Good advice about how to wash down sleeping bags at Tom Managan’s Hike Hacker:
… don’t wash the bag a lot, but do launder it at the end of the season before long-term storage, or after your annual two-week backpacking trip.
Where to wash it? Not at home: washing machines with agitators can tear up your bag. You need a front-loading, non-agitating machine found in a laundromat. Lots of folks these days have home-sized front-loaders, which can work in a pinch (though my wife, an unsurpassed authority on household cleaning, insists the tubs in home front-loaders aren’t big enough to thoroughly rinse the soap out of a sleeping bag).
Getting it dry really requires the large tumblers available at laundromats (home dyers are too small and get hot enough to damage your bag’s liner…). Since you have to go to the laundromat to dry it anyway, you might as well do the whole deed there.