I was talking to a neighbor recently about replacing the elements in my heater as they go out here frequently because of major lime in the water. He told me something interesting. He said he was not happy with the recovery rate of his electric water heater and that most electric heaters are designed to only have one element at a time come on. He said he fixed his situation by putting a thermostat on both the top and bottom elements so both could heat at once and now he has almost unlimited hot water. Is there any reason you should not do this?
My hot water heater came with a thermostat for each element. In the directions it stated that each thermostat should be within 10 degrees of each other and if different the top should be cooler then the bottom. I do not know if you can do this on heaters that were not designed this way or not. By the way I bought my heater about 15 years ago at lowes.
On most 40 US gallon hot water tanks I've seen the top heating element, typically 3000 watts, heats the upper part of the tank, then the upper thermostat flips over and the lower element, often also 3000 watts, heats the lower portion of the tank under control of the lower thermostat. Some tanks can be larger and have larger heaters, my daughters is AFIK 60 US gallons and the heaters are larger, possibly 4500 watts each? But the principle is the same as with our 40 gallon.
In many cases you can move one wire to allow both heaters to come on at the same time, each controlled by their individual thermostats. i.e. faster recovery because you are heating top and bottom of the tank at the same time.
As an example
The total amount of electricity used depends on the TOTAL amount of hot water drawn from the tank; no matter how slowly or quickly you heat it up and use it!
BUT: the wiring from the panel to the tank must be of sufficient size to carry 6000 watts instead of the usual 3000 watts. Since Watts = Volts times amps. Therefore Watts/Volts = Amps. When the voltage is 230. For 3000 watts: ... Amps = 3000/230 = 13. This would require #12AWG with a
20 amp breaker/fuse. For 6000 watts: ... Amps = 6000/230 = 26. This would require #10AWG with a
30 amp breaker/fuse.
So unless wired with the heavier wire it would be overloaded. Although it might seem to work OK there is also the possibility that the now under capacity 20 amp breaker will trip, correctly doing it's job of protecting the wiring against overload. Result no hot water! The breaker will be operating at 30% overload and may not last long?
Most ARE designed to only have one at a time. The reason is fast recovery. In this situation, the top element draws the max the branch circuit can supply AND the internal wiring of the heater is designed for.
When a limited supply of water at the top is heated for light usage, the upper thermostat transfers power from the upper heating element to the lower thermostat to heat the bulk of the water.
Before you change anything, be sure the branch circuit and internal heater wiring is suited to the task.
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