Insta hot draws 25a. /QUOTE]
I doubt that a flow switch will be of a size to handle 25A so there is probably going to be a solid state relay or triac to supply power to those heating elements. And probably more complicated electronic circuits to control that relay.
I'll bet that the installation instructions for that insta hot require a dedicated circuit and bigger than 20A so the problem is solved.
Any other trouble shooting or experimenting to solve the cause of the problem should not be charged to the customer.
That is what I was getting at, if dry contacts - there shouldn't be any interference in the controls from harmonics, if solid state controls - the possibilities of interference are more open.
I do agree putting the insta hot on it's own circuit like it is supposed to be will likely make the problem go away, some just might be curious as to why this happens anyway.