This humidifier is designed to create demand for expensive wick filters

All things considered the Holmes HM 3500 is a perfectly acceptable humidifier that will humidify about 2000 square feet and has some desirable features such as an appropriately sized water reservoir, the multi speed fan and humidistat. That having been said it is also true that it suffers from 2 design flaws. The first is that the tops of the reservoir tanks are angled to merge with the shape of the units fan cover, the result is that as the tanks need to be removed and inverted for filling you cannot place them unattended in the bottom of your sink and fill them as they would tip over. They must be held as they are filled and they do tend to get heavy and it takes some time to fill. This taken together with the relatively thin walled plastic moldings of its body parts that tend to cause rattling when it is running full blast could be overlooked if it were not for the rather sinister intentional design flaw of the wick filters that must be used in this unit.

When I first started using this unit I was surprised that after only 8 days of moderate use the filter had become hard and choked with what appeared to be mineral deposits. I tried to wash the filter but it is made of paper materials that cannot withstand much handling after it has been initially placed for use and is heavy with water. I had been using softened water in the unit and thought the mineral content should have been low enough to have more filter life than 8 days so as an experiment I then used only distilled water with no mineral content for the next filter and ran it again to see how long it would last. This time we were in a cold snap and the unit ran more than before resulting in a filter freeze up in about 5 days. My next question was how could the filter have become hard with mineral deposits when the water was free of minerals? The answer is that the deposits are not minerals but rather the activated baking soda and other compounds used by the manufacturer in making the filter. The next experiment I tried was to take a filter and try to neutralize this effect by soaking it in white vinegar until the baking soda at least was burned off. The result was a filter that lasted almost a month in heavy use with only softened water. Unless we are to believe that the manufacturer has done no product testing whatsoever the results of my experience can only be understood as an intentional act by this manufacturer to create a market for an expensive replacement part with that parts intentional premature failure by design.

I dislike waste and intentional waste most of all. My solution to this problem is to make my own filters of a wickable and washable material that can be reused. Taken as a whole unless you are prepared to goof around making your own filters you probably shouldn’t buy this or any other unit that uses disposable wick filters