I replaced my old thermo with a new digital one. Now my heater won't start. I've checked all the wiring and made sure I've done it right. I called a repairman who suggested a new board. He tried a couple he had and none worked. I think he should use the specified part for that heater and was trying to "rig it". Now he says I may need a new heater. I got rid of him. Any ideas on what is wrong??