1 Heat can dry your leather out too much, too fast leading to cracking and shrinking. Neither of these is reversible. Both of these will ruin your boots.
2 Heat can reactivate adhesives and cause soles to become loose. This can usually be fixed with a trip to the cobbler but you’ll be without your boots for a while.
3 Heat can shrink some types of soling and this may deform your shoes or boots. This, in turn, may severely alter the fit. Again, this can usually be fixed but we’re talking sustainability here: An ounce of prevention is worth a pound of cure (thanks Ben Franklin – you know: waste not, want not).
4 Heat can melt and distort internal components. Most boots have a thermoplastic toe counter to maintain the shape of the toe. This can shrink or get dented. Sometimes this is fixable, but typically not.
Keep your boots away from heat – campfires, fireplaces, radiators, ovens, closed up cars parked in the sun, or any other area that may become really hot for extended periods of time.