ROB: One has to ascertain that the Rov issuing the Psak to be lenient was/is FULLY knowledgeable with the ENTIRE mechanism of the device / apparatus / appliance. I don’t C”V want to belittle any Rov’s kavod, but it was known to occur in the past. Unknowledgeable technicians “experts” presented and explained different machines to Rabbonim, Gedolim in the wrong way. Totally. (I used to be very scientifically inclined, when I read certain seforim “A certain expert explained it to me…”, I had a hearty laugh)
There is a world of a difference between a frig, air conditioner and the likes where one may be gorem an earlier on or off (according to most / some). Even the water system pumps that will definitely be working due to one’s use, is also permitted. Too much lumdus – not for me.
But saying a hot water heater is a gorem is pulling it a bit too far. – The only way hot water is shooting out at the faucet, is because cold water is entering and pushing it out. Not a gorem at all (issur occurs at and by the motion of the faucet). Not even a she’aino miskaven – you do want it! If you turn off the cold water, your not going to get anything.
The only mechanism that should exist today, should be an air-pressured force going into the cold line feed, with the cold turned completely off before Shabbos.
I don’t know why nobody thought of it. (if people come out with such a device, KNOW – they got it from Little Froggie, on the Yeshiva World!!)