Fitting an optional relay into HWS booster circuit

I have a HWS (240VAC20A) with a manual booster switch inside house.
I wish to also fit a 24VDC powered relay into the circuit so as I can manually control a timer from a 24VDC source used to switch other 240VAC electrical items such as pumps and lights.
The boost switch is presently operated by switching the active wire.
I propose to splice a relay between the active IN and OUT wires from the switch.The relay is activated by 24VDC.
Would this cause any problems if the manual switch was on at the same time as the relay switched on?
Thanks