Wiring a hot water tank with two immersion heaters

[john77]

I've just replaced an old copper hot water cylinder with an unvented system made by UV Gold. The old one was small and only had one immersion heater; the newer one is larger and has two - one near the base and one higher up. This is our only source of hot water. The exisiting supply is 2.5mm T&E so the installer only connected the lower of the immersers. Most of the time this will be fine but when we have visitors I'd like to be able to run both at the same time for rapid reheating of the water after showers etc. I was planning to replace the supply cable with 4mm leading to two fused spurs but was wondering what is the best arrangement. Should I use a junction box to connect the two spurs to the supply cable or run a cable from the first spur box to the second ?

Thanks

John

 

[kai]

Assuming both elements are rated at no more than 3kW each, you really need a pair of independently run radial circuits, from the consumer unit, each with a B16 breaker, wired in a minimum of 2.5.sq.mm cable, to a pair of DP switched fused spurs, with a 13amp fuse in each. Connect one heater to each fused spur, and use accordingly. (MK-K330-WHI) is an ideal spur box to use - it has an easy-fit cord grip for the immersion leads to go.

 

[xr4x4]

You don't need to use a FCU, a 20a double pole switch will be fine.

 

[Taylortwocities]

Indeed, each heater must be on separate circuits.

Normally the bottom one is on the off peak supply but its not a must have, provided you don't mind the electricity bills!

 

[john77]

Thanks everyone for these suggestions, much appreciated.

John

 

[EFLImpudence]

You don't need to use a FCU, a 20a double pole switch will be fine.

I agree with xr4x4 but more than 'don't need' you 'should not' use a FCU.

 

[Taylortwocities]

You don't need to use a FCU, a 20a double pole switch will be fine.

I agree with xr4x4 but more than 'don't need' you 'should not' use a FCU.

Assuming each immersion heater has its own circuit, each with a 16 or 20amp CPD.

 

[EFLImpudence]

Assuming each immersion heater has its own circuit, each with a 16 or 20amp CPD.

Oh yes, I thought we had already established that.

Immersion on 13A fuse/switch is not a good idea so no alternative even if cable were 10mm².

 

[phatboy]

I'm not sure you are supposed to run both elements together. I have one element wired to off-peak, and the other on-peak. There is a BX2000 that ensures only ever 1 element is on.

It shouldn't take too long to re-heat on 1 3KW element, unless you have a huge tank

 

[john77]

Is there a reason why both elements shouldn't be run together ? The tank isn't huge (170 litres I think) but if people are waiting for showers it seems strange not to use them both. I don't have an off-peak supply.

 

[JohnW2]

I'm not sure you are supposed to run both elements together. I have one element wired to off-peak, and the other on-peak. There is a BX2000 that ensures only ever 1 element is on. It shouldn't take too long to re-heat on 1 3KW element, unless you have a huge tank

There's no reason why one cannot run both elements together, provided that they both have satisfactory electricity supplies.

I have such an arrangement, and (like the OP) the main reason I (rarely) have both on is to achieve more rapid re-heating. I don't have a huge cylinder, and I agree that one element will not take 'too long' to heat it (maybe an hour or so), but if we have a house full of people queuing up to have showers (and certainly if any insist on having baths!) their view of 'not too long' may not be the same as yours and mine (particularly when they include teenage daughters and their contemporaries - but I've fortunately grown out of that one now )!

Kind Regards, John

 

[phatboy]

Not sure on if there is a reason. There isnt a huge amount of point though in my opinion.

The bottom element heats water, that water rises to around the second element, causing the thermostat in the second element not coming on, so doing nothing.

You could wire the top one to quickly heat the top quarter of the tank only.

 

[JohnW2]

Not sure on if there is a reason. There isnt a huge amount of point though in my opinion. The bottom element heats water, that water rises to around the second element, causing the thermostat in the second element not coming on, so doing nothing.

As I've said, there's no reason. Water heated by the bottom element will certainly rise, but until it reaches the desired temperature (which will be setting of both element's thermostats), it will not turn the top element off.

You could wire the top one to quickly heat the top quarter of the tank only.

That is the classic use of a top element, and remains very useful for heating a small volume of water during the day, if needed, prior to the whole tank being heated by cheap electricity.

Kind Regards, John.

 

[Dan Robinson]

170 litres from cold with 3kW is gonna take nearly 3 hours to heat up.

 

[JohnW2]

170 litres from cold with 3kW is gonna take nearly 3 hours to heat up.

Probably - but, with both immersions on, one should be able to draw off some pretty hot water long before the entire contents of the cylinder has heated up, since the water heated by the top immersion will tend to stay at the top of the cylinder.

Kind Regards, John.