Dehumidifier or air conditioner: which machine solves your problem
Contents (9)
A dehumidifier and an air conditioner contain the same components — compressor, refrigerant, two heat exchangers, a fan — arranged to do genuinely different things. Shops sell them side by side in July, and European households buy the wrong one often enough that it is worth spending two minutes on the physics before spending several hundred euros on the box.
The one difference that decides everything: where the heat goes
A split air conditioner is a heat pump with its two coils in different places. The indoor unit absorbs heat from the room; the compressor raises its temperature; the outdoor unit dumps it into the street. Because the indoor coil runs below the air’s dew point, water condenses on it and drains away, so cooling also dries — a typical domestic split removes a litre or two an hour while it works. Net effect on the room: less heat, less moisture.
A dehumidifier is the same machine with both coils in one cabinet. Air is drawn over the cold coil, water condenses into a tank, and the now-dry air is passed over the hot coil and blown back into the room. Nothing leaves the building except the water. The latent heat released as the vapour condenses, plus the entire electrical input of the compressor and fan, stays exactly where you are standing. A dehumidifier warms the room it dries, in practice by one to two degrees.
That single fact answers most of the purchasing questions people actually have. It also explains why the standard summer disappointment — buying a dehumidifier to survive a heatwave — ends badly.
Match the machine to the symptom
| What you are experiencing | Right machine |
|---|---|
| Streaming condensation on window panes in winter, black mould in bedroom corners | Dehumidifier, plus a ventilation fix |
| Drying out a flat after a leak, or new plaster and screed | Dehumidifier (a builder’s unit if the volume is large) |
| A 28 °C, sticky night on a Mediterranean or Adriatic coast | Air conditioner — a dehumidifier would make the room warmer |
| 35 °C of dry inland heat | Air conditioner; there is little moisture to remove |
| Laundry drying indoors in an unheated flat | Dehumidifier, generally cheaper per load than a tumble dryer |
| A cellar, garage or boat at 8–12 °C | Desiccant dehumidifier; compressor models lose most of their output in the cold |
| Damp tide-marks low on a wall, or mould along one cold external corner only | Neither — that is a building defect, and a machine will only mask it |
Northern Europe’s damp problem is a winter problem
In Britain, Ireland, the Netherlands and the Nordic countries, household damp is overwhelmingly a cold-season condensation phenomenon rather than a heat phenomenon. A family generates several litres of water vapour a day simply by breathing, showering, cooking and drying clothes. That moisture stays airborne while the air is warm, then condenses the moment it touches a surface below its dew point: single-glazed panes, an uninsulated reveal, the cold corner where two external walls meet, the wall behind a wardrobe with no air gap.
Aim for 40–60% relative humidity in occupied rooms. The number that actually governs mould is not the humidity of the air in the middle of the room but the relative humidity at the coldest surface, which can sit far higher; mould germinates where surfaces are held above roughly 80% for days at a time. This is why a room can read a comfortable 55% on a hygrometer and still grow mould behind the sofa.
An air conditioner is close to useless against this. Most European splits are not rated to run in cooling mode with outdoor temperatures near or below about 10–15 °C, and the dry mode that dehumidifies so nicely in August works by cooling — which is the last thing a cold, damp November bedroom needs.
Compressor or desiccant
Domestic dehumidifiers come in two technologies, and the choice is decided by temperature, not by price.
| Compressor (refrigerant) | Desiccant | |
|---|---|---|
| Works well from | Roughly 15 °C upwards | Down to a few degrees above freezing |
| Typical draw | ≈ 150–350 W, cycling with a humidistat | ≈ 300–650 W, close to constant |
| Heat added to the room | Modest | Noticeably more — sometimes welcome in a cold room |
| Best for | Heated living space, summer and shoulder seasons | Unheated garages, cellars, holiday homes, garden rooms |
| Weight and noise | Heavier, compressor hum | Lighter, a steadier fan-and-heater note |
Reading the specification honestly
The headline number on the box — 12 L/day, 20 L/day — is measured at a standard hot and saturated laboratory condition, commonly 30 °C and 80% relative humidity. A British bedroom in November at 16 °C and 70% is a different physics problem, and the same compressor unit may deliver a quarter of its rated extraction there. Manufacturers who also publish a figure at 27 °C / 60% are being more useful; the gap between the two numbers tells you how quickly performance falls away.
There is also no energy class to fall back on. Household dehumidifiers are explicitly excluded from the scope of the EU energy labelling and ecodesign regulations that cover air conditioners, and the Commission’s own guidance states that no EU energy rating for dehumidifiers has been defined. You therefore cannot shop them the way you shop a split, where the classes are directly comparable — that framework is explained in our guide to the EU energy label for air conditioners. For a dehumidifier, compare stated power in watts, extraction at a stated test condition, tank capacity, whether continuous drainage is possible, and whether it has a real humidistat rather than a timer.
What each costs to run
Order-of-magnitude arithmetic, using an indicative European domestic tariff band of roughly 0.15–0.35 € per kWh — national prices vary widely and the bands here are illustrative, not quotations.
- Dehumidifier, 250 W, cycling about eight hours a day: ≈ 2 kWh, roughly 0.30–0.70 € a day. Run continuously through a damp winter that becomes a real line on the bill, which is why the humidistat setting matters.
- Split air conditioner, 3.5 kW class averaging about 0.8 kW over six hours: ≈ 4.8 kWh, roughly 0.70–1.70 € a day during the cooling season only.
The comparison is misleading if read as a league table, because the two machines are used in different months. In an Irish or Dutch home the dehumidifier may run on 150 days a year and the air conditioner on fifteen. The full tariff picture sits in our guide to running costs across Europe.
Two habits change the bill more than the model does. Run a dehumidifier in a closed room, with internal doors shut, or it will patiently dry the whole street’s worth of air passing through the flat. And set the humidistat to 50–55% and let it cycle, rather than to its minimum: chasing 35% costs a great deal of electricity for air that is now uncomfortably dry.
Ventilation is the actual cure
A dehumidifier treats a symptom well. Removing the moisture at source treats the cause and costs less: lids on saucepans, extractor fans that are actually used during and after showers, trickle vents left open, airbricks not blocked, tumble dryers vented or condensing, and enough background heating that surfaces stay above dew point. Mechanical ventilation with heat recovery does the job continuously in newer or retrofitted homes. Where mould follows a cold bridge or rising damp, the building needs the repair — no appliance substitutes for it, and running a dehumidifier for years is the more expensive way to lose.
Do you need both?
Plenty of European households genuinely do, and use them in opposite seasons. Coastal Mediterranean and Adriatic flats deal with muggy summers that call for cooling — see our split air conditioner page — and damp, poorly heated winters that call for drying. In the Atlantic north the balance tips heavily the other way: a dehumidifier earns its keep for half the year, while the air conditioner waits for the handful of days now arriving every July.
Frequently asked questions
Does an air conditioner dehumidify?
Yes, substantially, as a by-product of cooling: moisture condenses on the cold indoor coil and drains outside. Dry mode simply prioritises that by running the compressor with a low fan speed. But it only works while the machine is allowed to cool, so it is a summer tool, not an answer to winter condensation.
Will a dehumidifier cool my room?
No — it warms it, typically by one to two degrees, because all the electrical input and the latent heat of condensation are released inside the room. Drier air does feel more comfortable at a given temperature, so the perception is mixed, but the thermometer only moves upward.
What humidity level should I aim for?
40–60% in occupied rooms is the usual target: high enough to avoid dry airways and shrinking joinery, low enough to keep surfaces clear of mould. A hygrometer costing very little tells you whether you have a problem at all, and many households discover they do not.
Is it cheaper to run a dehumidifier than to heat the room?
Not as a heating strategy. The claim that drier air is cheaper to heat is true but small, and a dehumidifier is a poor electric heater — every watt does become heat, but you would get the same warmth from a plain heater for the same money. Buy one to control moisture, and count the warmth as a side effect.
Can a dehumidifier stop mould that has already appeared?
It stops it spreading by keeping surfaces below the humidity at which spores germinate, but existing growth has to be cleaned off, and any underlying defect — a cold bridge, a leak, rising damp, a blocked vent — has to be fixed. A machine that hides a building fault for three years is not a solution, it is a delay.
Sources
Read next
- AC noise rules in Europe: keeping the neighbours (and the law) calm
- Heating with your air conditioner through a European winter
- Portable vs split in Europe: renting, rules and the real math
- Cooling a server room or comms cabinet: why a comfort split is the wrong machine
- Multi-split systems: one outdoor unit, several rooms