Last Updated on September 1, 2026 by Umar Farooq
Run a dehumidifier in summer only when the room actually reads above 55–60% relative humidity — not because it is July. Warm air holds far more water than cold air, so summer is the season with the most moisture available, and in a humid climate a dehumidifier earns its keep from June to September without argument. In a dry summer, in a well-ventilated house, the same machine is an expensive fan.
The difference between those two situations is a reading on a hygrometer, and it costs about five pounds to find out which one you are in.
Rule 1: measure before you run
Summer humidity swings enormously — across the day, across a week of weather, and from room to room in the same house. A basement can be at 72% while the upstairs sitting room is at 48%, at the same moment, on the same afternoon.
Put a hygrometer in the room and watch it for a week before deciding anything:
- Consistently above 60% — run it. Mould can establish on a suitable surface within 24 to 48 hours up here, and that is the real cost of ignoring it.
- 55–60% — worth running on a humidistat set to 50%. It will cycle itself and cost very little.
- Below 50% — switch it off and put it away. There is nothing for it to do, and every hour it runs is heat added to a room you already think is too warm.
The rooms most likely to need it in summer are the ones with the coolest surfaces and the least air movement: basements, cellars, ground-floor back rooms, and anywhere north-facing that stays shut.
Rule 2: it will make a hot room more bearable, but not cooler
Worth being precise, because the marketing is not.
A dehumidifier warms the room slightly — both its coils are indoors, so the heat it removes from the air goes straight back in, plus the work of the compressor and fan. Yet it makes a muggy room feel dramatically better, because sweat can only cool you when it evaporates, and it evaporates badly into air already loaded with water.
The scale of that effect is worth knowing. On the National Weather Service heat index, air held at 30°C / 86°F feels like 38°C / 100°F at 80% humidity and like 30°C / 86°F at 45%. That is roughly 8°C of felt heat removed, with the thermometer not moving at all.
So the honest framing: it is a comfort appliance in summer, not a cooling one. If your complaint is that the room is genuinely 32°C, you want air conditioning. The full answer on cooling goes through where the line falls.
Rule 3: do not put it in the same room as the air conditioner
This is the mistake that quietly costs the most money, and I have yet to see it stated clearly anywhere.
If both machines run in the same room, the dehumidifier’s warm coil is dumping heat into exactly the air the air conditioner is paying to remove. Each is making the other work harder, and you are paying both bills.
Separate them. Put the dehumidifier where the moisture comes from — the basement, the utility room, the space where washing dries — and let the air conditioner handle the living space. Done that way they genuinely help each other: drier air is cheaper to cool, because the cooling coil no longer has to condense water out before it can drop the temperature.
Rule 4: shut the windows while it runs, and open them when it doesn’t
An open window during a run means you are dehumidifying the outdoors — a job with no end and a meter that keeps turning.
But ventilation is not the enemy, timing is. On a summer evening when the outside air is drier than the inside air, throwing the windows open shifts more moisture in twenty minutes than the machine manages in hours, and it is free. So: ventilate hard when the outside air is drier, close up and run the machine when it isn’t. More free options in how to dehumidify a room.
Rule 5: put it on a drain hose for the season
In summer a dehumidifier collects a great deal more water than it does the rest of the year, because there is far more of it available. A tank that lasts two days in March will fill overnight in August, and a full tank means a machine that stops.
Running a hose to a floor drain, a sink or a condensate pump turns it from something you tend into something that just works. It also removes the most common cause of a musty smell, because water never stands in the machine. Setting the hose up properly is twenty minutes and the single best upgrade to summer running.
The filter needs cleaning more often in summer too, because the interval is measured in running hours rather than weeks.
What to set it to
Fifty per cent, and leave it. Not 40%, and definitely not 35%.
The setpoint decides when the machine stops, not how hard it works, so a lower number does not dry the room faster — it just means the unit never switches off. Fifty sits comfortably below the 60% mould threshold and above the point where you pay for nothing. The settings guide has the few exceptions.
One genuine summer adjustment: fan on high while you bring a room under control, low once the reading is stable.
When to switch it off entirely
- The room has held below 50% for a fortnight.
- You are running air conditioning in the same room, which is already dehumidifying as a side effect of its cold coil.
- The windows are open all day anyway, in which case the machine is treating the neighbourhood.
Store it properly rather than shoving it in a cupboard: empty and dry the tank completely, wash the filter, leave the tank unsealed. Water standing for six months is how floats seize and how a permanent smell starts. If the space still needs drying through the cold months, that is a different machine and a different discussion.
Frequently asked questions
Should I run a dehumidifier in summer?
Only if the room reads above about 55–60% relative humidity. Summer air holds the most moisture, so many damp rooms do need it — but a room already sitting at 45% has nothing for the machine to remove, and running it just adds heat.
Does a dehumidifier help in hot weather?
It helps a great deal with how the heat feels, because dry air lets sweat evaporate properly. It does not lower the actual temperature, and will raise it slightly. On the heat index, dropping from 80% to 45% humidity at 30°C removes about 8°C of apparent temperature.
Is it better to use a dehumidifier or air conditioning in summer?
Air conditioning if the temperature itself is the problem; a dehumidifier if a moderate temperature feels unbearable because the air is damp. If you have both, put them in different rooms so they are not fighting each other.
What should I set my dehumidifier to in summer?
Fifty per cent. Lower settings do not dry the room faster — they only stop the machine switching off. Drop to 45% only if walls are cold or mould has already appeared.
Should I close windows when running a dehumidifier in summer?
Yes, while it runs. An open window means you are trying to dry outdoor air. Ventilate hard at the times of day when the outside air is drier than the inside air, then close up and run the machine.
Do dehumidifiers use a lot of electricity in summer?
They run more hours than in other seasons, so the seasonal cost is higher even though the machine is unchanged. Take the wattage from the rating plate, multiply by hours run and divide by 1,000 for kWh, then apply your own tariff.
Will a dehumidifier stop mould in summer?
It will stop the humidity that feeds mould, which is most domestic cases. It will not stop mould being fed by an actual leak, and it does not clean up mould that has already grown — that is a separate job.
Related guides: Can a dehumidifier cool a room? · Dehumidifier settings · Dehumidifier in winter · Dehumidifier drain hose setup
Sources
- NOAA National Weather Service — Heat Index — the apparent-temperature figures behind the comfort argument.
- EPA — Mold Course, Chapter 2 — the 60% threshold and how quickly mould establishes above it.
- ENERGY STAR — Dehumidifiers — the 30–50% target band and current efficiency ratings.
Where this guide and your appliance’s own manual disagree, follow the manual. And if yours has been running flat out since May without you ever checking the reading — go and look. There is a fair chance it finished the job in June and has been billing you ever since.
