Please do not take anything you read in any of my posts (but especially not this post) as engineering advice. My parents’ house has an on-demand water heater for energy efficiency reasons. This has a small drawback: you have to press a button to prime the water heater and then wait 2-5 minutes before showering. This turns into a somewhat bigger drawback for the one room for which it’s just Not Possible to wire up a button. The water heater company hypothetically sells a plug-and-play wire...
