Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: Does it help to turn off water heater?

Learn more physics!

Most recent answer: 08/04/2015
Q:
My mother and I have a disagreement concerning a conventional 40 gallon electric hot water tank.She says it saves energy shutting it off and turning it on as needed. I say it takes less energy to simply keep that 40 gallons hot with little shorter intervals of energy rather than heating a 40 gallon tank of 35 degree water to 120 degrees.
- Michael Cookson (age 48)
Burnham, Maine USA
A:

I think your mom is right. The amount of heat that the electric heater will need to supply is about equal to the extra thermal energy that flows out in the hot water plus the heat that leaks out to the room from the heater. If you assume that the water use is fixed, then the only thing that would change is the heat leak. If the heater is kept a little les hot when it won't be used for a long time, then the heat leak will be reduced a bit.

There can be problems with growth of bacteria in tanks that are kept warm but not hot, so before switching to your mom's system you might check with somebody who does this sort of work professionally.

To really save energy, you could switch to a heat-pump water heater. because these heat up more slowly, you have to leave them on all the time. The little extra heat leak is not important compared to the very large efficiency improvement.

Mike W.


(published on 08/04/2015)

Follow-up on this answer.