My van only puts out heat when it is riding, not idling.

Most likely the fan motor is not working. When driving, some air is forced into the scoop giving you some flow through the heater core but the fan does most of the work to draw air through the system.

you need to check the coolant level. if the level is low fill it and you should be ok. if the level is fine could be bad thermostat.

Agree with both above. With the ignition on motor not running, turn on your heater on full blast ( heat and fan ) if you dont hear blower working then start there. If it does come on, check coolant level and get thermostat diagnosed