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More efficient to reheat water or keep it at constant temp.?

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safro
#1
Jan3-11, 10:09 PM
P: 1
A relatively simple question from a physics ignoramus:

I have a kitchen water heater with a 4 liter capacity. I can opt to reheat the water to 195 degrees F every time I want hot water (roughly every 2 hours over the course of the day) or I can keep the water constantly at 195 degrees. Which option is more energy efficient?

Some pertinent information:
The temperature of the room where I keep the water heater is 65 degrees F. Unfortunately I don't have a measure of the insulation of the heater.

My own intuition from some barely recalled high school physics: energy input is the same in either case.
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rcgldr
#2
Jan3-11, 10:16 PM
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The rate of heat loss is proportional to the difference in temperature between water and the room. From that standpoint it would be better to reheat the water. This could be offset if for some reason the reheat process was less efficient than the maintain heat process, but I'm not sure if that could happen.
Andrew Mason
#3
Jan4-11, 12:26 AM
Sci Advisor
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P: 6,654
Quote Quote by safro View Post
A relatively simple question from a physics ignoramus:

I have a kitchen water heater with a 4 liter capacity. I can opt to reheat the water to 195 degrees F every time I want hot water (roughly every 2 hours over the course of the day) or I can keep the water constantly at 195 degrees. Which option is more energy efficient?

Some pertinent information:
The temperature of the room where I keep the water heater is 65 degrees F. Unfortunately I don't have a measure of the insulation of the heater.

My own intuition from some barely recalled high school physics: energy input is the same in either case.
If the tank is well insulated, it does not matter. If it is not well insulated but you use a heat source with the same efficiency to heat your home, it does not matter either.

AM

rcgldr
#4
Jan4-11, 01:07 AM
HW Helper
P: 7,054
More efficient to reheat water or keep it at constant temp.?

Quote Quote by Andrew Mason View Post
If the tank is well insulated, it does not matter. If it is not well insulated but you use a heat source with the same efficiency to heat your home, it does not matter either.
Unless it's 80 degrees outside, and air conditioning is being used to cool down the room (to 65 degrees?).
Jobrag
#5
Jan4-11, 06:21 AM
P: 475
If the tank is well insulated, it does not matter. If it is not well insulated but you use a heat source with the same efficiency to heat your home, it does not matter either.

Only if the central heating timer and the water heater timer are set the same, CH off Water on uses more energy.


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