In an earlier contribution to this discussion,
Chris Hogg wrote:

Our immersion heater has just stopped working. The heater itself is
not open-circuit, and the thermostat appears to be working. It's a
fairly old immersion, so there's no over-temperature or backup
thermostat. Investigation suggests that the relay in the electronic
time switch has failed, because although there's a click when it comes
on, there are no or only momentary volts on the output terminals of
the timer. It's rated at 3kW.

The resistance of the immersion heater itself is 12.8 ohms, measured
with a simple electronic multimeter. Some simple maths suggests that
with 240 volts applied, a resistive load of this amount would draw
18.75 amps, and be dissipating 4.5kW, i.e. half as much again as the
rating of the time switch, even though the cover-cap on the immersion
clearly says it's a 3kW immersion. If my measurements and calculations
are correct, it would explain why the relay on the time switch has
failed. But do/did 4.5kW immersion heaters exist, or could the
resistance have fallen over time (seems unlikely to me, though)? If
it's genuinely a 4.5kW heater, are time switches available rated at
that wattage? I've only ever seen 3kW ones.

Fortunately, we can also heat the water with gas (propane) via a
Vaillant boiler, but off-peak electricity is cheaper.

Don't forget that the reistance of the increases when it gets hot. So
although it may take a high initial current, that won't last long and it
probably draws the right steady-state current for 3kW (and the accuracy of
your resistance meter may not be wonderful).
--
Cheers,
Roger
______
Email address maintained for newsgroup use only, and not regularly
monitored.. Messages sent to it may not be read for several weeks.
PLEASE REPLY TO NEWSGROUP!