Immersion Heater help please

[jimhowert]

Hello, I hope somebody can give me some useful information. I have been searching this forum but cannot seem to find any post on the subject I require help on. Many of the posts are varied and contradictive about my question.

I have a hot water cylinder in my cupboard upstairs. It is run by an immersion heater. Last night it may hae failed to work, or work as good as it used to, as today there is no hot water, as to be more exact, the water is not hot, just warmish but not appropriate. The cylinder is 900 x 450mm direct. The heater is rated at 3KW.

To be short about it, how do I find out if the element has definetely failed or is failing. Some posts suggest it may just be the thermostat which is at fault.

I have a multimeter (bought from screwfix over a year ago).

I know how to use it but what is the exact or easiest procedure for testing for a faulty element, and or thermostat on my immersion?

Any help I would much love and could run down to B & Q this afternoon and get what I need, ie a new thermostat or whole new heater.

 

[spacethegas]

the stat is just a switch. you can buz/resistance test across the two connections whilst moving the temp up & down. mind you this will not tell you whether it is out of calibration.

the resistance across the element should be about 18 ohms.

 

[jimhowert]

Thank you.

So if the reading of resistance is higher then the heater has failed definetly?

 

[spacethegas]

any significant difference higher or lower.

 

[JohnD]

be sure to test the thermostat first, as it is the easiest thing to replace.

look at the head of the element as it might have tripped the Overheat sensor (they are sometimes resettable). post a pic of the head if it is not obvious.

If you test the stat and element and they seem OK, verify that you have power from the cables to the immersion head (use the insulated probes and be exceptionally careful not to touch anything live, and preferably do not be touching the metal pipes or cylinder either. there is a chance the fuse or switch or a cable connection might have failed.

make sure you have reset the multimeter from Low Resistance Test to AC volts or you will damage it.

btw I like to fix the instructions from immersion heater etc to the wall of the airing cupboard with a drawing pin, is helpful when checking connections or buying replacements.

 

[jimhowert]

Sorry I have only had the chance to get back to you, have been busy.

I still have no hot water. Luckily I have an electric shower so it's not to much of a rush to get it sorted. But I have the rest of the day off now and want to start this and finish it.

OK, so to clarify with you, firstly I need to test to see if the THERMOSTAT has failed. As mentioned, these are more easily replaced.

So I test for continuity between the Phase and Neutral terminals.

 

[seco services]

test if you have power on both sides of the stat if you have its working

 

[jimhowert]

yes I have just tested it and there is power between L and T terminals respectively.

So the stat is fine then definetly?

Next I test resistance between Live and Neutral terminals... ? I should be looking for about the 18ohms range?

 

[seco services]

isolate the mains power first.
check resistance between T N

 

[jimhowert]

I turned the power off and removed the fuse from the connection unit, about 1/2 metre next to the cylinder, before I did the continuity test for the stat. That was correct wasn't it, I am sure it is?!

OK, now I will test between T and N. and also L and N should be the same anyway I believe.

 

[seco services]

yes.
now test T N

 

[jimhowert]

Just tested between T and N terminals on low resistance, and it read 2.

Is this normal? I mean is the element faulty then for sure?

 

[seco services]

yep its knackered

 

[jimhowert]

Ok, will need to replace then.

I think it's a 27 inch heater (the longest ones) , but once it is out, I will measure to be extra sure.

Why does low resistance mean it's knackered though? I'd have thought it'd be the other way round, ie high resistance means it is knackered, from being scaled up, etc!...

 

[seco services]

if it a top entry element its 27" with a 18" stat.
its only the outer that gets scaled not the main element.