Hello,

I know new immersion heaters have a second thermal cut out so that if
the thermostat fails, the water does not boil but how recently was
this introduced?

For old immersion heaters, if the stat fails what happens? Does if
fail on, fail off, or is it fifty-fifty what happens?

I think my stat has broken. We heat our water overnight on economy
seven and this morning the water is scalding hot.

Our cylinder has two horizontally mounted immersion heaters: one at
the bottom and one two-thirds of the way up. Both immersion heaters
have the same writing on their covers, so I think they are identical.

I have isolated the electricity and removed the covers. If I turn the
dial on the thermostat of the top heater, I can hear it click on and
off as I vary the setting.

If I change the setting on the bottom thermostat it never clicks. Is
the absence of the click proof it is broken?

I didn't think to measure the resistance of the stat whilst it was hot
and in the cylinder and at the moment it out of the cylinder and cold.
I will put it back into the cylinder and then measure the resistance.

I know I can buy replacement stats but I am unsure which I need. The
writing on the immersion heater says it is a 14 inch heater. Looking
on plumbers' merchants' web sites it appears that 14 inch heaters have
seven inch thermostats but this one measures 11 or 12 inches (do you
measure the rod or the whole length?). So should I buy an 11 inch one?

The heater says "Backer maxistore anti corrosive, type 314c 825,
355mm, 14 inch, 3kW"

How long do the stats normally last. Is it worth buying a spare or
shall I wait for one to blow first?

Whilst browsing these web sites I see some heaters are for
"aggressive" water. Does that just mean hard water? At what point is
water classed as "aggressive"? This is just me wondering, I'm hoping
just to change the stat as the heater obviously works and draining the
cylinder to change a heater sounds daunting. I've also heard cylinders
can be damaged if you cross thread the heater.

I hadn't paid too much attention to immersion heaters until today. I
hadn't realised they are all 3 kW; I thought the larger ones were
higher rated. I realise that the 3kW limit is probably due to cable
ratings but if we can run bigger cables to cookers and electric
showers, why isn't it common practice to use bigger cables to
immersion heaters and use more powerful heaters? This would allow the
cylinder to heat up faster.

I had not realised that the top heater is only used for boost and the
bottom heater only used for overnight. I always thought they came on
together. I realise this is due to the cable again but it does seem
silly having a top heater that is never used.

The heaters are controlled by something like this:
http://www.screwfix.com/prods/44419/...rtz-Timeswitch

It must have a battery inside because it ticks away even when he power
is switched off. I always thought this seemed over the top but I
realise now that it must have to switch between the two heaters. I
presume it would not let you use boost heater at the same time as the
economy seven one.

Is there a reason that butyl cables are used with immersion heaters? I
know they have to be heat resistant but why not heat resistant PVC? I
always find PVC easier to cut.

Thanks,
Stephen.