immersion heaters

[Stephen]

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:

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.

[John]

> It must have a battery inside because it ticks away even when he power

Sounds like the contacts have welded themselves together.

If you don't use a lot of water then the top one can be used on its own to just heat some of the water.

As the heaters are horizontal the length of the thermostat isn't important to the way it operates.

I am sure more specific responses will soon follow.

[dennis]

The top heater is very good if you want a limited supply of hot water as it doesn't heat the entire tank. An example would be for a shower rather than a bath.

longer/bottom heater will be used at night on economy 7 (if it still exists) to heat the whole tank. If you run low you use the top one to heat a bit at the top on full rate electricity.

If you don't have economy rate electricity use the top one only for anything that doesn't require lots of hot water, or get a gas heater.

[<me9]

Also in two cases, boils the water in the (poorly mounted) header tank, which distorts, tipping the water onto anyone sleeping in the rooms below. That's why the cut out thermostat is now needed.

[NT]

sounds like it.

depends on the water temp. If the water's ice cold, no, if its scalding no, if its somewhere between the 2, yes.

you already know it failed

When replacing a heater you should have the tank full initially, once its loose then turn tap on and water supply of to drain it.

they arent

a) no need b) more cost

nothing to do with cable. The top one heats a smaller volume of water, so it gives you hot water quicker. Bottom one is run on E7, top one is used during the day if needed for a topup.

NT

[Stephen]

But don't they drain from the top? I thought cold water went in the bottom and hot water came out the top?

There's both here:

[Bob Minchin]

on a hot tap will not drain the tank. Turn on or opening the drain c*ck will. There is no need to remove the element to change the thermostat. If you are not using the top heater then why not swap the stats over for now and get a replacement at your leisure. £6-7 from toolstation and you are bound to find something else they sell to get to the £10 post free threshold. Maybe even buy 2 to get the protection of the new type on both your heaters?

Bob

[Stephen]

That's what I thought. Our posts must have crossed. I presume the drain c*ck is on the cold fill pipe? I'm not sure that mine has one. Even if it does, the pipe goes behind the cylinder, so I can't get to it without demolishing a wall ;(

I know. It was just the whole episode got me into thinking about immersion heaters so after my initial questions about thermostats, I started musing about everything else!

Good idea but I've already ordered two.

Great minds think a like and all that. I don't use the boost function there's always plenty of hot water left at the end of each day from the overnight warm, but even so, I thought for the cost of a couple of pounds, I would replace both so that both have the new safety cut out.

Thanks again. Stephen.

[Stephen]

Why are they so short relative to the length of the heater? Is it so they measure the temperature of the hotter water nearer the top of the heater, rather than the colder water below?

[Stephen]

We always have plenty of HW left at the end of the day. Perhaps I should use the top one on economy seven to just warm half a tank?!

[dennis]

If there is enough it will save you money. You can swap the wires and try it out.

[NT]

'heat resistant' only means its to some degree more heat resistant than the more common pvc, that doesnt make it comparable to high temp cables such as butyl rubber.

Then there's also the q of what happens when temp rating is exceeded, rubber and pvc behave somewhat differently.

NT

[NT]

There's to some extent differing meanings to 'high temp' in this situation. PVC flex can be rated to 90C, but running 13A through it _and_ connecting it to 70C heater connections will likely overheat it. The butyl rubber leads used on immersions have much thicker conductors to minimise self heating, and enable them to function ok when connected to a hot heater.

NT

[Stephen]

I'm happy to admit I may be confused because I am certainly not an expert but shouldn't TLC know better? Why are they advertising it as immersion cable if it would be dangerous to do so?

What happens to butyl and PVC cables when they overheat?

[Stephen]

Hello again,

Just to say that I have received two thermostats from BES. I believe they are made by Banico. If I have a slight grumble it is that only

60C is marked on and the other temperatures are "max" and "min", whatever they are. Also these are marked white on white, so they are difficult to see. OTOH I suppose you only have to set it once.

When I changed the bottom thermostat there was some fine white powder inside the immersion heater. Whatever could that have been? There was no powder in the otherwise identical top heater.

The head of the slotted nut that retains the cover on the one heater is a bit chewed up, making it difficult to grip with a screwdriver. Any suggestions on how to grind a new slot into it?

The new thermostats have a small metal disc on the rear, which I presume is the safety thermal cut out. Should this be touching the cylinder, because I'm afraid they don't in my heaters?

Thanks again, Stephen.

[NT]

What makes you think their immersion heater cable may be dangerous?

Short term pvc goes very soft, conductors migrate through it, and fires tend to happen. Long term is hardens and falls apart Rubber suffers no ill effect short term, long term it hardens and breaks.

NT

[Stephen]

Sorry, I thought you said in your earlier answer that even a 90 degree rated PVC cable might overheat when connected to an immersion heater, implying that PVC should not be used for this purpose?

So rubber has benefits in the short term but long term prospects are bad with either?

BTW, how accurate are the thermostats? If set to 60C will I get 60C, or 55-65C, or 50-70C, etc? I know I am supposed to set it to 60C to kill all the bugs, will that be high enough?

Thanks again.

[Grimly Curmudgeon]

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember Stephen saying something like:

Clue - Bath and Sink.

[NT]

If youre saying they're selling pvc flex as good for immersion heaters it would be interesting to see.

Of course overloaded or overheated cable has poor prospects, thats what overheated and overloaded mean.

Last time I looked at specs for one it had huge hysteresis.

NT

[Stephen]

This is what they say: "General Purpose - Pvc Heat Resistant Cables Application: Immersion heaters, Storage Heaters, Radiant fires and longer runs of low voltage lighting. Other applications that require flexible heat resistant cable Not be used in areas where the cable may come into contact with grease or oil. Current Rating: 16 Amps"

Sorry, I knew that, what I meant to say was the rubber has the advantage of being safer in short term situations but you are quite right, it shouldn't be overloaded in the first place.

I'm best to set it to 70C then just to be sure.