4 letters:
heat, heer

5 letters:
eater, hater, heats, heber, heger, jeter, liter, meter, peter, teter

6 letters:
beater, deater, deeter, dieter, hatter, hauter, hayter, healer, heat's, herter, hieber, hitter, hooter, hotter, hutter, jeeter, keator, keeter, keiter, neater, pieter, seater, teater, teeter, vietor, yeater, heater

7 letters:
cheater, haueter, heaters, huerter