I have an old Rheem RTE 9 Electric Tankless Water Heater, 3 GPM to give away. (https://www.amazon.com/Rheem-RTE-Electric-Tankless-Heater/dp/B003UHUSJI)

The heater itself is working, but the flow switch is stuck in the "on" position, which means that the heater is constantly on.

I believe this is the part that needs to be replaced; one of the customer reviews says that Rheem customer service linked to this part to be replaced.

https://www.amazon.com/Ecosmart-FSA-QC-MEDLRG-Replacement/dp/B00Z0Z5XXE

Under-sink water heater: needs work (Belle Sherman)