Washing will not ruin your goose down comforter, but only if done right. Cleaning a down comforter is not like doing regular laundry; goose down comforters requires special care to prevent clumping, flattening and removal of essential oils from feathers. Down comforters that are lumpy and heavy were probably not cleaned and dried properly. It is worth the extra time and expenses to have down comforters cleaned properly rather than risk having lumpy bedding. With proper cleaning, drying and protection, down comforters will last for many years.
Load the comforter into a large capacity front-loading washer. Down comforters become heavy with water and can damage top-loading washers, and the agitator of top-loading washers is rougher than the spinning action of front-loading washers. They can also become tangled or rip in washers that are too small.
Add a small amount of mild detergent -- no more than about 1/2 cup -- into the washer. Set the dial of the washer on the gentle or delicate cycle with warm water. Add an extra rinse cycle to ensure the detergent washes out of the comforter.
Load the comforter into a large capacity dryer. Set the dryer on low heat. Take the comforter out of the dryer about every 30 minutes and fluff it up.
Hold the comforter up to an overhead light to see if there are any clumps. Put it back in the dryer if there are any clumps. When the down is evenly spread out, it is probably dry, but running it through another cycle on low heat will help dry any trapped moisture. Drying may take a few hours, but make sure that it is completely dry to prevent mold growth in the down.