Hard water has been the scourge of central (furnace) humidifiers for decades. "Hard water" is essentially water containing dissolved minerals of the calcium and magnesium family. As the water in the humidifier evaporates, it leaves behind the calcium and magnesium carbonates which generally appear as a white crusty substance (mineral scale). The crusty deposits eventually build up over time, clogging the evaporative media, rendering the humidifier ineffective. This is especially true of traditional drum/sponge (or pad) style humidifiers.

Homeowners with severely hard water will generally purchase and install a water softener to prevent the deposits from clogging plumbing, water heaters, dish and clothes washers etc. Water softeners generally exchange sodium (Na+) or potassium (K+) for the calcium and magnesium, thereby preventing the formation of carbonates and mineral scale. While the process of water softening generally provides more usable water for showering and even drinking (unless you are restricted to a low sodium diet for health reasons), it can actually cause even greater problems for a humidifier. Instead of a build-up of hard water minerals, there will be a build-up of sodium. Sodium deposits in a humidifier can take the form of a white paste or powder. The powder can be blown into the duct work and distributed throughout the home (making a mess). Further, the sodium can react with free chloride ions in the water (common on both municipal and well water systems) to create salts which are corrosive to the humidifier's parts, especially drive motors.

Look at what hard water did to this drum humidifier!
This is exactly why I do not recommend them...

So how do you deal with hard water?

Of course, you should select a humidifier with an evaporative media that is less prone to hard water minerals. Generally speaking, you should avoid drum-style furnace humidifiers with an evaporative sponge or foam media. A flow-through style (good) or drum style with plastic disc evaporative media (best) are better alternatives. The plastic disc media in the Desert Spring furnace humidifier, for instance, is far less prone to mineral problems.

At first glace, one would think he/she could run out to the local hardware store and buy a water filter to remove the calcium, magnesium from the hard water, or sodium from the softened water and be done with the problem. Unfortunately, none of these minerals are removed from water by standard filters (sediment filters or activated carbon/charcoal filters). Reverse osmosis is general used as a point-of-use drinking water filter to remove these contaminants, however, the cost of a reverse osmosis unit at over US$175 makes it somewhat less than economically feasible, especially given that it will require periodic filter changes, etc. Never-the-less, it is an alternative.

There are a small handful (if you call 2 a handful) of manufacturers making special flushing units that are designed to reduce mineral deposits. The theory is that a few times a day, a timer will activated a valve which will open and drain the mineral-rich water in the humidifier's reservoir before the minerals can precipitate out of the water and form scale. Air King / Wait was the first company to introduce such a unit (as far as my research shows). It was a rather crude looking device but was a reasonably effective unit. It activated roughly every 12 hours and operated on a standard 110v circuit. Unfortunately, to my knowledge, Air King no longer manufactures this product - quite a shame. On the bright side, Desert Spring Products, the company that makes the humidifier that I am recommending, introduced its own flushing unit called the "AutoFlush" in 2003. It is a slick little box with a user-adjustable timer that allows the homeowner to select the flushing interval (every 12, 24, or 48 hours). The flushing cycle can there fore be set to accommodate the users needs for self-cleaning of the humidifier while also being considerate of water usage. At about $120, it isn't really cheap, but having used one for a little while now (I have very hard water), I can say that it definitely is well worth the money. Apparently, this little device also works equally well in a soft water environment. The only potential disadvantage of this product is that it required connection to some sort of nearby drain (floor drain etc.). Not all homes will have this option nearby. The AutoFlush by Desert Spring Products is best used with a drum style humidifier.

In 2003, a highly specialized filter cartridge that looks a lot like the type of filter you would use on your fridge ice maker also became available for dealing with hard water problems. It was designed specifically for central humidifiers by www.highqproducts.com. It contains a specialized filter media that attaches itself to hard water minerals preventing them from binding to each other and surfaces within the humidifier, therefore preventing scale build-up. At only about $30, I had to give it a try and I must say that I was impressed. It was quite effective at reducing mineral build-up on the evaporative surface of the humidifier (which is what you really want). As you might expect, since the filter does not actually remove all of the minerals, there is an accumulation of deposits in the humidifier reservoir, however, they proved much easier to clean than normal since they did not bind very effectively with the surface etc. All in all, it is worth the $30 every season or two if you have hard water, especially if you have hard water and no drain access to use the AutoFlush. This specialized filter can be used with any type of humidifier.

For the ultimate solution... Try combining the AutoFlush and this little filter. While there will always be some cleaning and maintenance involved, this combination (especially with Desert Spring's unique rotary disc humidifier) is the best solution for hard water available today.