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Your fog machine should come with an owner's manual that gives specific cleaning instructions. If you do some sort of cleaning routine that isn't recommended by the manufacturer, it could void your product warranty. If the cleaning techniques don't work, you may need to contact your manufacturer for a replacement, as long as your machine is still under warranty. So try not to do anything that may void the warranty. Most fog machines don't need to be cleaned regularly. In fact, fog machines typically don't need to be cleaned at all unless you have been putting low quality fog liquid products in your fog machine these lower quality products can cause the fog machine's heater to clog. You definitely don't need to clean your fog machine every year unless it starts exhibiting some symptoms indicating it needs a good cleaning. Cleaning a fog machine too often can cause it to stop working altogether. The main sign that your fog machine needs to be cleaned is when you start noticing a significant decrease in output of the fog effect. If your machine isn't producing fog as much as it used to, then it's probably time for a cleaning. Since you don't want to clean your fog machine too frequently, you should wait until you see a significant decrease in fog output not just a small change. If you use your fogger once a year (like at Halloween) and then put it away in storage for the rest of the year, you should clean the fog machine before you pack it up for storage. Don't wait until you bring it out the next year because a clogged fog machine that sits unused for months on end will most likely be permanently unusable when you try to use it again. Before you store your fog machine for an extended period of time, check its output to ensure you don't need to clean it before storing it.
Follow the manufacturer's cleaning instructions. Avoid overdoing it. Watch for decreased output. Clean before storing.