# Desiccant or Compressor Dehumidifier? The Cold-Room Test

> Compressor dehumidifiers stall in cold rooms because the coil has to get below freezing. Work out the dew point in your room and the answer picks itself.

Canonical (HTML, zitierfähig): https://getecoback.com/en/guide/desiccant-vs-compressor-dehumidifier.html

Both types pull water out of the air and both are sold as "dehumidifiers", but they do it by opposite methods, and the method breaks down at different temperatures. In a warm flat the choice barely matters. In an unheated back bedroom, a garage, a conservatory or a hallway that never sees the boiler — which is where British damp actually lives — it decides whether the machine works at all.

Here is the test, and you can run the arithmetic yourself below.

## A compressor dehumidifier is a fridge that has to beat the dew point

A compressor unit blows room air across a coil it has chilled with refrigerant. Water condenses on the coil and drips into the tank. For that to happen at all, **the coil has to be colder than the dew point of the air in the room** — the temperature at which that air is saturated. That is not a preference, it is the definition of condensation.

So the question becomes: how cold is the dew point in a room like yours? That is pure arithmetic from the temperature and the humidity, using the same Magnus formula every calculator on this site uses:

Room temperature | Dew point at 60% RH | Dew point at 70% RH |

20 °C (heated living room) | 12.0 °C | 14.4 °C |

15 °C (cool bedroom) | 7.3 °C | 9.6 °C |

12 °C (unheated room) | 4.5 °C | 6.7 °C |

10 °C (garage, utility) | 2.6 °C | 4.8 °C |

8 °C | 0.7 °C | 2.9 °C |

5 °C | −2.1 °C | 0.0 °C |

Read the 10 °C row. To take water out of a 10 °C room at 60% humidity, the coil has to sit below **2.6 °C** — and a coil does not run at the dew point, it runs well under it, because the air has to be chilled past saturation in the time it takes to cross the fins. In a room that cold, that means below zero. **Frost forms on the coil, the machine stops to melt it, and the hour you paid for becomes part drying and part defrosting.**

That is the whole reason the type matters. It is not a quality difference between brands. It is the dew point falling with the temperature until the machine is fighting ice instead of water.

**Work out your own room**
Room temperature (°C)

Relative humidity in the room (%)

Find the dew point

A cheap hygrometer gives you both numbers. The humidity reading is the one people guess wrong.

## A desiccant dehumidifier does not care about the dew point

A desiccant unit has no cold coil. Air passes through a wheel coated in a material that adsorbs water vapour directly, and a heater then drives the water back out of the wheel so it can keep going. Nothing has to be chilled below anything, so **the cold room that stalls a compressor is simply a normal working day for a desiccant machine**. It keeps extracting down to a few degrees above freezing.

Two consequences follow, and both are real:

- **It uses more electricity per hour.** The regeneration heater is the point of the design, and it is the bulk of the draw. A desiccant unit typically sits well above a comparable compressor unit on the plate.

- **It warms the room.** That heater's energy ends up in the air coming out of the machine. In a cold utility room people like this. It is not free heating — see the caveat further down.

## The honest running-cost comparison

At the Ofgem price cap for **1 October to 31 December 2026**, electricity on a Direct Debit tariff is **26.32p per kWh**. Scale the hours to your own run.

Machine | Plate | Per hour | 8 hours | Cost (£) |

Compressor dehumidifier, small | 200 W | 5.3p | 1.6 kWh | 0.42 |

Compressor dehumidifier, typical | 300 W | 7.9p | 2.4 kWh | 0.63 |

Desiccant dehumidifier, low setting | 330 W | 8.7p | 2.6 kWh | 0.69 |

Desiccant dehumidifier, high setting | 650 W | 17.1p | 5.2 kWh | 1.37 |

Unit rate only, standing charge excluded — that is a daily fixed cost you pay whether the machine runs or not. There is no VAT on electricity from 1 October 2026 to 31 March 2027. Rate from Ofgem, read 2026-09-17. Your tariff may differ; the sum is watts ÷ 1000 × hours × your rate.

Note what this table cannot tell you: **cost per hour is not cost per litre.** If the compressor unit in your 10 °C garage spends a third of each hour defrosting, its real cost per litre removed is half again what the wattage suggests, and the desiccant machine can end up cheaper for the job despite the bigger number on the plate. Rate the machines at the temperature of your room. The litres-per-day figure on the box is measured at conditions — warm and humid — that a British winter room does not meet.

**The heat is not a bonus.** A desiccant dehumidifier does put its electricity into the room as warmth, and it is tempting to count that as heating you would have paid for anyway. Resist it unless you were actually going to heat that room. A fan heater turns the same kilowatt-hour into the same heat, and if the room is one you normally leave cold, the warmth is simply spend you did not plan.

## So which one

Where it is going | Type | Why |

Heated living room or bedroom, 18–21 °C | Compressor | The dew point is 10–14 °C, comfortably above the coil. More water per unit of electricity. |

Cool bedroom that dips below 15 °C | Either, compressor if it stays above 15 °C | This is the crossover. Watch what the room actually does overnight rather than what you assume. |

Unheated room, hallway, spare room | Desiccant | Compressor extraction falls off exactly where you need it most, in the cold months. |

Garage, conservatory, caravan, boat, loft | Desiccant | Regularly under 10 °C. A compressor unit will ice up and mostly defrost. |

Drying laundry indoors in a heated room | Compressor | You are running it in warm, humid air, which is where the compressor design is at its best. |

## What neither of them fixes

A dehumidifier removes water from the air. If the water is arriving through the fabric of the building — up a wall from the ground, or in through a defect — the machine will run all winter, cost you every hour of it, and the wall will still be wet. Before buying either type, be sure you are looking at condensation and not one of the other two kinds of damp: [how to tell the three apart](https://getecoback.com/en/guide/rising-damp-penetrating-damp-or-condensation.html).

And if the humidity is high because of what happens in the room rather than what comes into it, the cheapest machine is sometimes no machine. [Ventilation does the same job for nothing](https://getecoback.com/en/guide/tilt-and-turn-windows-winter-condensation.html), when the outside air is colder and therefore drier than the air indoors — which in a British winter is most of the time.

## Sizing, once you have picked a type

Litres per day is the rating, and the honest version of it is that manufacturers measure at about 30 °C and 80% humidity, so real extraction in a cool British room is typically around half the number on the box. Our size ladder is here: [what size dehumidifier for a room this big](https://getecoback.com/en/guide/dehumidifier-20-sqm.html), with every rung and the rule behind it in [the open dataset](https://getecoback.com/daten.html).

## Frequently asked questions

### Which is better, desiccant or compressor?

Neither, on its own. Above about 15 °C a compressor unit removes more water per unit of electricity, so it wins in a heated room. Below that its coil ices up and it spends its time defrosting instead of drying, which is where a desiccant unit takes over. The room temperature decides, not the litres-per-day rating on the box.

### Why does a compressor dehumidifier stop working when it is cold?

It dries air by condensing water on a cold coil, so the coil has to be colder than the dew point of the room. At 10 °C and 60% relative humidity the dew point is 2.6 °C, so the coil must run below that, and in practice it drops below freezing. Frost forms, and the machine has to stop and melt it off.

### Do desiccant dehumidifiers use more electricity?

Per hour, usually yes, because they run a heater to regenerate the drying wheel. Per litre of water actually removed in a cold room they can use less, because a compressor unit in the same room spends part of every hour defrosting rather than extracting. Compare the machines at the temperature of your room, not at the rating conditions on the box.

### Will a dehumidifier heat the room?

Both types put nearly all the electricity they draw into the room as heat, and a desiccant unit adds the heat from its regeneration cycle on top, so its outlet air is noticeably warm. That is a side effect worth knowing about, not a reason to buy one as a heater: a fan heater delivers the same heat for the same electricity.

**What we have not done.** We have not tested these machines. Everything above is either arithmetic you can repeat — the dew points come from the Magnus formula, the costs from watts × hours × the Ofgem rate — or a statement about how the two designs work. Where a number depends on a specific model, we have given you the method instead of a figure, because the plate on your machine is the only one that applies to you.

---
Maschinenlesbare Übersicht: https://getecoback.com/for-agents.html · Sizing-Datensatz (CC BY 4.0): https://getecoback.com/sizing-data.json
MCP-Server für Assistenten: https://getecoback.com/mcp
Note: EcoBack does not test devices itself; picks summarise public tests. The site is funded via Amazon affiliate links on the HTML pages — this Markdown view deliberately contains none.