I have a intertherm model 015HA. The unit got to where it would not start unless I hit the front of the wire box. Now it won't come on at all unless I turn the fan on manual and set the heat. The fan has to run all the time but the heat will kick in and out. I think it is a relay but I don't know which one it is. Can anyone help?

On your wire diagram it should show a limit switch and blower control switch. When you look at them they look the same. The limit sitch will have a 115v on each post all the time(opens on temp rise). The blower control will have a 115v on one side only(close on temp rise) with out the blower on. The wire diagram should show the color of wires going to the blower control to help get the part #.