How does the Spark test the valves?

I want to connect a 24V input sensor on the spark outlet channels to switch 2 knx connect valves in different location. Cabling to those valves is to long for 24V. 
The valve testing always says that there is not current and switch it off --> Do i need a resistor on the outlet to simulate a valve? If yes, which value should the resistor have?

Thanks,

Peter