Electrical puzzle

[Penners]

About a month ago our 2-year-old immersion heater stopped working (normal immersion heater fitted into the secondary side of the indirect cylinder). I checked its connections (under the cap) and found that no power was getting through the thermostat, even though the water was cold. I also found that the multi-strand single core connection flex from the load side of the thermostat to the immersion heater terminal post was extremely corroded - it almost crumbled away. However, when I connected up the immersion heater directly to the electrical supply (with the thermostat out of circuit) it worked fine.

So I bought a new thermostat, soldered a new bit of flex on to the immersion heater terminal post, and fitted it all up. Worked fine, patted myself on the back (yeah - big mistake).

Yesterday (some 4 weeks later) it stopped working again, and I find it's the thermostat that's failed again.

Clearly something's wrong that I'm not spotting. I'm hoping that I don't have to drain down and replace the whole immersion heater, not simply on cost grounds, but also because it's an absolute b*gger to get rid of all the airlocks when I refill the system! However, what do you think?

Anyone got any ideas please?

 

[NT]

Thermostat failures are mainly down to 2 causes:
1. oxidation on the stat contacts causes high R causes high heat diss
2. oxidation on the wires connected to the stat, or screws not done up tight enough cause heating and failure.

Replace stat and replace the wires connecting to it with clean shiny ones, do the screws up very firmly, and all should be well.


NT

 

[Aidan]

So I bought a new thermostat, soldered a new bit of flex on to the immersion heater terminal post, and fitted it all up. Worked fine, patted myself on the back (yeah - big mistake).

Newer thermostats should now have a secondary manual-reset high-limit thermostat. This is normally reset by pushing in some little button. It hasn't operated, has it? If so, it would indicate some other problem, possibly the primary temperature control is set way too high.

The thermostat should be wired with heat resistant cable. I think it's just the PVC insulation that is different.

Otherwise, it might be that the immersion heater has failed and is drawing more current than the thermostat is rated for. I don't know how they usually fail; I think they get scaled up, overheat and either burn out or split the metal sheath. I think this would cause a leak to earth and would trip an RCD or a correctly rated MCB or fuse. You could check the resistance if you've got a multi meter, or else turn everything else off and try to check the power consumption at the meter.

Why soldered; they're usually screw terminals aren't they? If the terminal gets hot the solder might go awol.

 

[Penners]

Many thanks for those comments, NT and Aidan. I hadn't considered that the new stat might have a high limit cutout. I'll check it and see.

The instructions that came with it said that its temperature was factory-set and should be correct for most installations (60 I presume). However it has no numbers printed on the dial - just division marks. It came out of the box set to its maximum on the dial, so I left it like that. Do you think that was a mistake?

As to the soldering - no, the terminal posts on the immersion heater have no screw connectors - the cables are definitely soldered on. Hopefully my solder won't melt at 60 degrees!

 

[Aidan]

It came out of the box set to its maximum on the dial, so I left it like that. Do you think that was a mistake?

Probably; ideally you should have the stored hot water at 60 degC ish. You need to measure the temperature at a tap, when the the stored water is at the set temperature, and jiggle the dial as appropriate. They don't put numbers on now because then they'd have to calibrate them.

As to the soldering - no, the terminal posts on the immersion heater have no screw connectors - the cables are definitely soldered on. Hopefully my solder won't melt at 60 degrees.

I'd hope not, but the connector is directly connected to the metal resistance heating element which gets a lot hotter than 60 degC.

If the element gets scaled up, the scale acts as thermal insulation and the element can overheats until it fails (as seen on those Calgon adverts). You can get noises from localized boiling under the scale (>100 degC) & the heat is dispersed through a mineral insulation inside the element sheath, so the element is probably operating in the hundreds of degrees C.

If you get hard water, the element will scale up and should be regularly descaled, as with a kettle. If this hasn't been done, it's quite likely to be damaged. I think I'd get a new element, they're fairly cheap.

 

[Penners]

I think I'd get a new element

Thanks again, Aidan. I was trying to avoid fitting a new element, but I fear you're right.

The secondary system is badly designed and always gets full of hard-to-shift airlocks when it's been drained down. The only way I find that I can sometimes shift them is to press my hand hard on to the bath tap nozzle and turn on both taps. This forces mains pressure cold water up the hot water pipe and blows out the airlocks. But I always end up soaked!

I suppose I should be thankful that the cold water feed to the bath is at mains pressure.

 

[Aidan]

Thanks again, Aidan. I was trying to avoid fitting a new element, but I fear you're right.

http://bes.co.uk/products/103.asp

£12.40 + vat + postage for their most expensive and corrosion resistant immersion heater, complete with thermostat; not worth draining to check whether the old one needs replacing.

Suitable for use with 'very aggressive water'.
"You lookin' at me?" :twisted:

 

[Penners]

not worth draining to check whether the old one needs replacing

Absolutely! If it's getting drained, it's getting replaced!

Thanks for the website suggestion - very useful.

 

[NT]

I would want to know what the problem is before replacing everything. Heaters do scale up, corrode and split, and this can cause overcurrent and stat fryup. But that doesnt mean thats the cause. You can check for a split heater by measuring resistance from element to casing, with a multimeter. Multimeters wont tell you what the real world power consumption is due to 2 factors: water resistance is not the same at low v, and element resistance ditto.

If you do need to replace the heater and youre in a hard water area, definitely go for one of those alloy ones. They expand and crack the scale off. You dont need to descale them.

A multimeter reading should tell you whats going on here.


NT

 

[Penners]

You can check for a split heater by measuring resistance from element to casing, with a multimeter

Thanks NT, but if the heater was split, would it still work? Mine works OK - it's the thermostat that's given up the ghost (although I haven't yet had a chance to check for a high level cutout).

 

[NT]

You can check for a split heater by measuring resistance from element to casing, with a multimeter

Thanks NT, but if the heater was split, would it still work? Mine works OK - it's the thermostat that's given up the ghost (although I haven't yet had a chance to check for a high level cutout).

Split heaters often continue working for a long time. The element itself corrodes through, so after a while they run with the heating power flowing dfirectly through the water, with part of the original resistance wire still in circuit. Although it sounds scary to the unelectrical, in reality it does not cause any electrocutions.

As it deteriorates and splits along a great part of its length, there is less resistance wire in circuit, and the total power consumption may (or may not) increase. If it does, this could cause the thermostat to overheat, leading to premature failure.

Really the only thing is to get your multimeter onto it to see where the problem lies. If the heater element conducts eagerly from live to case, replacing it may solve your thermostat fryups. I say may though, as split elements also often behave themselves fairly well.


NT

 

[Penners]

Many thanks for the continuing advice, NT. I've decided that I'm just going to replace the whole thing - heater and thermostat. The modest cost isn't worth any further agro. I need to drain down at some stage to carry out a modification to the secondary circuit anyway, so I may as well bite the bullet and do the whole lot together.

Many thanks again.

 

[Me!]

Although it sounds scary to the unelectrical, in reality it does not cause any electrocutions

NT - Yup, you've got me scared! :shock:

Why doesn't it trip the earth leakage circuit breaker thingy? RCD?, is that what they are called?

 

[Penners]

Hi Me!

I think I can explain this in layman's terms. It's because the left-handed floggle-toggle agitates the snibber valve below its resonant thermonics. This ensures that the back-EMF does not have a contentious enough resistivity to belt you with 10,000 volts.

I hope that helps.

 

[NT]

Many thanks for the continuing advice, NT. I've decided that I'm just going to replace the whole thing - heater and thermostat. The modest cost isn't worth any further agro. I need to drain down at some stage to carry out a modification to the secondary circuit anyway, so I may as well bite the bullet and do the whole lot together.

Many thanks again.

The reason I suggested a multimeter is that it may save you the hassle. All you need do is switch power to heater off, take off the heater outer cover and put one probe on the heter live terminal and one on the heater metal case. On the ohms range it'll show you whats going on. 30 seconds of hard graft


NT

 

[NT]

Although it sounds scary to the unelectrical, in reality it does not cause any electrocutions

NT - Yup, you've got me scared! :shock:

Why doesn't it trip the earth leakage circuit breaker thingy? RCD?, is that what they are called?

In most houses there is no RCD. In cases where there is, if its been fitted by a skilled spark the immersion heater will be on the non-rcd side of the CU.

If the immersion heater is on the rcd side, or on a whole house RCD (eg with TT systems), then the combination is a recipe for trouble, since a split heater not only takes its own power out but also that to many other circuits.

Fixed heating appliances such as immersion heaters and cookers should be on the non-rcd side of a split CU because leakage is common and does not lead to electrocutions in practice. There are effective enough other safety measures in place that the death rate is zero per annum.

Electrode heating (ie the passage of mains direct through the water to heat it) is a standard method of heating used in industry, but has not been approved for domestic use. It was sometimes used for portable appliances in the 1930s, in which role it was genuinely dangerous.


NT

 

[Penners]

NT

On the ohms range it'll show you whats going on

I'm afraid my ignorance is such that I wouldn't be able to interpret what the needle was telling me. The only thing I use the ohms range for is to see whether there's a circuit or not - no needle movement: open circuit. Full needle movement: circuit.

Yup - that's how knowledgeable I am!

 

[NT]

Ah. Well you can always ask us. However there is a bigger problem: new meters dont have needles!

NT

 

[Aidan]

NT - Yup, you've got me scared! :shock:

Why doesn't it trip the earth leakage circuit breaker thingy? RCD?, is that what they are called?

If it's split, the electric resistance heater will be in contact with the water; the heater should be isolated and replaced asap. It is hazardous and they can kill.

Some, or all, of the current could flow from the heater element to the cylinder's metal casing. If the pipework is properly earthed, this would trip an RCD.

An RCD trips if the current going out down the live wire doesn't equal the current returning down the neutral wire; this would indicate a fault in that some current is flowing to earth, or to some other current path. Many consumer units/ fuseboards don't have RCDs. Some Period Properties may have Period electrics.

If the pipework is not earthed, the cylinder and attached pipework will be at mains voltage, 230V. If you should happen to touch the live parts and something that is connected to earth ( say a radiator, bath) current will flow through you to earth.

If it is earthed, the fuse/MCB should pop/trip if the current exceeds it's rating. If the break is near the end of the heater element, it is possible that the current will be low enough not to trip the MCB. The heater might still work with current flowing to earth, rather than to neutral. I've seen a kettle plug wired with the neutral wire connected to earth; the kettle worked.

I'm not an electrician.

 

[biffvernon]

Some Period Properties may have Period electrics.

Mine if definitely Period electrics. No RCD/MCB, just a good ol' fuse box with bits of wire strung through china thingies. You know where you are with stuff like that. But when my immersion heater has split asunder it just stopped working. No blown fuses or other excitements. It has puzzled me

I've seen a kettle plug wired with the neutral wire connected to earth; the kettle worked.

The Neutral does go to earth anyway after it leaves your house. There isn't an earth wire running all the way back to the power-station.[/quote]