How Do Dehumidifiers Work?

I’m sure it’s something most of us will be familiar with around this time of year, condensation building up on window sills is the most obvious one, and musty smells are another. Both are clear signs that you have excess moisture building up in your environment, and we see these scenarios across every sector in the industry.

As we start closing windows and doors and sealing up our homes for the colder months, dealing with the humidity left behind is the next challenge. The first thing I notice is the condensation that appears on window sills first thing in the morning. Normal activities such as showering, drying laundry indoors and cooking can release up to 12 litres of water vapour per day.

Unless this moisture is removed, it’s going nowhere fast.

It simply builds up indoors. Over time this will lead to condensation and eventually more serious problems such as mould growth and structural decay.

This is when we use a dehumidifier to pull moisture from the air and to protect materials such as timber, plasterboard and insulation from rot. Relieving these damp conditions will remove mould and dust mites from the environment and promote healthy indoor air and with dry air being cheaper to heat, this will have a huge effect on your energy bills too.

While they are essential for homes, at National Heater Shops we understand their importance in the professional sector as well. We can provide the equipment needed for flood restoration projects and construction sites and understand the differences in the technologies, such as refrigerant and desiccant.

Key Takeaways: How Dehumidifiers Control Humidity

• Controlling Humidity is Essential: It’s not just about comfort; it’s the only way to protect your home from condensation, mould growth and long term structural damage.
• The Ideal Humidity Range: You’re aiming for a Relative Humidity sweet spot of 45%-55% to maintain a healthy environment.
• Match Technology to Temperature: Refrigerant (Compressor) units for heated living spaces (above 15°C) and Desiccant units for unheated areas like garages or lofts.
• Protect Structural Integrity: Protect your home's timber frames and insulation from swelling and rotting by managing water vapour.
• It Is Easier to Heat Dry Air: Removing excess moisture lowers the thermal mass of the air. This now dry air is easier to heat and can even reduce energy costs.

What is a Dehumidifier?

We use dehumidifiers to manage relative humidity (RH). RH is the amount of water vapour present in the air in relation to the amount it can hold at that temperature. Warm air holds more moisture, while cold air holds less. As the air cools it reaches a point where excess vapour condenses into liquid water.

Your ideal RH range is typically around 40% to 60%, this promotes healthy indoor air and contributes to the structural integrity of your building. Once RH exceeds these levels the air becomes saturated, and this will inevitably lead to damp and mould growth.

How the Technology Works

The most common technology used is Refrigerant (Compressor) and can be found throughout the industrial and domestic sectors. These work by following a precise cycle:

• An intake fan draws moisture through a filter and over a series of cold evaporator coils.
• As it cools below its dew point, it changes state and turns into liquid water.
• Depending on the model, the water is either collected in a tank for manual removal or via a pump for continuous drainage (essential for 24/7 commercial use).
• The dry air is now released back into the room, lowering the RH of the space.

Versatility Across Sectors

Dehumidifiers are particularly useful in server rooms for protecting equipment such as rack servers, routers, fibre optic patch bays. I used to work in IT and we had a dehumidifier running in our server room initially due to a leaking roof. Even after the leak was fixed, we realised there was still a need for moisture to be controlled in there.

Our range also covers dehumidifiers used for construction and building drying. These professional units can be used to pull moisture deep from within a building's structure before deterioration begins.

Choosing the Right Technology

Selecting the wrong type of dehumidifier is more common than you might think as every environment is different. Choosing the wrong technology can not only result in poor performance, but can send the cost of running the unit soaring. It’s something we see quite often, someone is advised to buy the wrong type of unit and then later discovers the temperature is too low for the technology chosen.

The two main types are refrigerant (compressor) dehumidifiers and desiccant dehumidifiers. Both remove moisture from the air but use different technologies to do it and both are suited to the following environments:

• Refrigerant (Compressor) Dehumidifiers: These units are typically used for controlling indoor environments above 15°C. They use a compressor to circulate a refrigerant gas that cools internal metal coils to reduce moist air to its dew point when contact is made. In environments where the temperature is below 15°C the coils can start to frost over. This reduces productivity as the unit stops dehumidifying to melt the ice.
• Desiccant Dehumidifiers: These units have no cooling coils and are most suited for when temperatures are below 15°C. They often use a silica gel rotor that absorbs the moisture from the air. This is not too dissimilar to the packets of silica you find in shoeboxes for absorbing moisture. They are ideal for use in garages, classic car storage, unheated lofts, boats and remain effective all the way down to 0°C.

The Professional Verdict

A refrigerant model is the most cost effective tool for drying out a laundry room in a heated house. For use in unheated spaces, or for drying out a holiday home while you’re not there over the winter months, a desiccant model is the only reliable choice to prevent dampness.

Matching Dehumidifier Technology to Your Environment

Knowing the ambient temperature and moisture load for the room is essential for choosing a dehumidifier for its intended place of use. Using the wrong technology can lead to the unit switching on and off between cycles instead of efficiently pulling moisture from the air in one complete run.

Environments Above 15°C

A refrigerant (compressor) model is often the most economical solution for domestic spaces. For living spaces and kitchens that deal with spikes in humidity from boiling water or showering on a daily basis, RH levels can quickly reach 80%. A compressor unit is most suited for bringing levels back down to around 50% before condensation begins to settle on surfaces.

They’re also used throughout construction for drying out new plaster and floor screed. Professional units are often referred to as building dryers and can run constantly, pulling vast amounts of water vapour from the air to ensure materials dry correctly.

Unheated Environments Below 15°C

Desiccant systems can remain 100% efficient in temperatures all the way down to 0%. They are ideal for places such as garages, lofts and workshops where it is necessary to protect expensive tools from rust and structural timber from dry rot.

The Dry Air Bonus

As noted by the Energy Saving Trust, controlling moisture in any environment can be viewed as a money saving tactic. Water molecules suspended in the air have a high heat capacity that requires a significant amount of energy to heat up. Using a dehumidifier to remove moisture from the air not only makes it easier for your central heating, it saves you money while protecting your home from damp.

The Key Benefits of Using a Dehumidifier

Managing moisture indoors is an essential part of building maintenance and when left unchecked, can lead to damage that is often expensive to set right. Not long ago I noticed a huge build up in my Mum’s kitchen as she does not have an extractor fan. I bought her the Brolin BR10C to help keep the humidity under control. It’s a simple machine that is easy to operate and features a 2 litre removable water tank, so she doesn’t have to keep emptying it constantly.

Structural Care and Mould Reduction

Many of the materials found in our homes are hygroscopic, which means they act like sponges pulling water vapour from the air and holding it within the structure. I often view this as the silent threat posed by excess moisture.

• Eliminating Thermal Bridging Risks: By keeping relative humidity (RH) around 50%, you lower the dew point to prevent condensation forming on window frames or collecting in corners.
• Preventing Material Decay: High humidity can warp floors and lead to structural compromises in timber, resulting in wet and dry rot that can destroy a property.

Fighting Allergens & High Energy Bills

You’re not just protecting your home when managing humidity indoors, you’re physically changing the air that you heat and breathe inside your home. This goes a long way to promoting healthy indoor air quality and can even make the space more economical to heat.

• Bio-Aerosol and Allergen Control: When RH stays below 50%, indoor air quality (IAQ) improves dramatically. By keeping the air dry you remove the water needed in the air for dust mites to thrive while simultaneously creating a biological shield for use against mould growth.
• The Energy Saving Value: Damp air is harder to heat. If you can remove excess moisture from the air, you reduce the thermal mass your central heating has to deal with. Once removed, it will be quicker to heat and will also retain heat for longer.

Conclusion: Take Control of Your Indoor Climate

If you’ve underestimated the importance of excess moisture indoors, you will not be the first.

It’s one of those problems that most people overlook and only realise the importance of once damage is caused to the property. It can quickly move from a minor nuisance to a problem that not only affects the structure of the building but impacts energy bills and respiratory health too.

I’ve seen it many times myself – condensation forming on windows at morning time is often casually wiped off, but this is the first sign that humidity levels indoors are too high.

There is no dehumidifier on the market that offers a "one-size-fits-all" solution. Choosing the right model will ultimately come down to the unit's specification. At National Heater Shops, we offer an extensive range of dehumidifiers and our online support team can help you select the one that is most suitable for your needs.

Frequently Asked Questions About Dehumidifiers

What is the ideal relative humidity (RH) level for a building?
You should be aiming for somewhere in between 45% and 55% RH. The air is comfortable at this level and keeps you out of the danger zone where dust mites thrive and the conditions for mould growth are just right.
What are the warning signs of high humidity?
Well the obvious signs are the things you can see e.g condensation forming on window sills. You should also be aware of other signs that are not so obvious like wooden drawers that suddenly start to stick as the material expands.
Compressor vs. Desiccant: Which should I choose?
The choice between the two will always depend on the temperature of the space they are to be used in. Compressors are the more economical of the two and we recommend these for use in heated living spaces where the temperature is above 15°C. Desiccants become the only real alternative if the temperature drops below this or they are destined for use in cold or unheated spaces.
Where is the best place to position my dehumidifier?
This is a really good question because in my experience the position of the dehumidifier can make or break the unit's success. Placing a dehumidifier in a central location such as a hallway or landing often yields the best results by encouraging airflow.
Can a dehumidifier replace a tumble dryer for laundry?
A dehumidifier will certainly be cheaper to run than a tumble dryer and many now feature laundry modes designed for drying clothes hanging on an airer. This is also better for your building's integrity too as drying clothes on radiators can release up to 5 litres of water back into the air.