Economy 7 immersion heater suddenly not working

[Mike_Burnstead]

Hello all,

Help! Yesterday morning I woke up to find no hot water. I have an Economy 7 water tank fitted with two immersion heaters, one at the bottom of the tank and one about halfway up. The lower heater is wired to a circuit which is only live during cheap rate, and the upper is wired on a normal circuit which is live all the time. I believe all this is fairly standard.

The upper heater works fine but of course it only heats up half the tank.
I've tried wiring the lower heater up with the lead to the upper one, but it still didn't get hot. I've removed the thermostat that sits in the lower element and tested it with a continuity tester and a jug of hot water, and it seems to work fine. Whilst the lower heater was wired up with the live circuit for the top-up heater, both sides of the thermostat were live when tested with a voltmeter, so power is definitely getting to the element.

Would I be right in thinking it's a new immersion heater required? There's a datestamp on the water cylinder of May 2003 so it seems very early to have failed, though the water is pretty hard here. The other reason I'm a little suspicious is that both elements have the same resistance when checked with an ohmmeter, so there's obviously still a full circuit through the element.

Any ideas would be very much appreciated!

Cheers
Mike

 

[bab]

I would of set yes straight away if u hadent said the ohms test proved the same? sound's a daft question but is there an overheat button in your immersion heater. If u do have hard water & have to change the immersion heater u can order titainium ones 4 agressive water.When u wired up the bottom immersion with the top supply how long did u wait to see if it was workin. depending on the volume of the cylinder it could take a couple of hours.It may just be the supply to your bottom immersion thats failed. The timer or the breaker. Have u made the end of the flex for bottom immersion safe before it due to come on.

 

[Mike_Burnstead]

I would of set yes straight away if u hadent said the ohms test proved the same? sound's a daft question but is there an overheat button in your immersion heater. If u do have hard water & have to change the immersion heater u can order titainium ones 4 agressive water.

Sadly no overheat button on the thermostat. Also, testing with a simple neon screwdriver shows that both sides of the thermostat are live, so I don't think it's the 'stat stopping it heating up.

Shame, cos I don't fancy going through all the hassle of draining the tank and swapping the element!

 

[bab]

just edited 1st reply if u spoted that.

 

[Mike_Burnstead]

Thanks bab.

When the top element comes on (and in fact the bottom one too) then after just a few minutes I can hear it making a quiet sizzling noise, rather like a kettle. I didn't get that at all from the bottom one, though normally I can on those occasions when I'm padding about late at night near the airing cupboard where the tank sits. But I did leave it powered up for about three hours, and it wasn't getting even slightly warm on the large brass but at the end of the thermostat.

I've actually wired it back up now to the E7 outlet, so there's no danger from the exposed wiring.

 

[Mike_Burnstead]

Bad form to follow-up my own post I know, but I've found the problem...

I retested the resistance of the two elements and found that the lower one is in fact an open circuit, while the top one is around 20 ohms. So I guess it's definitely the lower heater that's failed, even though it's only around 2 years old. I only bought the place 6 months ago - d'oh!!

I'll go and buy a new hard-water-suitable element, a big spanner, suitable sealant, and a large slice of luck when it comes to pulling out the old one.


Thanks,
Mike

 

[oilman]

They're all suitable for hard water, the iconel (not titanium) type are rated for agressive water such as found in Devon or Scotland amongst other places. You shouldn' t need a sealer as the washer is adequate. Leave the tank full while you crack open the existing element, then drain the tank.

 

[Stoday]

Immersion heaters now have to have a thermal cutout as well as a thermostat.

Have another look just to make sure you haven't just got a cutout that's cut out. It won't be on the thermostat.

 

[Mike_Burnstead]

Well I drained the system, removed (without too much difficulty, luckily!!) the old heater and found it was warped and split along most of it's length. So I guess that was the problem... however, this morning, I still had no hot water at the bottom of the tank and the brass boss of the new heater was barely lukewarm to touch. AARRGHH!!!! I had the top-up heater on yesterday afternoon so I had some hot water, and that was scalding hot to touch.

Since it's a big tank (166 litres) is it likely that it was barely warm because one nights heat isn't enough to heat it up properly? I had the top up heater on for the afternoon so the top of the tank was already hot, so could it be some stupid effect caused by the plug of hot water sitting at the top of the tank? Really confused now - thought I'd cracked it yesterday!

 

[Softus]

One night is more than enough - you still have a fault. Have you checked that there is a live electrical supply to the element, through the (new?) thermostat?

 

[bab]

Mornin softus.Re split in element. If ur house is only as old as ur cylinder then u must of tripped out a mcb on the board supply the night storage immersion witout a dout because ur split wil of put L to Earth. is there a built in switch fuse spur on the flex outlet to the immersion. does the boost top imm/ switch on via the timer for the bottom imm/ or ar they controled seperate. some new installations dont put immersions through the Rcd so if there is a 13amp fuse some where which is very likley it will hav blown.

 

[Mike_Burnstead]

Yep, it was definitely live. Waited up till the night timing kicked in, and then checked the power on both sides of the 'stat. I still have the thermostat from the old one, so I can easily swap that in if there's a problem tonight.

Since the top half was hot, there'll be a cold/hot layer in the tank. I'm thinking that hot water rising from the lower element will punch through this layer, roll over at the top and start to sink from the sides, but then reach this barrier and not sink all the way down to the bottom?

I may be clutching at straws, but I'll run all the hot out tonight with a tap so the whole thing's cold and see what happens...

 

[bab]

Well u got live there so thats a good start. Just a thought. What sort of timmer controls ur night rate lecy is it out of sink with when you night rate comes on (long shot) Water layers in temp in cylinders hot at top down to cooler at bottom naturally. heats gotta rise Whats the new stat set too, they now come preset so if ur imm/was live 4 hrs even with the stat on 45 oc say u would still hav a full cyl/ off hot water.

 

[Mike_Burnstead]

Morning bab,

The house is about 15 years old so the hot water system is much newer (I only bought it in July). The power is a split distribution board, with 15A cartridge fuses. There are individual circuits for each heater - one on the nighttime side for the E7 heater, and one on the permanent side for the top-up. Originally the fuse for the top-up was blown but I've replaced it now and it heats up the top half happily.

Incidentally, the water this morning (from the top of the tank) was &*%$£% hot, and I mean HOT. That's why I'm thinking it may be some stupid convection thing where it wouldn't cycle back down to the bottom of the tank but instead stayed higher up.

 

[Softus]

Since the top half was hot, there'll be a cold/hot layer in the tank. I'm thinking that hot water rising from the lower element will punch through this layer, roll over at the top and start to sink from the sides, but then reach this barrier and not sink all the way down to the bottom?

This is a very unusual description of convection in a tank of water. The thing you perceive to be a barrier is not a barrier, but it is true that the water heated by the lower element will rise through the water above it at a rate determined by the temperature, and therefore density, of that higher water.

I may be clutching at straws, but I'll run all the hot out tonight with a tap so the whole thing's cold and see what happens...

That would waste water, electricity, and your time, and therefore would be tantamount to clutching at a straw and then eating it. If the boss of the element is cold, then the element is also cold. If you have a live supply to the element, whether or not it's through the 'stat, AND the neutral path is made back to the CU, through the DP switch, then the element is faulty.

If you need reassurance about the supply to the element, disconnect the flex from the heater and connect it up to a test lamp. If that lights up, then have a cup of tea and get a new element.

Isolate before working on the electrical connections.