The 24VAC is the "nominal" output from the furnace, probably running closer to 26VAC I suspect, but I haven't put a meter on it 28VAC.

The "24VAC" goes to a bridge rectifier salvaged from a computer PSU, and then past a 470uF capacitor to the buck. I've inserted the three extra diodes between the bridge rectifier and the 470uF capacitor.

The "24VAC" supply will fluctuate with line voltage. It should be pretty constant with the odd dip, but can have spikes just like the wall sockets do.

The buck regulator is a standard circuit from the LM2695S-ADJ datasheet. The datasheet says "45V absolute maximum; but only up to 40V for the operating range".

So we're under 40V, I'd just like a little more margin, or some protection against the odd spike -- thus the thought of perhaps a 40V zener.

Cheers