Can I place relay in a return wire?
You will not place the relay "across" the return wire. You will switch the return wire with the relay contact.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Switched neutral in low-powered heater element.

This will work fine. Make sure your relay contacts current rating are greater current than the load current and that the voltage rating is greater than or equal to the supply voltage.