I use this relay to power a duct booster fan. The relay coil is powered by the EAC terminals on my forced air unit. I had been using a pressure switch for this function, but the first pressure switch stopped turning the fan on after about a year of use, and it’s replacement wouldn’t turn the fan off after a few weeks.110vac is applied to the EAC terminals whenever the furnace’s blower is powered. The EAC terminals have insufficient current for my duct fan, thus the need for this relay. And the setup cost less than a quarter the price of that unreliable pressure switch. Happy!