Does a dehumidifier make a room hotter? Where the heat comes from and when it matters

Where the heat comes from

A portable dehumidifier adds heat to a room in two ways.

1. The electricity it uses. The power used by the compressor, the fan and the electronics all ends up as heat, and all of it is released into the room. A unit drawing 300 W releases about 300 W of heat, much like a small heater of the same power.
2. The heat hidden in water vapour. Turning liquid water into vapour takes energy, and that energy stays in the vapour as hidden, or “latent”, heat. When the dehumidifier condenses the vapour back into water, the same energy is released. It is roughly 2,440–2,470 kJ for every litre collected, depending on the temperature.

The second point is the one most people miss. That hidden heat was already in your room, carried by the humidity. The dehumidifier does not create it. It turns heat you cannot feel into heat you can feel and measure on a thermometer. Engineers call these latent heat and sensible heat.

So the honest summary is this: the room’s total energy rises only by the electricity the unit uses, but the heat you can feel rises by more than that, because the moisture’s hidden heat is turned into warmth. In the example below, 0.3 kW of electricity raises the heat you can feel by about 0.64 kW.

A worked example

This is an illustrative calculation, not a measurement from a real home. It assumes a compressor dehumidifier drawing 300 W while removing 0.5 litres of water an hour. Your unit’s power and the water it collects will differ, so treat the numbers as a way to understand the scale.

How the moisture figure is worked out: 0.5 litres × 2,440 kJ per litre = about 1,220 kJ an hour, which is about 0.34 kW. One kilowatt is about 3,412 BTU per hour, the unit AC capacity is usually given in.

For scale, an AC unit rated at 12,000 BTU per hour can remove about 3.5 kW of heat at its rated conditions. If that AC was already removing the same moisture, running the dehumidifier moves part of the work from one machine to the other. The AC has about 0.34 kW less moisture to remove but about 0.64 kW more warmth, so its total load rises by about 0.3 kW, the dehumidifier’s own electricity. The AC also uses its own electricity to remove that extra heat. If the dehumidifier is removing moisture the AC was leaving behind, the room ends up drier, and the AC has about 0.64 kW of extra warmth to remove, while the room’s total heat gain is still about the 0.3 kW of electricity.

Why the air coming out feels warm

Warm air at the outlet is normal. On its own, it is not a sign of a fault.

• Compressor (refrigerant) dehumidifiers pull room air over a cold coil, where water condenses. The same air then passes over a hot coil, which releases into it the heat collected at the cold coil plus the heat from the compressor and fan. The air leaves drier and warmer than it entered.
• Desiccant dehumidifiers pass air through a water-absorbing material. As the material takes up moisture, it releases heat, so the air warms as it dries. Portable desiccant units then dry the material again with a built-in electric heater, and that heat also ends up in the room. In large commercial desiccant systems, the air used for this drying step is typically heated to around 100 °C, though newer materials can work at 40 °C. This is why desiccant units usually add more heat to a room than compressor units.

The outlet is the warmest spot near the unit. The temperature a metre or two away, and across the room, is what tells you whether the room itself is getting warmer.

Room temperature, outlet air and how it feels are three different things

When people say a dehumidifier makes a room hotter, they may mean any of these:

There is one more trap. Warming air lowers its relative humidity even if no water is removed at all. A humidity sensor placed in the warm air near the outlet will read lower than the rest of the room. That lower number does not prove the room is drier.

The reliable signs of real drying are water collecting in the tank or drain, and a lower dew point across the room. Our guide to how a dehumidifier works explains dew point and includes a calculator. Our guide to measuring humidity at home explains where to place a hygrometer.

Does drier air make a room feel cooler?

Less than many people expect, at least in rooms kept around 26–28 °C.

A 2018 study in Energy and Buildings tested 20 people at air temperatures of 26, 28, 30 and 32 °C, each at 40%, 60% and 80% relative humidity. At 26 °C and 28 °C, changing the humidity made no significant difference to how warm people said they felt. At 30 °C and 32 °C, humidity had a significant effect, likely because sweat evaporates less easily in humid air. Cooler rooms were not tested, and the study was small: 20 people under controlled lab conditions.

In practice:

• In an air-conditioned room at around 26–28 °C, the study found that lowering humidity did not make people feel noticeably cooler. There is no evidence here that it would in cooler rooms either. The main reasons to dehumidify there are moisture problems: mold, musty smells, condensation and damp fabrics.
• In a warm room at 30 °C or more, drier air can help your body shed heat. But a dehumidifier also adds heat, so it is not a substitute for cooling.

For this reason we do not assume that a dehumidifier will let you raise your AC setting or cut your electricity bill. That depends on the home and has to be checked, not promised.

What decides how much warmer a room gets

How to keep the extra heat from becoming a problem

1. Use a humidity target, not continuous mode. Set the humidistat (the unit’s humidity setting) below 60%, for example around 50%, in line with U.S. EPA advice. The unit then stops once the room is dry enough. Less running time means less heat.
2. Run it while the AC is on in rooms that are air-conditioned anyway. The AC removes the added warmth. Our guide on using a dehumidifier with AC covers settings in more detail.
3. Point the outlet into the open room, not at you, a bed or the AC indoor unit. Warm air blown straight at the AC’s sensor may make it read the room as warmer than it is and run longer than needed.
4. Expect warming in small closed rooms with no AC, such as store rooms and wardrobe rooms. Check the temperature after a few hours, and stay within the operating temperature range in your unit’s manual.
5. Fix the source of moisture first where you can: leaks, wet laundry drying indoors, or doors left open to humid outdoor air. Less water to remove means less heat released. See when to use a dehumidifier for the situations that call for one.

A simple check you can do at home

If you want to see the effect in your own room, this is a test you can run. It is a suggestion, not a test we report results from.

1. Place a thermometer-hygrometer at about chest height in the middle of the room, at least a metre from the dehumidifier outlet.
2. Close the door. With the AC running as usual and the dehumidifier off, note the temperature and humidity at the start and after two hours.
3. On another day, at the same time of day and with the same AC setting, repeat the two hours with the dehumidifier running.
4. Note the temperature, the humidity and how much water is in the tank.

Sun, outdoor weather, AC cycling and people in the room all affect the result, so treat it as a rough guide.

Then work out the dew point before and after, using the calculator in our dehumidifier guide. A lower dew point shows real moisture removal. If the temperature rose by more than you are comfortable with, try a shorter runtime, a higher humidity target, or moving it to a room with AC.

When this approach will not solve the problem

• If the room is too hot, a dehumidifier will not fix it. It is not a cooler. A room that is too warm needs cooling or shading.
• If humidity stays high despite hours of running, the unit may be too small for the room, or moisture may be coming in faster than it can remove it. Our guide to what 20 litres per day means explains rated capacity.
• If you see mold, lowering humidity helps prevent new growth, but it does not remove mold that is already there. The U.S. EPA notes that the key to mold control is moisture control, and recommends keeping indoor humidity below 60%. See our guide to mold on walls for what to do first.

Frequently asked questions