Why is Todoroki scalded when his mother throws hot water on him? It's clear that Todoroki left side is burnt but isn't it also his flame side. If he is able to tolerate the high temperature he himself generates from his own flames shouldn't he also have been able to withstand the burn when his mother throws hot water on his face?
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.