I am in the process of installing duct booster fans on my HVAC. I also have an existing “sail switch” that operates my humidifier that uses a DC to power a solenoid to turn on water flow. I’m considering attaching a DC relay to the sail switch to complete an AC circuit to turn on the duct booster fans. Is there any problem with doing this? I’m assume in a normally open relay getting DC power to complete an AC circuit should not be a problem. Any comments or suggestions?

Where are you getting the DC voltage? the standard output on a heat/ac transformer is 24 Volts +/- a few volts AC. Either way, your sail switch relay has to be able to handle the load of the relays. The rating of the relay/switch may be in watts(W) or volt-amps(VA); add up the watts/va of each relay to be powered and compare to the sail switch rating. you have to install a higher rated transformer to handle the loads of the relays. Remember, the manufacturer is only going to install a big enough transformer to power what they install in the unit(45 VA); the extra devices may overload and kick out the transformer or over-current device (internal breaker or fuse).