My apartment is very humid, and I regularly use a dehumidifier. Is it OK to use the resulting water to water my indoor plants? Since it's basically distilled water it does lack minerals. I usually use tap water that I leave for a day or so to remove chlorine.
Provided that you are feeding the soil/plants the macro and micro nutrients they need there shouldn't be an issue. Epsom salt (1tbsp/gallon) every now and then helps with some of the micro nutrients.
Minerals for your plants can be supplemented via plant food, rock dust, azomite or greensand if needed or through the soil/potting mix of the plants unless you are using a sterile potting medium. Provided that you repot every now and then you should be fine.
Like everything else it depends on the plants and their needs. Some believe you should provide as many trace minerals as possible and let the plant decide what it needs.
Just over a hundred years ago, in 1915, a chap called M.C. Merrill did some experiments on this (that link may be paywalled, sorry) and found that, while plants would grow with distilled water, it was not a great choice. (If you prefer to read the very lengthy report of that study in its entirety, it's available for free in the Annals of the Missouri Botanical Garden.)
As Charles says, you'll probably be okay if you're adding liquid fertiliser, but your plants will likely be happier if you stick to tap water instead.
Disclaimer: I've done all of this in my own apartment too, and is based on my personal experimentation/research.
I recently grew plants hydroponically in distilled water. It worked fine, but I had to manually measure and re-add the required minerals for the plants to be healthy. If you can't or don't want to carefully re-add minerals I recommend you avoid distilled (or reverse osmosis or similar) water for plant survival.
I also trialled adding a product based on chloramine (marketed as "Pythoff") to the hydroponic setup, which is a chemical sometimes used in tap water (chlorine + ammonia), to stimulate root growth and it worked quite well. It basically works by giving the plant roots a sterile environment to grow in, removing both good and bad bacteria. Removing chlorine from tap water is absolutely not necessary for plant growth and will simply encourage bacteria and mould growth, for better or worse. Depending on the environment, you may not want to encourage this type of growth in your garden. If your apartment is very humid, it might be better to leave the chlorine in the water to help fight unhealthy microbeasties.
Currently I have an aquaponics setup with live fish, worms, snails and a beneficial bacterial colony. For this I cannot use chlorine or chloramine as these chemicals would kill all bacteria immediately. Without bacteria to process fish poop, ammonia would quickly build up to the point of becoming lethal to fish. Chloramine itself is also lethal to fish. Chlorine + fish poop = Chloramine. If you want live animals (including bacteria) in your water, you do have to remove the chlorine somehow, but a tiny amount of vitamin C will neutralise both chlorine and chloramine (http://www.beeradvocate.com/community/threads/vitamin-c-for-chlorine-chloramine-removal.76953/). However, building beneficial bacteria colonies requires specific environmental conditions (oxygen, light, temperature, time, food, substrate). If you can't reliably provide these conditions 24/7, you're more likely to be left with only bad bacteria/mould as it tends to outcompete the good bacteria outside these specific conditions. This bacteria/mould can make your plants sick in the long term, especially so if they are nutrient deprived (from persistent use of distilled water).
It may also be worth checking that your water is actually being treated with chlorine and not chloramine (check with your local water supplier). In the case of the latter, simply leaving it out may not be enough for the chemicals to break down/off-gas (this is why chloramine is used, as it tends to stick around).
Based on my own experience/experimentation however, I would recommend that you either simply use tap water directly on your plants, or commit to creating a setup that accommodates distilled/RO/de-gassed water in a healthy way.
It's common when searching for the answer to this question to see answers that say the condensate is full of heavy metals acquired as the water condenses over the metal components of the dehumidifier, gathers bacteriae, fungal spores and viruses from the air, may even be growing legionella. However, most of these things are highly unlikely to be harmful to your plant, and if you have plants that don't tolerate chlorine or fluoride such as Dracaena sanderiana (lucky bamboo) and tillandsias, then the water from a dehumidifier will likely be much better for your plant than your municipal tap water.
Most dehumidifier companies say the water is not safe to drink, but few have published analyses of the water produced. Quest Hydro is one company to have published some data, and have found that the water is even safe to drink ( though clearly it's not recommended ).
If you wish to be surer your unit is safe for your plants, you could test the water with a TDS meter to make sure that the ppm is less then your tap water, and check the pH to ensure it's in range for your plant.
I would like to point out that the water produced by your dehumidifier is not "distilled" but "condensed". Distilled water has been raised to a boil, converted to STEAM and then the steam is captured and allowed to cool, this water should be exceptionally clean, because impurities have been removed by this process. The water produced by your dehumidifier is referred to as 'condensate', has NOT been brought to a boil and is probably not very clean. It is probably all right to water your plants with it, but by no means should you drink the condensate. It is NOT pure "distilled" water by any means!