Hot Water
Question
I did an experiment. My hot water is usually set around 140 degrees. I turned it down and it took at least 4-5 hours before it was under 125 degrees. I think therefore that it is a good idea to run the hot water before Shabbat about 15 minutes. Certainly, AA's suggestion that you can lower it a short while before Shabbat doesn't work on my hot water heater.
Answer
Answered by Aaron AbadiSometimes it does, sometimes it doesn't. Sometimes your hot water is at 140 and often it is at 110. After 15 minutes you are already in a strong enough Safek, not to have to worry about it. It is very probable that even if it were at a high heat it would be OK. AA