Hello.
2 situations... a water heater with a 240VAC 2KW heating element fed by 240VAC; and the same water heater fed by 120VAC instead.

Its thermostat set to 140F. ---> Is this right : ? Both can reach the same water temperature of ~140F, but will take longer time if fed by 120VAC.

 

Its thermostat set to 140F. ---> Is this right : ? Both can reach the same water temperature of ~140F, but will take longer time if fed by 120VAC.

Yes.
If they both have the same heating element, then it will take four times as long for the 120V unit to heat to the same temperature, since power is proportional to the square of the voltage for a fixed resistance.

 

Certainly correct, and the recovery time will be longer. I recently did a similar change for a 30 gallon water heater with two 4500 watt hearers on 240volts, 9000 watts total when heating. That had caused welded contacts in the thermostats. So I put the two 240 volt-4500 watt heaters in series. So now it is 2250 watts. No complaints about inadequate recovery in three months, plus a greatly reduced power bill. Happy clients.

 

Hi. I believe the two 4500W heating elements in a standard water heater are sequential, never both on. In series should be as you say, less waste even at 240VAC. But unsure if there would be any sequential action, depending the thermostats wiring too.

 

Hi. I believe the two 4500W heating elements in a standard water heater are sequential, never both on. In series should be as you say, less waste even at 240VAC. But unsure if there would be any sequential action, depending the thermostats wiring too.
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I responded to an emergency, which was when steam and hot water were coming out the safety vent. BOTH thermostat switches had welded shut contacts, so it was a 9 KW steam generator. Yes, the heaters had been staged but evidently the bottom contacts had been welded for some time. I do all kinds of jobs now!!