Hot water heaters. Wouldn't make more sense to keep the temp higher than 120 degrees?

Question

Due to insulating properties of water (and those of the water heater) wouldn't it be more energy efficient to keep the temperature of the water heater at around 150 degrees or more? When you take a bath or shower you be using less hot water since you would increase the ratio of cold/hot water to reach the same temperature. For example you would use more hot water to achieve an overall water temp of 105 degrees it your water heater was set at 120 degrees. I realize that it would take more energy to initially heat the water to a higher temp, but keeping water at say 150 degrees wouldn't take considerably more heat than keeping it at 120 degrees. Also since you wouldn't be using quite a much hot water, the heater wouldn't be having to heat up as much cold water to replace the hot water that was used. I'd be interested in hearing about the maths and science involved in this. And the reason why 120 degrees is considered the better option. I'm not interested in the safety aspects of it, just the scientific reasoning. Thanks.

2009-03-16T08:48:17Z

Edit: I see what you are saying, but water retains heat much better than air. I would think that this property would allow it to retain heat better, even if the difference between ambient temp and water is higher

Light Knight · Accepted Answer

It is the difference between the temperature of the water and the outside air that would be a problem.  The greater this delta the more energy you need to pour in to maintain the desired temperature.

An analogy to this is your home.  Lets say you maintain an inside temperature of 70°.  With an outside temperature of 40° the heater kicks on and off easily maintaining the 30° difference.  Now imagine if you were to try to keep a 60° difference instead of the 30°.  If you tried to keep your home heated to 100° or if the outside temp dropped down to the single digits, you would expect a very high heating bill.

Your optimum energy efficiency is more a function of a properly sized hot water tank.  Too small and it can not keep up.  Too large and you are keeping more water hot than you will use.
However the new "Flash", "Tank-less" or "On Demand" water heaters are the way to go for conserving energy.

hipp5 · Answer

The flaw in your reasoning is that it DOES take significantly more energy to keep hotter water hot. The relationship is not linear so doubling the temperature of water more than doubles the energy required to keep it that way.