a lot of it

On especially cold or stormy days, when Izuku's bones burn and ache from years of horrific injuries, he uses Fa-Jin to heat himself up. It may not be as strong as it once was when the Vestiges were still with him, but it's in a perfect sweetspot to both be used as a combat quirk and as a personal heater. The only unfortunate thing is that a lot of the Hero Course girls also see him as a personal heater.