I currently have a Noritz NRC1111NG installed with a Redytemp recirc pump. The pump has manual decora momentary rocker switches in the kitchen and two baths. When one of these switches are closed (they're all in parallel to the Redytemp controller) the pump turns on and circulates hot water, causing the tankless to light off. The pump turns off once the sensor at the furthest hot water run (where the dedicated return pipe taps off) reaches an adjustable 110 deg F setpoint. The flow through the water heater stops, and the water heater turns off in response.

What I would like to do, to further optimize this arrangement for more-automatic operation, is close a 4th switch (relay), causing the pump to start up anytime hot water is used and the temperature at sensor is less than 110 degrees F. If the water is already hot at the sensor, the pump won't start up anyway, part of the Redytemp controller logic. But if it is cooler, and somebody runs the water for a minute somewhere without manually triggering the pump, short cycling will be avoided and the water heater that is already on will efficiently complete a longer burn cycle by circulating hot water up to the kitchen faucet at the end of the recirc loop.

Makes sense, right? So where do I hook up a (24v?) relay within the Noritz to close the circuit at the Redytemp recirc controller when the heater fires up?