I have installed several 9 volt and buzzer type water alarms (very similar to smoke alarms, I suppose) in a critical crawl space. However, to maximize the coverage, I added several more sensors to each....merely by attaching wires to pairs of copper strips and placing these under carpets and on concrete floors where water from leaks would accumulate....then running these back to barrier strips with jumpers to which the power wires have been attached from the 9 volt alarm circuit.
The sensors all work fine, but is there any possible fire or heating risk from these sensors under carpet and so on? Of course the circuit will not be closed unless of moisture present and then the alarm will sound....but just want to be sure the senors sitting connected to 9 volt battery but not closed are not going to be a problem.
Perhaps not really an electrical issue in this forum, but I will post also in the appliance forum.