Immersion problems AGAIN!

[AdamW]

My immersion stat failed a few days ago, the heater just went and went so I had hot water, just it was very hot. So I switched the heater off and only had it on for short periods (20 mins-1 hr).

Today I replaced the stat, with one of those safety cut-out ones. £10 worth, so should be good as stats go. Anyway, despite having it set to maximum temperature the water was coming out luke warm at best. I checked and the wires were both live (i.e. the element was on).

So, I tried bypassing it by putting a chock-block in, just to check the thermostat wasn't at fault. It wasn't The water still comes out tepid.

The only thing I can think of is that the element burned out due to running at too high temperatures. Can't be scale, as that wouldn't be so sudden would it? The cylinder and heater are probably 19 years old, the age of the flat. The water is heated purely by immersion, no central heating here.

What do you guys think:

a) Burned out element
b) Scale, or
c) Something else?

As a matter of interest, how long does it usually take to heat a cylinder of water to 60 degrees? I am not sure what capacity the cylinder is or I would do the maths!

 

[TexMex]

That's a weird one eh? Heaters usually Fail OC. Sometimes they fail SC. but I've never heard of one failing somewhere in the middle You havn't got an overtemperature cut out built into the thing, that's playing up, have you?

For the maths excercise,
Assuming a Heater element of 3KW with an efficiency of 90% (pure guess)
you will be delivering about 2.7KW of heat
which equates to about 38.7 kg-calories/min

Assuming your hot water cylinder to be 25 Gallons (a small one), and an ambient temperature of 20 Degrees C
you are looking to raise 113.5 Kg by 40 Degrees (ie 4540 kg-calories)
This should therefore take about 117 minutes. (4540/38.7)
If your cylinder is 50 Gallons (A big one) it will be double this fugure (Almost 4 Hours).

This sounds a bit on the long side to me, but this doesn't take into account the temperature gradient in the cylinder (you will get hot water, long before the whole tank is hot). Also, some of my assumptions are probably way off mark, and I haven't checked through the maths, so I may have c*cked it up somewhere.

Anyhow, I'm tired, so I'm off to get some kip.

 

[oilman]

You'd better check all the electrical connections in the line as you might have one that is high resistance and getting hot, or the cable may have a problem somewhere near to the heater.

 

[DP]

AdamW, if you test the element to earth, you will get a reading when using an insulation tester (a multimeter might indicate something). I am quite sure the metal seath is split exposing the element to water. You do not get a shock because of the earth connection. Water is not heating because element is failing to reach working temperature.

 

[AdamW]

This should therefore take about 117 minutes. (4540/38.7)

Cheers for that Tex! What are OC and SC by the way?

I checked all the connections, including ones I hadn't touched (just in case I had inadvertently knocked it) and no problems there.

I am quite sure the metal seath is split exposing the element to water.

This does sound like the most likely culprit. I get a very gentle bubbling noise when the element is turn it on, that could be electrolysis! Will be changing the element this evening. These things aren't expensive, but unexpected jobs can be annoying. On the positive side, the cold power shower this morning has rendered my thoughts pure!

Anyone want some hydrogen and oxygen gas?

 

[IanDB]

OC

Open Circuit

SC

Short Circuit

Sounds like a good one - enjoy your Friday evening! I could think of better things to do but hot water could take priority.

 

[AdamW]

AdamW, if you test the element to earth, you will get a reading when using an insulation tester (a multimeter might indicate something). I am quite sure the metal seath is split exposing the element to water. You do not get a shock because of the earth connection. Water is not heating because element is failing to reach working temperature.

Turns out DP was quite correct. When I removed the old immersion, the sheath around the element had corroded away in one section, leaving the element wire exposed. I will put a picture on when I get the chance.

Got the new heater in, works a dream. Hot water in no time.

Thanks for the help guys!