Quick answer
Set a basement dehumidifier to 45–50% relative humidity in summer. Below 40% the unit runs almost non-stop for no benefit, and above 60% mould and dust mites take hold. Run a hose to the floor drain so it never stops on a full bucket, confirm the reading with a separate hygrometer, and turn it down or off in winter.
Set your dehumidifier to 50% in the basement to start, and drop it to 45% only if the room still smells musty after a week. That number is a relative humidity target: the unit runs until the air reaches it, then cycles on and off to hold it. The dial matters less than three other things, though: whether the unit can drain, whether it is big enough, and whether its built-in sensor is telling the truth.
Why 45–50% is the right basement dehumidifier setting
- Below 40%: wasted electricity. The compressor rarely shuts off, a cool basement in July may never get there, and nothing stored downstairs needs air that dry.
- 45–50%: the target. It is dry enough to stop mould and the musty smell, with a few points of margin for an inaccurate sensor.
- 50–60%: tolerable for a few days during a heat wave, but not a setting to choose.
- Above 60%: mould grows on cardboard, drywall paper and carpet backing, dust mites multiply, and cold ducts and water pipes start to drip, which is explained in why ducts sweat in summer.
Health Canada's guidance for homes lines up with this: about 50% in summer and about 30% in winter.
The built-in humidistat can be off by five points or more, because the sensor sits inside the cabinet next to the coil. Put a separate digital hygrometer across the room at chest height. If it shows 58% while the dehumidifier display says 50%, lower the setting until the hygrometer reads 50%, and keep the basement windows shut while it runs.
Continuous drain to the floor drain, a pump, or the bucket
A portable dehumidifier shuts itself off when the bucket fills, and in a damp basement that can take less than a day. Once that happens the setting is irrelevant, because the unit is not running. Pick the drain method before you fuss over the dial.
- Gravity hose to the floor drain: most units have a garden-hose thread on the back. The dehumidifier has to sit higher than the drain, on a shelf or blocks, and the hose must slope downhill the whole way with no loops. The steady trickle also keeps the floor-drain trap full, which prevents the problem described in why vents smell like sewage.
- Condensate pump: when there is no floor drain nearby, a built-in or add-on pump lifts the water to a laundry tub. Pumps fail quietly, so test it at the start of every season.
- Bucket: workable only if someone empties it daily. A week at the cottage is a week with no dehumidification.
What size dehumidifier does a basement need, in litres per day?
Size by how damp the basement is first and by floor area second. Portable units sold in Canada are rated by the litres, or pints, of water they remove in a day at a cool test condition. The ratings were re-based a few years ago, so an older 70-pint unit and a current 50-pint unit are about the same machine.
- About 10 L per day (20–22 pint): one finished room, or a small basement that is only slightly damp.
- About 14–16 L per day (30–35 pint): a typical 70–110 square metre (750–1,200 square foot) basement that feels damp and smells musty in July.
- About 23–24 L per day (50 pint): a large or open basement, visible damp on the walls, or laundry drying downstairs.
- Roughly 30–60 L per day: a ducted whole-home dehumidifier, which serves every floor and drains itself.
Capacity falls as the room gets cooler, and the coil on a basic unit can frost over in a cold basement unless it has auto-defrost. If a properly sized unit with a drain hose runs all day and the hygrometer still will not drop under 55%, either the basement needs more capacity or liquid water is getting in. The causes are laid out in why a basement is humid in summer.
What to do with a dehumidifier in winter
Turn it down or switch it off once the furnace is running. Cold outdoor air holds very little water, and once it leaks into the house and is heated, basement humidity usually falls under 40% with no help. A unit left at 50% will simply never start, and a portable in a cold room can ice its coil.
Before you put it away, empty and dry the bucket and wash the filter so nothing grows in it over the winter. Wet windows in January are a ventilation issue for the HRV or the bathroom fans to solve, not a job for the basement dehumidifier. Plug it back in around late May, when the hygrometer climbs past 50% again.
How a whole-home dehumidifier is controlled from the thermostat
A ducted whole-home dehumidifier sits beside the furnace, connects to the return duct and is set once: 45–50% on its own control, or on a thermostat that has a dehumidifier accessory terminal. When humidity rises above the setting it starts, can bring on the furnace fan to move the dry air through the house, and drains to the floor drain. It also works on the cool, damp days of June and September when the air conditioner is not running at all.
Installed cost is $1,500–$3,000, depending on capacity, the duct connections and how far the drain has to run. That is worth pricing when a portable unit has been running flat out for two summers, or when the humidity problem reaches the main floor of a house near Lake Ontario. ZK Mechanical sizes and installs whole-home dehumidifiers and gives you the figure in writing first; call (647) 801-1252 or ask for a quote. Other humidity questions are answered under air quality and ducts.