Quick answer: Yes. Running an air conditioner (aircon) and a dehumidifier at the same time, in the same room, is safe, and it is often the best way to keep a space cool and dry. An AC removes moisture only while it is cooling, so on mild, humid days, or when the AC is oversized, it switches off before the humidity comes down. A dehumidifier fills that gap. The trade-off is heat: a dehumidifier warms the room slightly, so the AC runs a little more. Set the dehumidifier to about 50 % RH and keep the AC on its normal temperature.
Quick answer
- Safe to run together; they do different jobs.
- Most useful in humid weather when little cooling is needed.
- Keep the dehumidifier's warm exhaust away from the AC thermostat or return.
Why the AC alone can leave the air humid
An air conditioner cools air over a cold coil. When the coil is below the air's dew point, water condenses and drains away. But the AC is controlled by temperature, not humidity:
- Mild, humid days: the room reaches its temperature setpoint quickly, the AC stops, and moisture keeps coming in.
- Oversized AC: it cools the room in short bursts. The coil barely gets cold and wet before it stops, so little water is removed, and some of it evaporates back off the coil when the fan keeps running.
- Fan set to "on": air blowing across a wet coil after the compressor stops re-evaporates water into the room.
Most AC units put roughly 70–80 % of their capacity into lowering temperature and only 20–30 % into removing moisture. If the moisture load is high and the cooling load is low, the AC cannot keep up with the water.
What the dehumidifier changes
A dehumidifier is controlled by humidity. It keeps removing water whether or not the room needs cooling, and it is indifferent to the AC's on-off cycles.
It also adds heat. Condensing water releases about 0.68 kWh of heat per litre, and the dehumidifier's own electrical input also ends up as heat.
Example: a dehumidifier removing 30 litres a day, running all day, releases about 0.85 kW from condensation plus its electrical input, say 0.5 kW: roughly 1.35 kW of heat into the room. An AC with a typical efficiency (removing about 3.5 kW of heat for every 1 kW of electricity) needs about 0.4 kW extra to remove it. That is the real cost of running both, and it is usually far less than over-cooling the room to force the AC to dehumidify.
Does a dehumidifier help the AC?
In humid weather, yes, in three ways:
- Comfort at a higher setpoint. At 50 % RH most people feel comfortable one or two degrees warmer than at 70 % RH, so the AC setpoint can go up.
- Shorter AC run time for drying. The AC no longer has to over-cool the room just to wring out water.
- Less mould and musty smell, because the coil and room surfaces stay drier.
It doesn't make the AC cool faster. The dehumidifier's heat is a small extra load the AC has to remove (see the example above). Whether the pair uses less energy overall depends on how much over-cooling it replaces.
When running both makes sense
| Situation | Run both? |
|---|---|
| Humid, mild weather (spring, autumn, rainy days) | Yes; the AC barely runs, so the dehumidifier does the drying |
| Basements and rooms below ground | Yes; cool surfaces and damp walls keep humidity high |
| Oversized AC with short cycles | Yes, as a fix until the AC or its controls are changed |
| Hot, dry weather | Usually no; the AC alone keeps RH low |
| Rooms with wet processes, pools, laundries, wet storage | Yes, and usually a commercial unit sized for the load |
Best settings
- Dehumidifier: about 45–50 % RH. The US EPA advises keeping indoor humidity below 60 %, ideally 30–50 % (US EPA).
- AC: normal temperature setpoint; there is no need to lower it to dry the air.
- AC fan: "auto", not "on", so water is not blown back off the coil.
- Placement: keep the dehumidifier's warm discharge away from the AC thermostat and return grille; otherwise the AC thinks the room is warmer than it is.
- Doors and windows: closed, or outdoor air brings the moisture back.
- Drainage: use a continuous drain hose in rooms where the dehumidifier runs for long periods.
See best dehumidifier settings to prevent mold for setpoints by room.
Air conditioner and dehumidifier in one: combo options
Search for an "air conditioner with dehumidifier" and you will find four different things:
| Option | What it is | Good for | Limit |
|---|---|---|---|
| AC "dry mode" | The AC runs slowly to keep the coil wet | A few humid hours | Still cools; usually no humidity setpoint |
| Portable AC with a dehumidify setting | One box that cools or dehumidifies | Single rooms | Usually does one or the other at a time |
| Whole-building dehumidifier in the ducted HVAC | A separate dehumidifier connected to the air system, controlled on humidity | Homes and buildings with ducted air | Needs design and installation |
| Dedicated outdoor air system (DOAS) | Dries ventilation air before it enters | Commercial buildings with high fresh-air loads | Larger project |
For a single room, a separate dehumidifier next to an existing AC is usually the simplest and most controllable option.
Same room or different rooms?
- Same room: fine; place the dehumidifier away from the AC's thermostat and return grille, with its discharge across the room.
- Different rooms: the dehumidifier only dries the room it is in, plus whatever air mixes in through open doors. Close the door of the room you want dry.
- Whole house with a ducted AC: one portable dehumidifier won't dry every room; a ducted dehumidifier is the whole-house option.
AC "dry mode" vs a dehumidifier
Many split ACs have a dry mode that runs the compressor slowly with a low fan speed so the coil stays cold and wet. It removes more water per hour of running than cooling mode, but it still cools the room and is not controlled to a humidity setpoint on most units. It works for a few hours on a humid day; a dehumidifier is better when you need humidity held steady all day or in a room that should not be cooled.
"Air cooler" is different
If by "air cooler" you mean an evaporative (swamp) cooler, it cools by evaporating water into the air, so it raises humidity. Running a dehumidifier against an evaporative cooler works against it. Evaporative coolers suit hot, dry climates; the Australian Government's household energy guide explains where they fit (energy.gov.au).
In commercial buildings
In offices, shops, warehouses and production rooms, the same problem appears on a larger scale: a temperature-controlled HVAC system cannot hold humidity when the cooling load is low but moisture keeps coming in from people, ventilation, processes or wet goods. The usual fixes are:
- a dedicated dehumidifier controlled on humidity, sized at the room condition;
- coordinated controls so the HVAC and dehumidifier do not fight (one controller, separate temperature and humidity setpoints);
- including the dehumidifier's heat in the cooling design.
Our industrial dehumidifiers and ceiling-mounted and ducted dehumidifiers cover 26 to 960 L/day (rated at 30 °C / 80 % RH). See how to size a dehumidifier.
FAQ
Frequently asked questions
Is it bad to run a dehumidifier and AC at the same time?
No. It is safe and often the most effective way to keep a room cool and dry. Expect the AC to run a little more, because the dehumidifier adds some heat.
Does a dehumidifier make the AC work harder?
Slightly. It adds the heat of condensation and its own power. But dry air feels cooler, so many people can raise the AC setpoint a degree or two.
Should I run a dehumidifier in the summer with AC?
Yes, if indoor humidity stays above about 50–60 % with the AC running, especially on humid days with little cooling demand or in basements.
Can I use a dehumidifier with aircon?
Yes. In the UK, Asia and Australia "aircon" just means air conditioner; the same advice applies: dehumidifier at about 50 % RH, aircon at its normal temperature, fan on auto.
Is there an air conditioner with a built-in dehumidifier?
Every AC removes some moisture, and many have a dry mode, but few hold a humidity setpoint. For steady humidity control, use a separate or ducted dehumidifier.
What is the purpose of a dehumidifier if I have AC?
To remove moisture the AC leaves behind, for example in mild humid weather, in basements, or when the AC is oversized and cycles off before it dries the air.
Which should I turn on first?
It doesn't matter. Many people start the AC to cool the room and let the dehumidifier hold humidity afterwards.
Where should the dehumidifier go?
In the dampest area, with space around it, and away from the AC thermostat and return grille.
Can a dehumidifier replace an air conditioner?
No. It removes water but adds heat. It makes a room feel more comfortable, not cooler.
Sources
- US EPA, A Brief Guide to Mold, Moisture and Your Home.
- Australian Government, Energy saving guide: hot arid living.
- ASHRAE Handbook—Fundamentals, Psychrometrics.
