Using a 24 volt ac relay with a 16 volt ac doorbell circuit
Use a multi-voltage relay
The good news is that you're not stuck with single-voltage relay coils; a RIBU1C's low-voltage coil is rated for anywhere from 10-30VAC, for instance.  That, a plastic &quot;handy box&quot; with ½&quot; KOs on it, and some wirenuts and cableclamps, will get you what you need.