I used hot water because I did want to make felt, so that the raw edges would not unravel. Also, to kill any moth eggs or bugs that might be there.
When I collected fabrics it was the hottest week of that summer, but the nice people at the Sally Army let me bring my dogs in with me to cool off in their ac. My little dog Charlie had a great time sneaking under the racks of clothes and making people jump when he put his cold wet nose on their legs!