The limit switch on your heating unit goes off if something gets too hot. Usually that means that there isn’t enough airflow through the system to push out the heat. First, I would suggest checking your filter to make sure it is clean and fits snugly. If that doesn’t solve the problem, call your contractor back to double-check that the correctly-sized limit was installed. If your contractor is not helpful, you need to consider getting a second opinion from a reputable HVAC contractor, like ARS/Rescue Rooter.