How to best adjust my water heater temperature?

October 8, 2008 8:25 AM

How should I adjust the two elements of my water heater?

I have a new electric water heater with two elements, one near the top and one near the bottom. Each has its own temperature control. Both are currently set at 140 degrees. I am thinking about conservation of energy, and besides setting the temperatures as low as is comfortable, would it make sense to set the upper element's temperature higher than the lower one?

My thinking is that a) hot water rises, and b) the outgoing water comes out of the top of the tank, so it seems that I could set the upper temperature a little higher than the lower, since the water in the lower part of the tank gets used more rarely (only when there is heavy sustained demand). On the occasions that demand is heavy enough to use the water in the lower half of the tank as well, it is fine if it is a few degrees cooler.

This all sounds good to me. The only hesitation I have is that perhaps the water in the lower part of the tank will stay there too long, as it gets used rarely, and that I will have water a few weeks old or more hanging around in the tank. Maybe Brownian motion and the general current of incoming water will be enough to negate this....maybe not?

posted by SNACKeR to home & garden (6 answers total)

IANA engineer, but I think your water heater is insulated enough that setting the controls at different temperatures will only mean that the one with the higher cut-off runs longer to heat the entire tank. Hot water may rise, but heat will still flow from the hot end to the cold end of the tank. I don't think you'll see any practical effect from this, it will probably be less efficient than setting them at the same temperature if anything.
posted by ghost of a past number at 9:04 AM on October 8, 2008

IMPORTANT NOTE: Most electric water heaters have two thermostats... one near the top of the tank and one near the bottom. It is important that these both be adjusted to the same temperature. Otherwise, one element may never go on causing premature wear to the other. Also, the usable output of hot water or the time it takes to heat the tank may decrease... depending on which thermostat is set higher!

From here; he mentions 130 degrees as the minimum temp, but the DOE suggests 120 degrees to save energy; the lower temp also reduces the risk of scalding which is especially important if you have little SNACKeRs running around.
posted by TedW at 9:36 AM on October 8, 2008

If the heaters come with two independent thermostats but it is recommended that you always keep them set to the same thing, what is the point of having two separate controls? Redundancy? Something else?
posted by mmascolino at 10:07 AM on October 8, 2008

@mmascolino: yes, this is what started me off thinking about this...

"Tank temperature should be no less than 130 degrees to prevent bacterial growth, such as Legionnaires disease." - now that is useful information!

I am not sold on the argument that the elements should not be set to different temperatures because that will result in one working harder than the other. Even if the upper one is set to a higher temp, it will not necessarily work harder as it is heating water that has already been heated by the lower element and has subsequently risen.
posted by SNACKeR at 10:50 AM on October 8, 2008

The two thermostats are interlinked so that only one element is on at a time. The upper element heats the water at the top of the tank until it reaches its limit and turns off. Then the lower element heats the rest of the tank until until it reaches its limit and both are off. They should both be set to the same temperature.

Assume that both thermostats are equal and the tank has reached equilibrium at 140 degrees. When you turn on a hot water faucet, cold water enters the bottom of the tank through the dip tube. The cold water turns on the lower element and begins preheating the lower tank. The upper element stays off because there is still 140-degree water at the top of the tank. If the faucet remains on, eventually the upper thermostat drops below 140 degrees which turns on the upper element and turns off the lower element. The upper element provides faster recovery of hot water at the top of the tank. When the faucet is turned off, the upper element remains on for a little while until the upper tank is again at 140 degrees. It then turns off and the lower element turns on to heat the remainder of the tank.

Now consider the case in which the lower thermostat is at 140 and the top is at 130. Since hot water rises, the lower element will remain on until the entire tank is at 140 degrees. When the faucet is turned on, the lower element will turn on, but the upper element will stay off until the water at the top of the tank drops to 130 degrees. This delays the hot water recovery at the top of the tank and means the faucet temperature starts at 140 degrees but then drops to 130 degrees, which can be annoying for adjustment at the faucet.

In the case in which the upper thermostat is 140 and the lower is 130, the upper element heats the entire tank to 140 degrees and the lower element rarely turns on. When you turn on the faucet, neither element will turn on because the cold water is going into the bottom of the tank. the upper thermostat doesn't detect the cold water and the lower thermostat won't turn on until it cools from 140 degrees to 130 degrees. Again you lose some time to pre-heat the water.

So the conclusion is that both thermostats should be set to the same temperature for optimum results.
posted by JackFlash at 12:23 PM on October 8, 2008 [3 favorites]

Thanks for the thorough explanation and analysis! Now I can go and enjoy a hot shower.
posted by SNACKeR at 6:16 PM on October 8, 2008