I am having trouble in the heater. When you switch the heater on the AC is also working ? Any reason.

You have a short between "Y" (AC) and "W" (Heat)which usually are the yellow and white wire. An overload could have melted the insulation and shorted them out or the t-stat could be bad. Cut power off and take wires loose from both t-stat and at furnace connection and use an ohm meter to see if the wires are shorted. If not then check between W and Y on t-stat. If so, replace t-stat. If wire is shorted see if you have an extra wire and use it instead of Y or W. Y hooks to a wire going to outside condensing unit. DrTom4444