Do You Use Hot Or Cold Water In a Humidifier? (Explained!)

I once got a call from my friend asking me why her humidifier was malfunctioning. When I went to check it out, I found out she’d been pouring hot water into it.

Like my friend, you might be tempted to put hot water in a humidifier’s water tank to produce a nice warm mist. That’s a big mistake that will cost you your humidifier — and could even harm your health.

Read on and I will explain whether you should use hot or cold water in a humidifier!

Should You Put Hot or Cold Water into a Humidifier?

In general, you should always use room temperature or cool water in a humidifier. Making hot mist with hot water might seem sensible, but that will cause issues for your unit — just like it did for my friend.

Most portable humidifiers are designed to work with cold water. That’s the case even if you have a warm mist humidifier that ends up boiling water.

Using room temperature or cool water allows your humidifier to work more efficiently and prevents damage to the unit.

However, there are some types of whole-house humidifiers that actually require hot water.

I know this all might be a bit confusing, so let’s take a closer look at the most common humidifier types and what kind of water you should use with them.

Cool Mist Vs Warm Mist

Always use cool or cold water with portable warm mist and cool mist humidifiers, no matter their size. Doing so ensures the devices’ proper operation and humidifies dry air most efficiently.

I like to compare the hot vs. cold water situation to screwdrivers. When you pour hot water into a humidifier, you’re essentially handing it a flat-head screwdriver and telling it to drive in a Philips-head screw.

The tool is technically right, sure, but it’s not going to work very well!

Cool mist humidifiers — such as evaporative and ultrasonic humidifiers — just aren’t designed to work with hot water. Putting hot water in them will decrease their efficiency and could even damage the device. Additionally, the warm water tank can turn cold mist humidifiers into a breeding ground for bacteria and mold.

Warm mist humidifiers use a heating process to create hot steam, but you should let the machine boil the water instead of adding pre-heated water to it. You might think you’re helping your humidifier, but hot water can carry more minerals than cool water. You’ll only cake up the humidifier’s internals with mineral buildup.

I recommend using cool or room temperature water with every cool mist or warm mist humidifier. That way you can protect both your and your humidifier’s health.

Whole-House Humidifiers

Whole-house humidifiers, like Aprilaire, are an exception to the “always use cold water” rule. A whole-house humidifier — such as a bypass or drum humidifier — will actually work more efficiently when connected to a hot water line.

This is because whole-house humidifiers are typically furnace humidifiers that rely on heat to generate water vapor that humidifies your home. The water evaporates quicker and more efficiently when you connect the unit to hot water.

It all comes down to basic design. Whole-home humidifiers are built to use hot water — personal portable units are not.

That said, whole-house and furnace humidifiers can work with either hot or cold water. I wouldn’t worry if an HVAC technician happens to connect your unit to a cold water line.

Humidifier for Good Sleep

Why is Water Temperature Important to Humidifiers?

Water is your humidifier’s fuel. As the units’ entire performance depends on water, it only makes sense that using the right water temperature is essential. That’s why I recommend always putting cold water into your humidifiers.

But what happens if you do put hot water in a non-furnace humidifier?

In short, you will likely see many negative effects from running a humidifier with hot water. Here are the most common problems I’ve seen hot water cause for humidifiers:

Damage to the Humidifier

As I mentioned earlier, an evaporative or ultrasonic cool mist humidifier is not designed to work with hot water. Pouring heated water into the water tank could cause damage to the device and even break it.

Since these units are intended to work with cold water, manufacturers don’t use heat-resistant materials to make them. Hot water can warp or melt the surfaces and components in cool mist humidifiers, rendering them useless.

This is what happened to my friend. The hot water melted the water lines in her unit and she had to buy a new one.

Even if your cold mist humidifier can withstand the temperature, its filter may not. Hot water can damage the filter, and will definitely clog it up faster than cold water.

As a result, your machine can’t filter out pollutants and minerals as effectively, reducing indoor air quality.

Excessive Mineral Buildup

Hot water — whether tap water or boiled water — has a higher mineral content than cold water. Hot tap water will leach minerals from water pipes, while boiling water concentrates the existing mineral content.

Adding hot water to your device will cause mineral deposits to build up inside it. That is true for both cool mist and warm mist humidifiers.

Hot water won’t damage the components of a warm mist humidifier, but the minerals will quickly cover its heating element and inner parts. This makes the warm humidifier work harder to maintain an appropriate humidity level and reduces its lifespan.

Mold and Bacterial Growth

If there’s anything mold and bacteria love, it’s high temperatures and damp environments. Using hot water in your humidifier can turn it into a breeding ground for all kinds of harmful pollutants.

Your humidifier will then spew the mold spores and bacteria that grow inside it into your home’s indoor air along with the warm steam. Airborne contaminants can cause breathing difficulty, irritate your nasal passages, aggravate common cold symptoms, and trigger asthma attacks.

These pollutants can be very dangerous for people living with breathing issues, like me. That’s why I always stress how important avoiding putting hot water in humidifiers is.

Chemical Leaching

Hot water added to a cool mist humidifier can do more than damage its plastic parts. Since they’re not heat-resistant, the hot water could corrode them and leach harmful chemicals off the plastic.

If introduced to your indoor air, they may seriously harm your health. Some plastic chemicals, like BPA, may even cause cancer. (1)

Burn Risks

Curious children or pets can be drawn to inspect a running humidifier. My dog, for example, finds my humidifier extremely fascinating for some reason.

If a child or pet happens to accidentally knock over a humidifier filled with hot water, the spill could cause serious injuries, scalds, and burns.

I always recommend people avoid using any hot water humidifiers around children and animals. That applies to warm mist humidifiers as well as adding hot water to any other humidifier.


Last but not least, you should consider your humidifier’s warranty. The manufacturer has not intended portable humidifier units to work with hot water. In fact, I wouldn’t be surprised if the owner’s manual warns you against using hot water in them.

Putting hot water in the humidifier will likely void its warranty. If you then accidentally break your expensive device, you can’t get it fixed and will have to spend money on a new unit.

I’ve wrangled with questionable warranty policies enough to be able to tell you that ignoring the small print can cost you a lot of money.

White Humidifier on Top of Bedside Table with Books

What Kind of Water Can You Use in a Humidifier?

When debating whether to use cold or hot water in a humidifier, always go for room temperature, cool, or cold water. Hot water, whether warm or boiling, is simply not safe to use.

But there are other factors that you should consider as well when it comes to your choice of water. Here’s my breakdown of different types of water and whether you can use them in a humidifier.

Tap Water

My #1 piece of advice for all humidifier owners is to never use tap water in the unit. Tap water often carries bacteria and minerals that can get distributed into the surrounding air through the humidifier. This gets even worse if you use hot tap water, as it can leach even more pollutants out of water pipes.

Purified water

Purified bottled water is better than tap water, as it has had its bacterial pollutants removed during bottling. However, the purification process doesn’t necessarily reduce the water’s mineral content, so purified water is still not ideal for humidifiers.

Filtered water

Water filters, like a Brita filter, are fairly effective at removing contaminants like lead and calcium from water, but they can’t remove all dissolved solids, such as sodium. You may still see mineral buildup and white dust spreading around your home with the cold water vapor if you use filtered water in a humidifier. Just do what I do and drink your Brita-filtered water — don’t put it in your humidifier.

Boiled Water

You should never use boiled water in a humidifier. Boiling may kill bacteria, but it actually increases mineral content in the water as some of the water evaporates during boiling. Boiled water will only clog up your machine with mineral deposits.

Distilled water

Distilled water is the best water for humidifiers. It has no bacteria or minerals in it, so you can safely use it without worrying about introducing contaminants to your indoor air. I recommend always using distilled water in your humidifier to maintain healthy humidity levels, high air quality, and your device’s condition alike.

Common Mistakes When Using Humidifiers

Using the wrong kind of water is just one of the many mistakes people make when using humidifiers. I’ve seen all kinds of other snafus — and I’ve made my fair share of them, as well.

Here are some of the most common user mistakes to watch out for:

Letting the Humidity Rise Too High

Humidifiers help raise the relative humidity levels in your home to healthy levels, but they could also make the air too humid. Excessive humidity is more difficult to breathe and contributes to the growth of mold, dust mites, and other pollutants. I accidentally got my curtains moldy when I bought my first humidifier years ago.

Ignoring Levels of Humidity at Home

Too many homeowners forget to monitor humidity levels. If you do not monitor your home’s humidity level, it is easy for the air to become too moist. I recommend buying a hygrometer or a humidifier with a built-in humidistat to monitor air moisture in your home.

Not Cleaning The Humidifier Regularly

Not cleaning your humidifier regularly can lead to mold and bacteria growing inside the unit. Thoroughly clean your humidifier at least once a week and replace filters regularly to avoid mineral and bacterial buildup. You can clean the unit too much, though — I give mine a wash every other day.

Letting Water Sit In The Humidifier

Sitting water can become stagnant and turn into a breeding ground for all sorts of harmful microbes and fungi. Change the water in your humidifier daily and allow it to dry properly to reduce the growth of dangerous contaminants.

Dehumidy Room

People Also Ask (FAQ)

What do you use in an Aprilaire humidifier, hot or cold water?

The manufacturer recommends connecting Aprilaire humidifiers to hot water to improve their humidifying performance. However, the units can work with either hot or cold water, and with soft or hard water.

Can you put ice in a cool mist humidifier?

You should not put ice or ice water in a cool mist humidifier, or any other humidifier. The cold ice can freeze and damage the unit’s components and it can even dry out your home air further. I recommend using an air conditioner to cool down your home.

What happens if you don’t use distilled water in the humidifier?

Not using distilled water in a humidifier can lead to mineral buildup and mold growth inside the unit. The humidifier may also spread white mineral dust all over, which is both annoying and can harm your health.

How do you make distilled water at home?

You can make distilled water at home by boiling a pot of water and using an ice-cold metal container to condense the water vapor. However, making distilled water yourself can be tricky, so I recommend buying bottled distilled water.

What can I put in my humidifier to kill bacteria?

You should not add anything other than distilled water to your humidifier. To kill bacteria, I recommend cleaning the unit with a mixture of water and white vinegar at least once a week.


So, should you use hot, warm, or cold water best for a humidifier? The short answer is — always opt for cold water.

Hot water can not only damage the humidifier, but it could also lead to negative health effects. I hate to think what my friend was breathing in as the hot water melted her humidifier’s tubes.

Pour only cool, distilled water into your humidifier and you can safely add moisture to your home’s air to maintain a cozy environment.



Josh Mitchell

Josh Mitchell

My name is Josh and I am obsessed with DIY and improving my family home. HVAC topics can be tricky for homeowners so I decided to share my knowledge on the subject. When I am not working on DIY projects, you can find me at the beach or my local coffee shop.