Immersion help please

[STI]

Sorry a bit of a long intro to give the background info. In my daughters new flat they have a pressurised hot water system with a large hot water tank. The tank hs two seperate heating elements, one at the top and one at the bottom. Each element is on a seperate switch. In the CU there is a MCB marked immersion heater and a seperate MCB with its own main switch marked night time immersion heater.

The two on/off switches which are in the airing cupboard have the red neons lights built in. The one switches on and the light comes on the other never seems to have the light on. The switches are independant and in the CU each of the MCB's only have 1 set of wires coming out of them. So i guess the elements are set up as one for the normal rate electricity and the other for the night rate.

The problems started when we put the night time switch on. When i checked in the morning the Night rate MCB had tripped . We called in the supplier (EON) who came out andchanged the meter. However although this stopped the tripping it did not heat up the water. The one at the top was working but could not supply sufficient hot water for two teenage girls. So i changed the top element to the bottom of the tank and moved the night rate up to the top. All went well for a day but now the day rate MCB is tripping. However ity stayed on all yesterday but the hot water did not heat up.

I am confused is it possible the bottom element holder has a problem and is tripping waterever is inserted in it and why does the night rate switch never seem to light up, yes i have got up at 1-00 am to see if it lights up

Can anyone help

Thanks
Steve

 

[flameport]

The setup you describe is a normal E7 hot water arrangement, the bottom element heating the water overnight on cheaper rate, and the top one for 'top ups' during the day if needed.

When you say 'element holder' and you have swapped the elements, what exactly have you moved?
Changing the elements would involve draining down the tank and removing the entire heater assembly from the tank.
If you have just moved a rod like item which slides into the heater, this is just the thermostat.

Either the element(s) or thermostat(s) are likely to be faulty. You need a multimeter to find out.

 

[STI]

Hi,

Yes you are correct its the thermostats (rods) i have moved. Along with the wiring attached to them so i had hoped i had moved the Night rate which never seems to work to the top and the day rate to the bottom. I do have a multi meter and i have checked for votage at the thermostat and switch for the day time set up They were there I havent at the night rate as i didnt trust myself checking for volts at 1 am but during the day their are no volts there.

 

[Taylortwocities]

The bottom one is probably on the off peak. The light will only come on in the dead of night. the time will depend on the tariff you have.

As an extra thought.

In the middle of the thermostat is a safety trip - little red button - that pops out if the the stat gets too hot.
Have a look. It may just need resetting (pushing back in).

 

[STI]

Hi,

I will check the button although i didnt see it when i changed the thermostats around. Our rate kicks in according to EON at 12.30 am and unfortunately i did get up in the dead of night 1-00 am to check but it, the little red light, didnt come on.

Fortunately at my age i often need to get up at 1 a.m any way and at 3 am and at....

 

[flameport]

As you have a multimeter, there are some easy tests you can do to see what is working (or not).

With the power OFF and the meter on continuity / low ohms range:

Remove the thermostat and check between the two terminals. With the temperature dial set to a high number, you should have 0 or a very low reading. Turn the dial to the lowest setting and you should have no reading/infinite. You may be able to hear the thermostat click as the dial is turned.
If not, the thermostat has failed and needs to be replaced. Fortunately they are easy to fit and less than £10.

The heater will have 3 terminals, L, N and E (E will be connected to the metal frame). With the wiring disconnected:
There should be no reading (infinite) between L + E and also N + E. Any reading here means the element has failed.
Between L + N you should have a fairly low reading, probably 10 or 20 ohms. No reading means the element has failed.

(repeat for the other thermostat and heater)

Other things to check:
Do the switches in the airing cupboard have fuses in them?
Check the terminals at the switches for loose or burnt wires.
You could also check the connections inside the CU, but be aware that the incoming wires from the meter will still be live even with the main switch off.

 

[JohnD]

sometimes neon indicators fail, so they are not a reliable way of knowing that a circuit is dead.

 

[STI]

Flameport, thanks for the tests i will try these out.

Does the fact that by moving the previously working thermostat to the same element that tripped the night time MCB when that was in that slot and it is now tripping its MCB point to a likely fail of that element, Could that not only trip the MCB but also damage the temp stat. Or is this a leap to far.

 

[flameport]

If the MCB is tripping, it is almost certainly an failed element, probably shorted to earth.

A thermostat connected to such an element could also fail, as when powered, the fault current which flows before the MCB trips (typically 100s of amps) can destroy the contacts in the thermostat, either welding them together or burning the contacts away.