IMMersion heater problem

[litody]

I got my hands on a multimeter.

Top and bottom elements read 19 and 18 ohms. So I figured it must be the timer.

So I did continuity testing across live in on timer and live out on timer for both elements. Tested switching of main element(bottom element) and boost element (top one). Continuity worked as it should. i.e. only beeped me when respective element was in switched on mode by the main dial or boost dial. So timer switching looks 100% fine.

Neutral continuity test in timer beeped all the time so its pass through with no switching.

So now I'm confused. Elements read OK and timer switching reads OK. What a B****R.

So next test was to swap the live out leads from the timer with each other so that top element becomes main and bottom element becomes boost.
With just the top element connected it trips the main RCD switch when timer is in on time period or when booster switch is on.

So disconnected top element and re-connected bottom element (timer live out leads still swapped). Now no RCD tripping. Bottom element comes on when booster switching is on. So booster switching is not causing a problem and is working fine

So after all this testing it seems IT MUST BE TOP ELEMENT which is faulty even though its ohms reading is as expected, i.e. 19ohms.

So what do you think?

Next question is how do I know what length element to order? The top element is a heatrae sadia Gold Dot and has NO model number/markings on it. The bottom element is a Redring P14T. Are they normally both same length? Thermostat is 7 in so I'm guessing 11in or 14in element.

TIA

 

[EFLImpudence]

Measure the continuity between each element terminal, L & N, and earth (the tank body).

It should be open circuit - if not, that's the fault.

 

[litody]

meter has gone back to owner.

Are you suggesting terminal has corroded through to tank? It'll need a new element if thats the case. Besides, I've been told meter doesn't have enough juice to test continuity through water. You'd need full 240 volts and that causes RCD to trip before you'd have a chance to meter it.

 

[EFLImpudence]

No, the elements corrode and bend and the actual filament contacts the casing of the element which is screwed to the tank causing an earth fault.

It cannot have a dead short if it still reads 19Ω or so L to N but the insulation may have broken down or got wet enough to cause an earth fault enough to trip an RCD - 8,000Ω - or more allowing for other earth leakage elsewhere.

Ideally you need an insulation tester but it could be measurable on a multimeter.

 

[litody]

OK I undestand. But either way its a new element required so I'll get one and fit it.

I was hoping it was the timer as that would be a whole lot less hassle to replace (but more expensive).

 

[litody]

And I would just like to thank everyone who has contributed. I've learnt a lot.

I'll report back when I've done the deed.

Happy holidays to all.

 

[JohnD]

If, as expected, it is the RCD that trips then it is, as expected, breakdown of insulation in the element.

You have had a lot of red herrings.

probably your elements go into the cylinder horizontally, so they have to be short enough not to touch the other side. In any case, with a dual element cylinder, the upper one is only intended to heat up a few gallons for the sink, quickly, not a bathfull.

Slacken off the huge nut before you empty the cylinder. The weight and pressure of the water will reduce the risk that the thin copper of the cylinder bends or tears. A box spanner is better than a flat one.

running the hot tap does not empty the cylinder.

 

[litody]

On emptying cyclinder my undestanding is as follows:

There is a red inline tap which is on pipe leading into bottom of tank.
I always call this the hotwater off tap as turning it stops hot water to taps.

And of course I have a main cold water stop tap in kitchen whichs stops feed to header tank and all taps.

So am I right in thinking that I should just turn off red hot water tap which actually stops tank refill from header. Open hot water tap in bath and then drain tank from bottom emptying tap?

 

[mogget]

No! the hot water comes out of the top of the tank!

If you just run the hot water till it stops the tank will still be full of water. If you're lucky there'll be a drain cock at the bottom of the tank. You need to use that.

Perhaps I misread your post, but it's best to be clear that just running the hot water will not empty the tank. You need to use the drain cock.