On Mar 10, 3:00*pm, Davej wrote:
Had a bad lower element in my water heater and noticed that the
diagram indicated "non-simultaneous" operation. I'm just a little
curious about this scheme. Seems odd to me because the hot water is
going to rise to the top of the tank either way, right?

Hi Dave:

Yes hot water rises. But the way I understand, see a recent post about
this same subject; the top heater under the control of the upper
thermostat heats the upper part of the tank first.

When temp. there reaches the preset the upper thermostat operates and
flips the connection over to the bottom heater under control of its
thermostat.

Heating of the lower water then continues until the whole tank is full
of hot water. The lower thermostat then opens and heating stops. This
is sometimes referred to as 'Flip-flop' operation. Or, in other words
the top thermostat is a 'changeover' type while the bottom is 'on/
off', type. Note 1.

The design principle being that the two heaters are not on
simultaneously. Also that even if there is fairly heavy usage of hot
water at least the top half of the tank is recovering its temperature
before trying to heat the whole tank again!

Notes: From the various (North American 230 volt style) tanks seen and
numerous repairs, mainly to 40 US gallon style tanks performed here;
1) It would be possible to use an upper style (changeover type)
thermostat as an on/off type in the lower position 'in an emergency.
2) It is possible (by moving one wire) for both heaters to come on
each under control of its own thermostat; BUT that doubles the amount
of electric current flowing during the heating period, so the wiring
and circuit breaker or fuses must safely sized. The 'total' amount of
electrcity used, to heat up the same mass of water, will be the same.
The advantage being that the tank will 'recover' to preset temperature
more quickly.

BTW with your bottom element burnt out; you will only have roughly
half the amount (the upper half ) of hot water ready for use. Are you
sure it was the lower element? Although it is more usually one of the
heaters, not the thermostats, But a dud thermostat can also result in
no current reaching the heating element; especially if something under
that cover has got wet!

Any help? Cheers.