I have an evaporative cooler with 120VAC control that I want to switch on and off using a relay that is activated when the cool call comes from my thermostat. I plan on just using 1 relay to activate the whole cooler control (rather than multiple relays to control the different aspects of the cooler), so this is a simple on/off when cooling call comes from the thermostat.
All of the wiring for the thermostat goes to the air handler and then heat, fan, etc. kick on from there. I have a C wire running to the thermostat. My thought was that I just put the relay in line with the Y wire (run Y from thermostat to relay, then out from relay to the air handler). This puts 24VAC across the relay (which is what the relay is rated for), but does not throw the switch.