LED indicators for HVAC (24 VAC)
Better still is a simple series diode plus current-limit resistor.







simulate this circuit &ndash; Schematic created using CircuitLab

There can be advantages to putting a reverse-polarity diode in parallel with a LED when that LED is being fed from an AC source but there are usually specific reasons for doing so.

The problem with a single series resistor feeding a LED with a reverse-polarity diode in parallel is that the resistor is dissipating power for most of the entire AC half-cycle.

If you use a series resistor plus a diode also in series, the resistor is dissipating power for only half of the time.  This reduces the power by half and either lets you use a smaller resistor OR the resistor runs cooler.

You can add a resistor in parallel with the LED if you want to trim the turn-on voltage of the LED.  This can help stop the LED from illuminating with small leakage currents.