Immersion Heater

[The Medway Handyman]

Hi

I need to check a 3kw x 27" immersion heater element to see if it's working. If I connect my multi meter across the terminals, what resistance should I find?

[Owain]

Current will be approx 3000 / 240 = 12.9A

V / I = R therefore 240 / 12.9 = 18 ohms

Owain

[Adrian]

Something lowish (in the region of tens of ohms) - but greater than a short-circuit (zero resistance).

If you've got a failure then it's as likely to be the thermostat (can be tested using the meter and 'tweaking' the dial until it opens or closes - while disconnected from the mains supply, of course...

Alternatively - wire a mains lightbulb across the heater element and reconnect the mains. If the lamp lights, but you don't get any heat from the immersion then it's the element at fault...

Hope this helps Adrian

[Andrew Mawson]

IME immersion heaters tend to fail as the sheath gets punctured usually due to a build up of calcium deposits, and the first signs are not a change in resistance of the element, but rather an increase in leakage current. Isolate both terminals of the element, check that there is a lowish (10's of ohms) resistance between them, and also a very high (megohms) resistance from the copper tank to the two terminals. I have some flats where electric immersion heaters are the only hot water source, and they don't last very long at all, even the better ones intended for hard water regions.

AWEM

[Bob Minchin]

Andrew, Have you tried the ones with stainless steel jackets. They should be a lot more resistant (to corrosion that is).

Regards

Bob

[Ed Sirett]

A little under 20 Ohms.

[Phil L]

Why bother? - sling it in the skip and fit a new one which will cost about

[Andrew Mawson]

message

resistance

Yes Bob, that's what's been in for the last 8 months with fingers crossed !

AWEM

[meow2222]

L-N around 20 ohms, but it may be less. L-E should be o/c, if it conducts its electrically leaky and will pop an RCD - but will work fine for a bit on a non-rcd supply, assuming a decent earth connection. No decent earth and you'll really have a problem.

NT

[gasman pete]

Ohm's Law

to keep for further reference

pete

[The Medway Handyman]

[Ian French]

The power dissipated in a resistive element is :-

Power = ( V * V ) / R, so transposing the equation we get R = ( V * V ) / P.

So, if the mains voltage is 240V and the power is 3KW the resistance should be

(240 * 240) / 3000 = 19.2 ohms.

I have done a quick check on this using my electric kettle, which incidently is 2.4KW , and reads 25 ohms on my multimeter, near enough ?

Ian.

[Andrew Gabriel]

Probably significantly less when cold.

[Dave Liquorice]

Finger trouble on part of the owner? They use all the hotwater in the morning then don't switch it back on, or "boost" it, again for hotwater later?

Is this an E7 instalation? If and assuming no "operator error" the boost contacts in the timer/switch might be najjered or the boost immersion element if it is a two element system or both. My first check would be for mains reaching the correct element when the boost is operated.

[meow2222]

yes. But wise to realise Ohm's law is not a law, and many diy type resistances change their R greatly depending on temp, thus making ohms law calcs out by anything upto a factor of 10.

NT

[meow2222]

yes. But wise to realise Ohm's law is not a law, and many diy type resistances change their R greatly depending on temp, thus making ohms law calcs out by anything upto a factor of 10. This applies particularly to light bulbs and some heating elephants.

NT

[Bob Mannix]

Oh yes it is! The fact that R is changing doesn't affect the law, it just affects the current to voltage relationship. For DC situations V=IR and continues to do so as R changes.

[Andy Wade]

Not usually. Nichrome has a very low low temperature coefficient of resistivity - about 60 ppm per degree, according to Kay and Laby. That's only a 3% change in resistance over 500 deg.

[l.milbourn]

As I suspect you know, "V=IR" is not and has never been Ohm's Law despite what nasa et al seem to think. V= IR is the definition of resistance and that is why you are correct. Ohm's Law states that for a metallic conductor at constant temperature, V is directly proportional to I (ie R is constant). As I have spent all morning trying to explain to my L6 Physics groups, nothing actually obeys the law but many things approximate to it.

Lawrence

[meow2222]

Hence it isnt a law, its just called a law.

NT