Re: Immersion heater and time switch query.

[Roger Mills]

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).

[js.b1]

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").

[fred]

In article , Roger Mills writes

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.

[Andrew Gabriel]

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.

[Chris Hogg]

Thanks for the suggestions. I don't have a low-value resistor to check the meter with, but I have measured the resistance of our electric kettle (across the plug terminals, as it's one of those 'cordless' kettles), at 32.2 ohms. For 240 volts, that gives a wattage of just under 1800 watts; the kettle is rated at 1850/2000 watts, so good enough agreement there, suggesting the meter is not under-reading.

I've rechecked the immersion heater resistance, disconnecting both ends in case there was anything in parallel, and cleaning the terminals, and get the same resistance at 12.8 ohms. I've just checked the MCB in the Consumer Unit, and it's rated at 16 amps. But with that resistance, the immersion would be drawing 18.75 amps. So why doesn't the MCB trip?

As has been pointed out already, the resistance of nichrome heater wire doesn't change massively with increasing temperature (tables I have indicate 7% increase at 500C. Don't know what temperature the heater inside an immersion element gets to, but it can't be too high with all that water in close proximity). I don't see how a mineral insulated element could 'fail' by going low resistance. An increase in resistance, due to ageing or thinning, perhaps. And if the insulation is deteriorating and there's an earth leakage, I would expect the whole house RCD to trip (100ma, time delayed, overhead mains supply).

I am puzzled!

I can't easily replace either the thermostat or the immersion itself, because some genius 'plumber' has arranged the hot water outlet pipe to go across the top of the tank, straight over the heater and 'stat giving about four inches clearance!

I think I'll explore the option of the Sangamo time switch, as js.B1 suggests.

[fred]

In article , Chris Hogg writes

I think you're following all the right lines on fault finding and agree that low resistance seems an unlikely fault mode.

The next step I suppose is to check the current drawn with your domestic electrickery meter, a 10 or 15 minute run with a bit of maths should show if it is running overcurrent.

In terms of the MCB not tripping, 2.75A is not a desperately high overload for a 16A breaker and I reckon it could carry this for some time, if not indefinitely, without tripping. Have a google for trip characteristics on MCBs for more info, fast trip on high overload and moderate on 50% overload but indefinite on 20%.

In terms of replacement and having the outlet pipe poorly run, I wouldn't worry too much, turning off the header tank and draining the hot water as far as it will go will let you cut out the H/W pipe for access and replace it later. Worry more about the immersion removal, there's a good piece in the uk.d-i-y faq on it, mainly on breaking the seal with the cylinder full and using a box spanner for leverage.

Good luck.

[ARWadsworth]

For type B MCBs the non-trip current is 1.13 times the MCB rating. So 18.08A for a 16A MCB

Adam

[fred]

In article , ARWadsworth writes

Cheers Adam, I couldn't be arsed looking at the curves to check. Just downloaded a spec on Wylex type B MCBs from TLC and it looks as if 1.13 times is the guaranteed not to trip limit but the tolerance band on the curves suggest it might carry up to about 1.4 times before being guaranteed to trip by 60mins which is probably fair for the thermal part of the trip. The o/p's overload at 1.17 times seems to be well within the 'might trip, might not' band.

[Chris Hogg]

That's a useful tip, thanks. I'll need to get power to the immersion first, so it'll have to wait until I get a new time clock.

Re-routing the outlet pipe is no great problem, although a bit of a fiddle at the back of the airing cupboard. But replacing the 'stat and heater is potentially much more fraught. I'm aware that it can go badly wrong, with the tank becoming buckled, and at worst, torn, with a replacement required and at short notice. I may pass on that one.