH notes that controlling moisture is key to minimizing mold growth, so a dehumidifier helps by removing moisture, but cleanup is a separate job.
Is a dehumidifier dangerous?
It is safe when used as the maker directs, but there are hazards.
One manual warns of electric shock and of explosion or combustion from flammable R32 refrigerant, and says not to modify the unit, service the refrigeration system or run it near open flames.
Use a grounded outlet protected by a GFCI and no extension cord.
Sources
- National Institutes of Health, Moisture and Mold Remediation Standard Operating Procedures (2023), retrieved .
- ENERGY STAR, Dehumidifiers (product guidance), retrieved .
- Legend Brands (Dri-Eaz), LGR 130 Dehumidifier Owner’s Manual #108997, retrieved .
- Therma-Stor Products Group, Owner’s Manual: Phoenix D385 desiccant dehumidifier (TS-387, revised 7/07), retrieved .
- Mohammed, Ahmadi, Ma and Bigham, Desiccants enabling energy-efficient buildings: A review (accepted manuscript, DOE OSTI), retrieved .
- U.S. EPA, A Brief Guide to Mold, Moisture and Your Home, retrieved .
- U.S. EPA, Moisture Control Guidance for Building Design, Construction and Maintenance (EPA 402-F-13053, December 2013), retrieved .
- National Weather Service El Paso: Vapor Pressure calculator formula (weather.gov, wxcalc), retrieved .
- National Weather Service El Paso: Mixing Ratio calculator formula (weather.gov, wxcalc), retrieved .
- IICRC, ANSI/IICRC S500 Standard for Professional Water Damage Restoration (standard page), retrieved .
Expert review
Written by the HyreRestore Research Desk. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Operation, ratings, operating range and safety notes for a restoration refrigerant unit were read from one manufacturer’s owner’s manual (Dri-Eaz LGR 130).
Household guidance and sizing come from ENERGY STAR’s dehumidifier page, and the role of dehumidifiers in drying from the NIH moisture procedure. The grain figures in the example are computed from NWS formulas at sea-level pressure and are not measurements.
We could not load a manufacturer white paper defining LGR in general, so no general definition of the term is given. DOE’s Energy Saver dehumidifier page returned an error and is not used. No prices and no durations are given.
Suggest a correction. We fix errors and say what changed.