Prolly gonna burn out his heating unit. If he's using gas, not only will his bill be thru the roof, but too much stress on whatever turns the gas to flame will eventually weaken the steel from which its likely made, which could lead to a gas leak and massive fire hazard. If its electrical, then its likely using a resistive heating element (like what you get with regular electric stoves); either the contacts between it and the mains will weaken and break loose, ruining the element, or it could weaken, deform, and short out, starting a fire.