"The manager has told me he wants 25 goals from me and it's a big ask," Heffernan told City's official website.
"I was pleased with scoring 21 in a relegated side (last season) but now I am joining a team with eyes on promotion, which is very exciting."
"I know the pressure will be on to succeed in the coming season, but it will be much better than the pressure at the wrong end of the table."