Before you begin
What affects the dehumidifier size you need
Measure the area of the connected space the machine will actually serve.
Choose severe only for persistent moisture, not a single cold morning.
Answer the temperature question for how cold the space gets while the machine would be running, not for how warm it is when you are in it.
Reading the answer
Understanding dehumidifier capacity
The litres-per-day figure is the size of machine to buy, not the water you should expect it to collect in an Irish winter.
Above 15°C a compressor is the cheaper machine to run. Between 10 and 15°C it has to be a large one. Below 10°C only a desiccant is dependable.
Boundaries
Assumptions and limitations
Working assumptions
- The size is rounded up to a machine actually sold, and the two kinds of machine are not sold in the same sizes: compressors from 10 to 30 L/day, desiccants only from 8 to 11.
- Running cost uses the manufacturer's published draw at 20°C and 60% humidity for a machine of the size recommended, rather than one figure for every compressor or one for every desiccant.
- The litres-per-day requirement itself — the floor-area bands and the damp multiplier applied to them — is a Verdanyx model.
Where to be careful
- A dehumidifier does not repair leaks, rising damp, failed gutters or thermal bridges.
- Persistent mould needs the moisture source addressed as well as cleaning and ventilation.
- The type of machine, its running draw and the sizes on the ladder are manufacturer figures. How many litres a day the room is judged to need is not: that part is still a Verdanyx model of floor area and damp severity.
- A desiccant’s litres-per-day rating is measured in cooler, drier air than a compressor’s, so the two figures are not directly comparable and the size shown for a desiccant is the class of machine rather than an exact match.
Worked example
The same question, answered end to end
A 20 m² heated room with moderate damp and no laundry drying, on a 36c unit rate.
What was entered
- 20 m² of floor area to be dried, including any room left open to it.
- Moderate damp: condensation on the windows most mornings.
- The room is kept at 15°C or warmer, so a compressor is the cheaper machine.
- No laundry regularly dried in the space. Electricity at 36c per kWh.
How it is worked out
A 20 m² room bands to 10 L/day, and moderate damp multiplies that by 1.25, giving a raw requirement of 12.5 L/day.
That figure is rounded up to the next size on the 8, 10, 12, 16, 20, 25, 30 ladder, giving 16 L/day, because those are the machines actually sold.
At 15°C or warmer a compressor works, and it is the cheaper machine to run.
A 16 L/day compressor is priced on the published draw for its size class, 216 W, or 0.22 kWh an hour.
0.22 kWh at 36c is about 8c an hour. Six hours a day for seven days is 9.24 kWh, or €3.33.
What the tool returns
- Size to buy
- 16 L/day
- Type of machine
- Compressor
- Cost per hour to run
- €0.08
- At six hours every day
- €3.33/week
Common questions
Questions about this tool
Why might a 20 L machine collect much less than 20 litres?
The rating is normally measured in warm, very humid laboratory air. Extraction falls sharply in cooler real rooms.
Why does a cool room sometimes get a desiccant and sometimes a compressor?
Between 10 and 15°C a compressor only keeps collecting if it is a larger machine, around 20 L/day or more. Where a desiccant can cover the room it is the simpler buy; where the room needs more than the largest desiccant made, the answer steps up to one of those larger compressors.
What indoor humidity should I aim for?
About 50–60% relative humidity is a practical target. Sustained readings above roughly 70% support mould growth.