If you don't have a washing machine in your appartment, if your hotel doesn't offer this service or whenever you need, here there is a list of the two major companies that manage the wash and dry in Siena. Here it's the list where they are located.

LAVANDERIA BARBARA
Average Prices: 3 to 5,50 euros to wash and 3 to 5,50 euros to dry
Opening time: open Mon-Sat from 9.00 to 20.00
Address: via di Pantaneto, 129

ONDA BLU
Average Prices: 3 to 5,50 euros to wash and 3 to 5,50 euros to dry
Opening time: open all week long form 8.00 to 22.00
Address: via del Casato di Sotto, 17