Why is Todoroki scalded when his mother throws hot water on him?
I don't think Todoroki necessarily is impervious to heat. Midoriya notices when they fight that Todoroki's use of his cold power is limited to how much cold he himself can stand, and that if he would not refuse to use his heat powers he would be able to remove this limit. So if he's not immune to the cold he may not be immune to heat. However, his father does have the flaming facial hair which doesn't seem to bother him. I think it's most likely that he's immune to the flames and the ice that he creates but when he uses it too much or is affected by something in the environment he's affected normally.