Immersions not heating direct unvented cylinder

[MarkRainbow]

I contest the claim that you "have to use" 230V.


Immersions elements should never be changed speculatively, because each change stresses the cylinder. I've never seen a faulty element that couldn't be diagnosed with the most basic of multimeters.

Customer has a small unvented direct cylinder with two immersions and is getting no hot water.
I think one of the immersions must have failed previously but no-one realised.

Voltage across the element terminals is 240V and both elements have a resitance of 20 Ohms.

Have disconnected the supply leads to the immersions and connected it to a 3kw hot air gun and that runs fine.

So I can only assume that the elements have failed, despite showing a good resistance value. Are there any other tests I can do to confirm this?

 

[Harry Bloomfield]

Stat failed? Stat over-heat having tripped?

 

[bernardgreen]

Where exactly did you find 240 Volts at the elements ?

If this was at the same terminals as where the 20 Ohm was measured then the impossible has been achieved or the meter is giving incorrect results.

 

[MeldrewsMate]

Yes.

You can check the continuity across each of the thermostats

 

[MarkRainbow]

I put the multi-meter across the terminals of the element, so the voltage is getting past the thermostat and the thermal cut out.

I turned of the power, swithched the muli-meter to the auto-ranging Ohms setting and put the test leads in exactly the same place.. on the wires that come out of the heating element.

I didn't disconnect the wiring from the controller however... But I would have thought the relays controlling the output to the boost and main immersion elements would be double pole?

I'm not there anymore unfortunately to do this.

 

[Harry Bloomfield]

I would make a guess at you perhaps misreading your auto-ranging meter - actually reading 20K Ohms, rather than 20 Ohms.

 

[ianmcd]

I would guess you should stop messing around with something you are not qualified to work on

 

[MeldrewsMate]

Customer has a small unvented direct cylinder with two immersions and is getting no hot water.

Does no hot water mean 'there is no hot water issuing from the hot taps'?
Could it mean 'the water coming from the hot taps is cold'? That would make less sense than the former, because you are convinced that you had a) 24vac ACROSS the heater element, and b) the heater element has a nominal resistance of 20 ohm.
The conclusion from this is that about 12 Ampere would flow when ON, generating about 3 kW of heat. Where do you think that heat has gone?

Perhaps an easier test is better suited to your talents.
Switch OFF (not 'OF') the immersion heaters, note the electrical consumption from the meter (how many kWh used in one minute)
Turn ON one of (that's 'of', not 'off') the imm heaters that has passed your test. What is the consumption now? 3 kWh / 60 minutes = 0.05 kWh in 1 minute). The consumption increase will be significant enough to avoid any 'noise' due to other appliances switching on or off (except microwaves, etc).

Hope this helps, MM

 

[ianmcd]

Or be radical and get a G3 qualified plumber or engineer in to work on it which you are required to do by law, you can not legally work on this yourself , but what could possibly go wrong, it is only a hot water tank afterall

 

[MarkRainbow]

Or be radical and get a G3 qualified plumber or engineer in to work on it which you are required to do by law, you can not legally work on this yourself , but what could possibly go wrong, it is only a hot water tank afterall

Decend from your pulpit Ian. I am G3 and more skilled with a multi-meter than most electricians, but I'm not too proud to ask for advice when something doesn't make sense to me. We can all overlook something and say Ahah! when someone is kind enough to point it out to us.

 

[MarkRainbow]

I would make a guess at you perhaps misreading your auto-ranging meter - actually reading 20K Ohms, rather than 20 Ohms.

It definitely read 20Ω and 0.020 when on the KΩ scale.

I think that either I wasn't measuring what I thought I was measuring because the elements were still connected to the contoller, or it's because I'm using 9v DC from the battery inthe multimeter to measure what is in reality an impedence, because in use, the element sees AC current...

 

[fixitflav]

It definitely read 20Ω and 0.020 when on the KΩ scale.

I think that either I wasn't measuring what I thought I was measuring because the elements were still connected to the contoller, or it's because I'm using 9v DC from the battery inthe multimeter to measure what is in reality an impedence, because in use, the element sees AC current...

I doubt there's a significant difference between resistance and impedance of an immersion. It will have zero capacitance and negligible inductance.

 

[ianmcd]

Decend from your pulpit Ian. I am G3 and more skilled with a multi-meter than most electricians, but I'm not too proud to ask for advice when something doesn't make sense to me. We can all overlook something and say Ahah! when someone is kind enough to point it out to us.

I can assure you that I am not on any pulpit, if you are as you say

more skilled with a multi-meter than most electricians

and you are G3 qualified
Why are you on a DIY site quoting absolute drivel, asking for advice to do your job ?

 

[endecotp]

Are there any other tests I can do to confirm this?

Clamp meter to measure the current.

 

[Harry Bloomfield]

I think that either I wasn't measuring what I thought I was measuring because the elements were still connected to the contoller, or it's because I'm using 9v DC from the battery inthe multimeter to measure what is in reality an impedence, because in use, the element sees AC current...

It is still a resistive element rather than an inductive load, so the need is to measure straight forward 'resistance'.