Could be, test it...if you have an OHM meter, set it to resistance and disconnect the humidstat. With the humidistat set all way on, you should get 0 ohms, if not it is bad. You most likely have a wire not connected somewhere, You need to have 24 volts at the solenoid when the heater is on and the humidistat > than existing humidity %. That is the bottom line. So check first at the electrical plug that the transformer is plugged into, if it does not use a wall transformer, test the furnace circuit board to see that you are getting 24Volts from the HUMIDIFIER TERMINALS when the HEAT IS ON. Then check the humidistat like I stated above. Solenoid is good cuz you replaced it.
Answers & Comments
Could be, test it...if you have an OHM meter, set it to resistance and disconnect the humidstat. With the humidistat set all way on, you should get 0 ohms, if not it is bad. You most likely have a wire not connected somewhere, You need to have 24 volts at the solenoid when the heater is on and the humidistat > than existing humidity %. That is the bottom line. So check first at the electrical plug that the transformer is plugged into, if it does not use a wall transformer, test the furnace circuit board to see that you are getting 24Volts from the HUMIDIFIER TERMINALS when the HEAT IS ON. Then check the humidistat like I stated above. Solenoid is good cuz you replaced it.