How Long Should You Leave a Dehumidifier On?

One of the most common questions we get is “how long should I run my dehumidifier?”

It’s a great question. The goal of a dehumidifier is to maintain a lower relative humidity to prevent mould, mildew and dust mites, but leaving it on 24/7 on can rack up a nasty energy bill.

Ideally, a domestic home in the UK should have a maximum humidity of 55%. Dust mites struggle to survive at relative humidity of 50% or lower, so this is often a better target. The problem is that average humidity in a UK winter can be considerably higher than this, so a dehumidifier needs to be run consistently to maintain healthy levels.

Unfortunately, there’s no one-size-fits-all answer for how much time your dehumidifier needs to run though. The answer broadly depends on:

  • Temperature
  • Starting relative humidity
  • Target relative humidity
  • Your dehumidifier’s rate of extraction
  • Whether your dehumidifier has an automatic humidistat

Let’s take a closer look at each of these to help you decide how long to run your dehumidifier.

A Word About Extraction Capacities

Before we go any further, it’s important to understand what extraction capacities really mean.

Every dehumidifier is listed with a maximum extraction rate. These tend to range between 6-20 litres per day, but there are some domestic dehumidifiers with 25 litres per day extraction rates.

A dehumidifier will almost never remove this amount of water in a day. The quoted numbers are the maximum the machine can pull out of the air in ideal conditions (usually at 30 degrees Celsius and a humidity of 80% for refrigerant models).

To put this in perspective, these conditions would be more likely in a tropical rainforest than a flat in the UK! That’s why a dehumidifier will actually remove much less water in an average day.

That doesn’t mean extraction rates are completely useless though. You should still look for higher extraction rates in areas of extreme damp or for larger homes, but don’t expect to get anywhere near the quoted maximum.

Note: When we talk about humidity, we really mean relative humidity. This is how much water the air is holding compared to its saturation level. In other words, 100% humidity means the water is full and ready to condense. As warmer air has a higher saturation point than cold, the relative humidity increases as the temperature drops. A relative humidity of 40-65% is usually considered healthy.

Choosing the Right Dehumidifier

We’ve written a full guide to choosing a dehumidifier, but the type of dehumidifier can affect how quickly a dehumidifier pulls moisture from the air.

Refrigerant dehumidifiers work best in warmer conditions. They are the most common type in the UK and great for heated homes. They get less efficient as the temperature drops, however, which means they need to run for longer.

Desiccant dehumidifiers are a better choice for cooler conditions, such as garages or conservatories. They are less energy efficient though, so we usually don’t recommend them for warmer environments.

Mould on the corner of a room

Take Into Account the Humidity of the Room

It sounds obvious, but the damper a room is the longer it’ll take for a dehumidifier to reduce the relative humidity to a target level.

Certain rooms are often damper than others. Bathrooms and kitchens, for example, can have extremely high relative humidity as there are multiple water sources.

Bedrooms and living rooms can also become damp though. If you dry your laundry inside, have lots of potted plants, or even breathe with your mouth open at night, relative humidity could rise.

The First Run

When you first run your dehumidifier, the air is likely to be damp (I’m guessing that’s why you bought it!) It’s a good idea to set the dehumidifier to its most powerful setting and run it until the humidity reaches your desired target.

How long this takes depends on the level of humidity and the efficiency of your dehumidifier. If you’ve bought the right size dehumidifier for the area you want to dehumidify, and there are no major sources of damp, most models will reduce the humidity to healthy levels in a few days. It can take up to a week though – and even longer if there are sources of damp.

As the humidity is high during this stage, the tank will fill up quickly. Be prepared to empty it regularly until the humidity has dropped to your goal. You can also use a continuous drainage hose if you don’t want to empty the machine manually.

Note: This doesn’t apply if the room has been flooded. You’ll need specialist equipment to tackle this sort of damp.

Maintaining a Lower Relative Humidity

Once your home, boat or garage has reached a humidity of 55% or less, you probably won’t need to run it continuously on high power. Instead, you can rely on your dehumidifier’s built in humidistat (assuming it has one).

Many dehumidifiers have an automatic function that switches off the machine when a target humidity is reached, then turns it back on when it rises. This is great for maintaining your desired level of humidity without needing to worry about how long the machine should run for. It also minimises energy costs, as the dehumidifier is only running when you need it.

The downside is that these humidistats are often inaccurate. They are usually fine for domestic use, but if you need to maintain a specific relative humidity they probably won’t be up to the job.

If your dehumidifier doesn’t have an automatic function, you’ll need to experiment to find out how long the dehumidifier needs to run each day to maintain the right level of humidity. Many dehumidifiers have a timer function, so once you know how long to run it for you can set it to come on at some point during the day.

Always keep in mind that dehumidifiers cost money to run. If you don’t want to spend a lot of money, use a smart meter to know exactly how much energy you’re using.

Note: If your dehumidifier doesn’t display the current humidity, you’ll need to purchase a separate humidistat. These are relatively cheap, so are definitely worth buying.

Summary

The time required for a dehumidifier to reduce relative humidity depends on a number of factors. This is why it’s impossible to give a single answer to the question “how long should I leave my dehumidifier on?”

The best way to find out how long to run a dehumidifier is to experiment. Set a target humidity – 55% is a good start – and see how long the dehumidifier needs to run each day to maintain this target level.

Of course, if you have an automatic dehumidifier, the machine takes care of this for you. That’s why automatic models are often the best choice for saving energy. If you’re struggling to choose a model, see our top picks depending on your budget and requirements.

Do you have any other questions about how long to run a dehumidifier? Let us know in the comments section.

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