# Why can't I put hot water in my humidifier? **URL:** \ **Category:** Factual Questions\ **Created:** [December 16, 2009, 3:20am UTC](https://boards.straightdope.com/t/why-cant-i-put-hot-water-in-my-humidifier/521443 "2009-12-16T03:20:56Z")\ **Posts on this page:** 19\ **Page:** 1
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [December 16, 2009, 3:20am UTC](https://boards.straightdope.com/t/why-cant-i-put-hot-water-in-my-humidifier/521443/1 "2009-12-16T03:20:56Z")
I have an evaporative humidifier–no heating element, no ultrasonic mist, just a filter to draw the water up and a fan to blow it into the air. It comes with an ostensibly permanent filter, which is several layers of metal grid (like mini chain-link fences) with some sort of material embedded that is supposed to make it bacteria resistant. The instructions say to fill it with cool water and specifically say not to use hot water. These kinds of instructions always give you these inscrutable proscriptions without telling you why. Why can’t I use hot water? ---
**Author:** ![kdeus](https://avatars.discourse-cdn.com/v4/letter/k/b487fb/32.png) [@kdeus](https://boards.straightdope.com/u/kdeus)\ **Post date:** [December 16, 2009, 3:29am UTC](https://boards.straightdope.com/t/why-cant-i-put-hot-water-in-my-humidifier/521443/2 "2009-12-16T03:29:01Z")
A theory: Hot water heaters hold water (duh) and heat it (duh) both of which a conducive to bad tastes and fungus/mold/ mildew/ metal from the water heater/ other nasty stuff in the water. Cold water has spent less time in your system, having faster turnover, and may be considered “cleaner” than hot water. This is what I heard regarding “Why use cold water in coffee machines & for cooking?” ---
**Author:** ![raindog](https://avatars.discourse-cdn.com/v4/letter/r/2bfe46/32.png) [@raindog](https://boards.straightdope.com/u/raindog)\ **Post date:** [December 16, 2009, 5:59am UTC](https://boards.straightdope.com/t/why-cant-i-put-hot-water-in-my-humidifier/521443/3 "2009-12-16T05:59:55Z")
Is this a central system connected to a furnace or air handler? ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [December 16, 2009, 11:25am UTC](https://boards.straightdope.com/t/why-cant-i-put-hot-water-in-my-humidifier/521443/4 "2009-12-16T11:25:05Z")
> [@raindog](#): > > Is this a central system connected to a furnace or air handler? No, it’s a [portable unit](http://www.fans-ceilingfans.com/hunter/humidifiers/hunter_humidifier_model_33257.htm). The filter is the [second one on this page](http://www.fans-ceilingfans.com/hunter/humidifiers/filters_permawicks_endurawicks.htm). ---
**Author:** ![Harmonious\_Discord](https://avatars.discourse-cdn.com/v4/letter/h/74df32/32.png) [@Harmonious\_Discord](https://boards.straightdope.com/u/Harmonious_Discord)\ **Post date:** [December 16, 2009, 12:57pm UTC](https://boards.straightdope.com/t/why-cant-i-put-hot-water-in-my-humidifier/521443/5 "2009-12-16T12:57:35Z")
It’s burn hazard if it spills on somebody, so it’s also a potential lawsuit. The hot water can also soften the plastics so the unit deforms. ---
**Author:** ![Sparky812](https://avatars.discourse-cdn.com/v4/letter/s/eb9ed0/32.png) [@Sparky812](https://boards.straightdope.com/u/Sparky812)\ **Post date:** [December 16, 2009, 1:13pm UTC](https://boards.straightdope.com/t/why-cant-i-put-hot-water-in-my-humidifier/521443/6 "2009-12-16T13:13:23Z")
> [@CookingWithGas](#): > > It comes with an ostensibly permanent filter The filter is not permanent and will require replacement depending on hours of use and water quality. The filter will eventually deteriorate and/or scale with minerals, sediment, etc. depending on your calcium hardness, Ph, alkalinity, lead contamination, etc.. Check your owner’s manual and/or search for replacement filters for your make/model. > [@kdeus](#): > > Cold water has spent less time in your system, having faster turnover, and may be considered “cleaner” than hot water. ``` Right! Hot water tends to contain more contaminants due to it's standing time in your pipes and hot water tank as well as it's lower Ph (more acidic) ``` ---
**Author:** ![raindog](https://avatars.discourse-cdn.com/v4/letter/r/2bfe46/32.png) [@raindog](https://boards.straightdope.com/u/raindog)\ **Post date:** [December 16, 2009, 4:22pm UTC](https://boards.straightdope.com/t/why-cant-i-put-hot-water-in-my-humidifier/521443/7 "2009-12-16T16:22:38Z")
I see no reason why you can’t use hot water. I do not believe that hot water has more contaminants (enough to make any practical difference), nor will it warp plastic below 130°. I’m hard pressed to believe that they make this distinction to avoid lawsuits. The filter media you link to is the same material used in central systems and they are virtually never permanent. Depending on your water quality they will get limed up and need replacing. I don’t know why they say you can’t use hot water and I’m reluctant to say it’s ok as they may have some reason I’m unaware of. This much I know: that filter media is the same media used in central humidifiers, and when they’re installed in heat pumps we use hot water right from the water heater. So I can tell you that the filter media will be fine with hot water. Maybe the guts of the system don’t like hot water. ---
**Author:** ![troub](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/troub/32/14802_2.png) [@troub](https://boards.straightdope.com/u/troub)\ **Post date:** [December 16, 2009, 4:51pm UTC](https://boards.straightdope.com/t/why-cant-i-put-hot-water-in-my-humidifier/521443/8 "2009-12-16T16:51:31Z")
It looks from the illustration in the manual that the tank sort drains into a tray at the bottom, from which the water is wicked up by the filter. Is that correct? maybe they don’t want you to use hot water so that the steam doesn’t rise from the base pan and condense on the fan, motor, or other parts in the main housing? ---
**Author:** ![raindog](https://avatars.discourse-cdn.com/v4/letter/r/2bfe46/32.png) [@raindog](https://boards.straightdope.com/u/raindog)\ **Post date:** [December 16, 2009, 4:57pm UTC](https://boards.straightdope.com/t/why-cant-i-put-hot-water-in-my-humidifier/521443/9 "2009-12-16T16:57:27Z")
> [@troub](#): > > It looks from the illustration in the manual that the tank sort drains into a tray at the bottom, from which the water is wicked up by the filter. Is that correct? maybe they don’t want you to use hot water so that the steam doesn’t rise from the base pan and condense on the fan, motor, or other parts in the main housing? Good idea. That may be true. The other thing that puzzles me about the OP is that the hot water will cool to ambient conditions long before it is all converted to water vapor, so I’m not so sure hot water will gain much. ---
**Author:** ![DirkGntly](https://avatars.discourse-cdn.com/v4/letter/d/d9b06d/32.png) [@DirkGntly](https://boards.straightdope.com/u/DirkGntly)\ **Post date:** [December 16, 2009, 10:00pm UTC](https://boards.straightdope.com/t/why-cant-i-put-hot-water-in-my-humidifier/521443/10 "2009-12-16T22:00:55Z")
If your filter uses activated carbon (charcoal) as a filtering agent (and it likely does), then hot water breaks down the carbon portion of the filter. This means your filter FAILS, and fails QUICKLY…the broken-down carbon will plug up the more “mechanical” parts of the filter, rendering it useless. And that “specially treated to prevent the growth of micro-organisms” part is probably silver oxide, which kills bacteria - and is also not hot-water friendly. ---
**Author:** ![raindog](https://avatars.discourse-cdn.com/v4/letter/r/2bfe46/32.png) [@raindog](https://boards.straightdope.com/u/raindog)\ **Post date:** [December 16, 2009, 11:30pm UTC](https://boards.straightdope.com/t/why-cant-i-put-hot-water-in-my-humidifier/521443/11 "2009-12-16T23:30:38Z")
> [@DirkGntly](#): > > If your filter uses activated carbon (charcoal) as a filtering agent (and it likely does), then hot water breaks down the carbon portion of the filter. This means your filter FAILS, and fails QUICKLY…the broken-down carbon will plug up the more “mechanical” parts of the filter, rendering it useless. And that “specially treated to prevent the growth of micro-organisms” part is probably silver oxide, which kills bacteria - and is also not hot-water friendly. You’re speaking about air filters. The OP linked to pads for humidifiers. ---
**Author:** ![Magicicada](https://avatars.discourse-cdn.com/v4/letter/m/c67d28/32.png) [@Magicicada](https://boards.straightdope.com/u/Magicicada)\ **Post date:** [December 17, 2009, 3:52am UTC](https://boards.straightdope.com/t/why-cant-i-put-hot-water-in-my-humidifier/521443/12 "2009-12-17T03:52:25Z")
CookingWith Gas–May I sidetrack a bit? Does it seem to you like this humidifier cools the surrounding area, maybe due to the fan blowing out not-very-warm air? This was just not what I had in mind when I bought a humidifier to use in the middle of winter (the only season in these parts when anyone would actually want to add moisture to the air). ---
**Author:** ![ecenur](https://avatars.discourse-cdn.com/v4/letter/e/dec6dc/32.png) [@ecenur](https://boards.straightdope.com/u/ecenur)\ **Post date:** [November 15, 2010, 8:26pm UTC](https://boards.straightdope.com/t/why-cant-i-put-hot-water-in-my-humidifier/521443/13 "2010-11-15T20:26:26Z")
I just recently had the same problem, started touching things I wasn’t supposed too, the best thing to do is get someone who understands it is to check it out or tell you why. I have to end up getting a new furnace filter now because of that, which is also good because I really needed a new furnace and a clean furnace filter. ---
**Author:** ![lazybratsche](https://avatars.discourse-cdn.com/v4/letter/l/ba8739/32.png) [@lazybratsche](https://boards.straightdope.com/u/lazybratsche)\ **Post date:** [November 15, 2010, 9:12pm UTC](https://boards.straightdope.com/t/why-cant-i-put-hot-water-in-my-humidifier/521443/14 "2010-11-15T21:12:03Z")
> [@Sparky812](#): > > Right! Hot water tends to contain more contaminants due to it’s standing time in your pipes and hot water tank as well as it’s lower Ph (more acidic) Anybody have a cite for this? I’ve heard the claim repeatedly, but never backed up by actual water quality data. How many contaminants are in a plumbing system anyways, and how much difference does hot water make? I’m sure it depends on a number of factors like local water quality and the age of a given plumbing system. I’m just surprised that there isn’t any good hard data on the subject (at least that I’ve found). ---
**Author:** ![boytyperanma](https://avatars.discourse-cdn.com/v4/letter/b/4491bb/32.png) [@boytyperanma](https://boards.straightdope.com/u/boytyperanma)\ **Post date:** [November 15, 2010, 9:59pm UTC](https://boards.straightdope.com/t/why-cant-i-put-hot-water-in-my-humidifier/521443/15 "2010-11-15T21:59:28Z")
It’s probably just easier for them to say ‘cool water’ then to get into the details of how hot the water can be. Water no warmer then 130 degrees results in people seeing them as complex instructions rather then simple. Maybe they only tested the equipment with cool water and don’t want any responsibility for unpredicted events that could happen with hot water. ---
**Author:** ![commasense](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/commasense/32/3017_2.png) [@commasense](https://boards.straightdope.com/u/commasense)\ **Post date:** [November 16, 2010, 5:42am UTC](https://boards.straightdope.com/t/why-cant-i-put-hot-water-in-my-humidifier/521443/16 "2010-11-16T05:42:20Z")
No cite, but I’ve always assumed that repeatedly using warm water might eventually weaken the glue that holds the tanks together. ---
**Author:** ![outlierrn](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/outlierrn/32/3005_2.png) [@outlierrn](https://boards.straightdope.com/u/outlierrn)\ **Post date:** [November 16, 2010, 5:57am UTC](https://boards.straightdope.com/t/why-cant-i-put-hot-water-in-my-humidifier/521443/17 "2010-11-16T05:57:14Z")
Maybe it confuses the > [@](#): > > This humidifier has a Digital Humidistat which enables you to set desired maximum humidity level. Once this level is exceeded, the unit will shut off until the humidity level drops. ??? ---
**Author:** ![BigT](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bigt/32/12044_2.png) [@BigT](https://boards.straightdope.com/u/BigT)\ **Post date:** [November 16, 2010, 6:05am UTC](https://boards.straightdope.com/t/why-cant-i-put-hot-water-in-my-humidifier/521443/18 "2010-11-16T06:05:26Z")
> [@outlierrn](#): > > Maybe it confuses the > > Quote: This humidifier has a Digital Humidistat which enables you to set desired maximum humidity level. Once this level is exceeded, the unit will shut off until the humidity level drops. > > ??? I’ve seen this warning on much simpler devices that do not have such a feature, so I don’t think that is it. ---
**Author:** ![Tastes\_of\_Chocolate](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tastes_of_chocolate/32/3232_2.png) [@Tastes\_of\_Chocolate](https://boards.straightdope.com/u/Tastes_of_Chocolate)\ **Post date:** [November 16, 2010, 6:11pm UTC](https://boards.straightdope.com/t/why-cant-i-put-hot-water-in-my-humidifier/521443/19 "2010-11-16T18:11:10Z")
Another possibility. We use similar style humidifiers. The tank has a valve at the bottom, that allows the water to flow out of the tank and into the bottom pan. It looks like that valve is also part of the opening that you use to refill the tank. Anyway, on ours that assembly has a couple of rubber gaskets, to prevent water from pouring out of the refill opening when you turn the tank upside down. Using hot water could soften the gaskets, allowing the 2 gallons of water in your tank to pour all over the floor.