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Choosing the right water temperature for your humidifier is more important than you might think.
So which option is truly best? With my years of experience as an HVAC expert, I’ll share info about using hot or cold water in a humidifier, helping you make the right choice for your needs.
Key takeaways
Should you put hot or cold water into a humidifier?
In general, you should always use room temperature or cool water in a humidifier.
BRITA describes the temperature of water as 78 degrees Fahrenheit for room temperature water.
Making hot mist with hot water might seem sensible, but that can cause issues for your unit.
Most portable humidifiers are designed to work with cold water only.
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.
Related article: Humidifiers for sinus issues
Portable cool mist versus warm mist humidifiers
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 versus 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.
Important Note
Despite the fact that warm mist humidifiers use a heating process to create hot steam, 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.
Related article: Humidifiers versus dehumidifiers
Whole-house humidifiers are an exception
Whole-house humidifiers, like those from 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 attach to your home HVAC system and rely on heat to generate water vapor that humidifies your home.
The water evaporates more quickly and 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 — but 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.
Learn more: Top whole house humidifiers for dry air
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.
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:
Hot water can cause irreparable 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.
Important Note
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.
Hot water may lead to 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.
These are two reasons why you should only use distilled water within your humidifier.
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 love high temperatures
If there’s anything mold and bacteria love, it’s high temperatures and damp environments, which numerous studies have shown.
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.
Consumer Reports found that even models that boil water and market themselves as "germ-free vaporizers" can spew bacteria into the air if the wrong water is used or the tank is dirty.
Hot water causes damaged plastic and 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. Researchers have discovered that plastic chemicals like BPA may even cause cancer.
Important Note
Most humidifier warranties will be void if there is evidence of misuse.
Accidental knock overs leads to 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.
Small children are especially at risk of burns, and hospital data has shown that the vapor from these machines can lead to severe burns requiring skin grafts.
For all humidifiers, keep out of reach of children and pet.
That applies to warm mist humidifiers as well as adding hot water to any other humidifier.
What kind of water can you use in a humidifier?
Room temperature distilled water is the best choice for your humidifier.
Never use 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.
Boiled water has a high mineral content
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.
Purified water has a normal mineral content
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 has some mineral content
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.
Distilled water is the best water for humidifiers
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.
Summary of types of water for humidifiers
Type of Water | Remarks |
|---|---|
Distilled | – No Bacteria |
Purified | – No Bacteria |
Demineralized | – Some Bacteria |
Bottled Water | – No Bacteria |
Boiled Water | – No Bacteria |
Tap Water | – Low-High Bacteria |
Other common mistakes you could be making with humidifiers
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 makes it more difficult to breathe and contributes to the growth of mold, dust mites, and other pollutants.
Here is a chart I made to help you find the ideal relative humidity for your home, by goal:
Goal | Below 40% | 40% - 50 % | Over 60% |
|---|---|---|---|
Best for total home comfort | |||
Ideal for allergy sufferers | |||
Manage asthma symptoms | |||
Avoid respiratory illnesses | |||
Stop the spread of viruses | |||
Negate static electricity | |||
Discourage bacteria and mold growth | |||
Fight dust mites |
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.
| Hygrometer | Humidistat |
|---|---|---|
Purpose | Measures humidity levels | Controls humidity levels |
Measurement capability | Gives real-time readings | Monitors and regulates based on set point |
Control capability | No direct control function | Can adjust devices |
Accuracy | High accuracy for readings | Moderate accuracy, focused on regulation |
Typical use | Monitoring indoor humidity | Automatic regulation of HVAC devices |
Cost | Low | Mid to high |
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 humidifier 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.
FAQs
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, cold water is the way to go (generally)
The short answer is to always opt for cold or room-temperature distilled water.
Hot water can not only damage the humidifier, but it could also lead to negative health effects.
