Help with wiring a MAIN unvented 170 direct cylinder

[scottyboyiow]

Good morning all,

As the subject suggests, we are looking at buying the above water cylinder, to replace our current hot water tank. Having read up on the above cylinder it comes with two 3kW elements, instead of the one element that the old one has. My question is- Why the two elements? At the moment there is 20A supply where the tank has to go, but if both elements need wiring this will obviously need looking at. My thoughts are that the two elements are possibly for an economy 7/10 tariff? However there is no such tariff where the tank is going, only standard tariff. If both elements need wiring, how is this set up? E.g. Off of a 32A MCB into two FCU's? Or two seperate MCB's, one for each element? Hope this all makes sense!

 

[Ian2008]

The two elements are usually one for E7 and one for daytime boost.
Also you will need an installer with G3 qualification to install this for you.
This is not suitable for diy installation.

 

[scottyboyiow]

Thankyou I understand this is not for a DIY install. I thought it would be for a Eco 7 setup, as this is not needed it looks like i just have a back up element. The plumber is installing the tank, just needed to know what the two elements were for, for the electrical side of things. Thanks again.

 

[Taylortwocities]

I am assuming that you will be heating the cylinder from your normal boiler?

If so the immersion would just be for emergency. You could only use one but you have to decide if you want to heat the whole tank (use the bottom one) or the top one (a few sinks full).

But I agree with Ian, the witring of the immersion must be done by a competent person, there are MANDATORY electrical interlocks to consider to prevemnt the cylinder producing boiling water in the event of a fault.

 

[holmslaw]

..

 

[scottyboyiow]

@Taylortwocities - Sorry mate may not have made it clear, but as the subject says, its a direct cylinder so no boiler involved, heated straight from the immersions! I am intrigued as to what these mandatory interlocks are though? As mentioned not bought the thing yet so any info is greatly recieved? As said the plumber is installing the tank but he 'doesn't do the electrics' which is fair enough, so its info on just the electrical side I was after. From what I have gathered so far, and from phoning MAIN technical helpline, its upto the customer whether they wish so wire up both elements or not but they would recommend both, for a couple of reasons:
1. Heats up tank quicker
2. A backup element if one goes down.

What I needed peoples opinions on was whether it was best to have a seperate feed for each element, or the same feed for both. I.E Higer rated MCB, bigger cable but split down into 2 FCU's? Any thoughts are appreciated.

 

[Taylortwocities]

It is not a matter of

1. Heats up tank quicker
2. A backup element if one goes down.

The top element will heat up just the top part of the tank.
The bottom will heat the whole tank and its expensive to have the bottom one running all the time.
Usually the bottom is on an off-peak tariff and the top is just used for a boost.

You need two separate feeds from the consumer unit @ 16 or 20amp.
Keep them separate or there will be problems if on element fails (and they do).

You cannot increase the amperage of an MCB without changing the cable size for a bigger one. What do you think thet the MCB is for?

BUT you must must comply with the regulations for PWVs and you don't even know what they are.

Sorry to be harsh but if you are doing the electrical works for a customer then you shouldn't need to come on a DIY web site for advice.

 

[scottyboyiow]

Oh dear Taylortwocities- Your too eager to do the whole diyer shouldnt do this, shoudnt do that lark and making assumptions, if you actually read my post you will see I am well aware what the MCB does, and know that its to ensure a cable does not exceed the amount of current it can safely carry, hence saying I would need to install a bigger cable. This is not for a customer but for my place of work, hence the 'we are looking at buying' part in my first post. As mentioned there is no ECO tariff, and we are well aware that IF there was it would be cheaper to have an element running on that tariff, however there are no storage heaters, so correct me if I am wrong, but would it not cost us MORE to have an ECO tariff purely for this tank? Considering most electricity is used during the day? I will admit I am not upto date with the regulations for PWV's, however I am hoping that the plumber is, but if the regulations for PWV's somehow effect the way in which one or both elements are wired I would appreciate if you could inform me? I was purely asking for peoples opinions on the best way to wire the elements considering there was NO eco tariff.

I am also pretty sure that considering my situation, and the fact that I could just wire up the bottom element (well aware of the theory that heat rises), would it not be best for the top one to be wired incase the bottom on fails? Obviously you would not get as much hot water but is some not better then none?

Appreciate anyones opinions, but dont try and force the whole diyer thing down my throat as unless you can quote something that I have said that is wrong, all I have been doing is asking for opinions of the BEST way to wire the two elements up based on other peoples experiences. As I am sure that is how people progress and learn?

 

[scottyboyiow]

And please do not role your eyes at me, I find it rather rude

And please read my posts properly before attacking, mucho gracias

 

[ban-all-sheds]

I once saw one totally destroy the equivalent of a 3 bed bungalow, bits of 4x2 and roof tiles were found over 100 feet away.

 

[bernardgreen]

all I have been doing is asking for opinions of the BEST way to wire the two elements

The best and ONLY way to wire is in compliance with the regulations in place to reduce the risk of the tank exploding when the control of the elements fails to remove power when the water is close to boiling.

As it is at "your place of work" the health and safety regulations apply and if an accident happens then you would be in a very difficult situation with claims for loss of property, injury, loss of earnings and compensation being laid against you.

EDIT... the video shows the structural damage, it doesn't mention the scalding injuries that a mass of super heated water will cause to people in the building who will also suffer compressive injuries.

 

[scottyboyiow]

Well the two elements obviously have thermostats on, but is there some other control then? Would you like to guide me to which regs in the 17th ed you refer too? If the vessel starts pressuring and boiling will the cut outs that come installed not cut out and the plumbers bit allow the tank to depressurise?

 

[holmslaw]

..

 

[kai]

If it has two 3kw immersions.
You will need two B16 circuits off the consumer unit, wired in minimum of 2.5.sq.mm cable, to a pair of 13amp DP switched fuse-connection units in the cylinder cupboard, and then 1.5.sq.mm immersion flex from each FCU to each element.
The MK K330WHI is one suitable switch to use - many immersion manufacturers state that a 13amp fuse is required in the feed to their immersions.
Earth bonding of the cylinder must also be adequate.

 

[flameport]

As mentioned there is no ECO tariff, and we are well aware that IF there was it would be cheaper to have an element running on that tariff, however there are no storage heaters, so correct me if I am wrong, but would it not cost us MORE to have an ECO tariff purely for this tank?

If you only have the E7 for the hot water, it is not likely to be worthwhile, as the small amount of electricity used overnight won't justify the additional costs during the day.

However if you have no storage heaters, and no boiler to heat the water either - how is the property heated?