kmwkmw:
Thank you, everyone, for the helpful answers.LarryD: I'm using the relays to switch 24 volts AC (thermostat inputs to a radiant floor heat valve controller). I assumed that for AC I needed to use mechanical relays rather than some sort of solid state alternative, but if I'm wrong about that please let me know -- I'll be pleasantly surprised. I'm relatively new to this stuff.
Solid state relays exist... google it.