I wanted to add a new electric baseboard heater, 1500 w in series with an old 2500 w baseboard. As I stared the work l noticed the wire used is a 14.2 wire. Is this a problem. Must I rewire everything?
The limits on wire size are based on amps, not watts, so we would need to know the amps the heaters require before determining the wiring requirements. 14 gauge wire is good for a 15 amp breaker, but for a continuous load you are supposed to derate to 80%, which is 12 amps. On a 120V circuit that would be 1450 watts and on a 240V circuit that would be 2900 W.
Also, how and why would you wire these in series? Virtually nothing in household wiring should be in series (except for switches). A simple resistive load like an electric heater would probably work if wired in series but it would dramatically reduce the heat output of both and is certainly a violation of the manufacturer's installation procedure.
You can't series the two together. They would have to be paralleled. But maybe that is what you meant. They would be imbalanced and not work properly if wired in series.
And then the 1500 watts would overload a 15 amp circuit and 14 gauge wire. Not to mention adding the 2500 watt.
You would need a circuit rated for 4000 watts divided by the nominal voltage (120 or 240) multiplied by 125% for resistive heating. This comes to a 25 amp circuit at 240 volts #10 wire and a 45 amp circuit at 120 volts with #8 wire. (Given you must wire the heaters in parallel.)