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
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If the thermostat is removable you can drop in a safety/main
thermostat such as a Sunvic (just get the right length).

If the timer has failed, Sangamo do a 1-gang 16A version with push-
button 30min-1hr-2hr boost on the front often cheap on Ebay UK
(although it only has 7hr or some such low figure battery backup).
Better than the cheapies anyway re relay quality - I've found the
electromechanical type do not like "juttering powercuts" for some
reason.

If the element has gone, check with an Insulation Resistance meter not
just plain resistance.
If it is hovering around 1.5-2.1Megohms the element is on its way out.
Best way of removing is a proper box spanner and tighten/loosen,
combined with a fine screwdriver to "chop out" the gasket all the way
around which once partly gone will permit removal of the element. Fit
a isolation valves whilst in there - but not before any vent pipe for
obvious reasons (element running, vent pipe closed off, things can get
the wrong kind of "exciting").

In article , Roger Mills
writes

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).

For water heating I think resistance variation will be small.

I've just measured the cold resistance of a spare 3kW element and
although it is a little high at 21.4ohms it looks as if the o/p has a
duff element _or_ the meter is suspect, measuring a test resistor will
check it out.
--
fred
BBC3, ITV2/3/4, channels going to the DOGs

In article ,
fred writes:
In article , Roger Mills
writes

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).

For water heating I think resistance variation will be small.

I've just measured the cold resistance of a spare 3kW element and
although it is a little high at 21.4ohms it looks as if the o/p has a
duff element _or_ the meter is suspect, measuring a test resistor will
check it out.

Heating wires are normally alloys such as nichrome, and the change
in resistance with changing temperature is tiny compared with that
of metalic elements such as tungsten, copper, aluminium, etc.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]