[solved]Opensprinkler with 2 pumps

Would like to use the Opensprinkler to switch two pumps alternately via intermediate relays (and pressure regulators).
For this purpose, the control voltage at the relay must be identical to the control voltage of the Opensprinkler.

My measurement results:
Input Opensprinkler + Relay: 28.8 VAC (identical to the 24VAC converter).
Output contact Opensprinkler (1-8): 29.8 VAC (each).
Where does the difference come from and how can the problem be solved?

HW: 2.1 AC, FW: 2.1.5 (1)