Water and heat are actually good for the down and feathers inside your comforter. Mild detergent and low – medium heat is ok, but strong detergents and high heat may take the oils out of the down, leaving them dry and susceptible to breaking and crumbling.
Water and heat revitalize your down and feathers, making them more fluffy. The main problem with washing some down comforters is that the cotton cover can lose some of its down proof ability. Down comforter fabric is carefully woven to give it downproofness. The best of these fabrics retain downproofness after washing but some do not. The threads loosen after being washed, allowing down and feathers to more easily escape. Read the care label sewn into your comforter to see if it is dry clean only or machine washable. Dry clean only comforters may experience some loss of downproofness after wash, but it may only be a little and should not be too much.
If you decide to wash your comforter, only use a large capacity front end loader. Home machines with an agitator (vertical pole in top load washing machines) will almost certainly do damage to both the fabric cover and down and feathers inside. Be sure to dry thoroughly. This will take several hours / cycles through your dryer. If you do not have a front load machine at home, you can either take it to a Laundromat or professional cleaner. Most are experience in washing down comforters.
I have washed a lot of down comforters, both those marked dry clean only and those marked machine washable. In truth a loss of downproofness was noticeable in those marked dry clean only, but only because I was looking. I believe most people would not even notice a few extra white things floating about. So my recommendation would be to have it cleaned by a professional if it’s an expensive item, if not, then just wash it yourself and see.