I have successfully washed down comforters in the washing machine and dried them in the dryer. I have also washed numerous down coats. One down coat I have I've washed at least 2x a season for the past 10 years and it's perfectly okay. I have washed down and feather pillows in the washing machine also.
The "trick" to washing and drying down articles is to thoroughly dry them. Just when you "think" the item is dry, take it out of the dryer for a check to see if all of the down inside the article is dry. Down holds water and moisture longer than most other fabrics and materials. I dry the article on the low setting on my dryer. Sometimes when I dry my feather pillows it can take anywhere from 2 1/2 hours to 3 hours before they are throughly dry. If in doubt, dry them some more.
Just be sure to use a mild detergent and warm or cold water depending on what color the comforter is.
I have even washed my feather pillows in hot water on regular agitation! And they came out just fine and dandy after drying.
For my coats I usually use the delicate setting. I have 3 down coats that I wash every season.
Before you put the down item away for storage in the summer please let the item finish drying by placing it over your shower rod for about 2 days minimum so all of the moisture has left the item or you could have mildew problems when you take the item out in the fall to wear or put on your bed.