hot water on shabbat

Question

I just spent an hour going over the Q&A forum and I'm a bit confused. If a hot water heater can be set to lower than 125 degrees, can water be used in a private home because it is considered "gemara" and similar to opening your door in the summer causing the air conditioner to go on? or is assur because every drop replenishes the tank and is therefor cooked on Shabbat? Please advise, Thanks