Tempest Unvented Cylinder - Immersion Heater?

[Chris1012]

Evening all,

We have had a Tempest 250l unvented cylinder for just under a year which runs in connection with a boiler.
It is scheduled to come one once a day (6am) for 1hr and it provides the house with hot water for the rest of the day.

I have read on some forums it may be cheaper to run via immersion if kept on 24hrs.

Cannot see how that is possible but I wanted to test the immersion as a backup.
I currently have it have switched on the power for the immersion within the cylinder for the past 4 hrs and since them the boiler remains on.
The boiler gas light is on however it doesn't actually go into full mode just humms constantly.
Is this normal as I assumed with the immersion it simply required electricity and nothing else?

 

[ShaunCorbs]

Need to turn the boiler off / hw program else your robbing heat out of it with the pump running

 

[Chris1012]

Thanks for the response. Why is the boiler still coming on even though I have set the nest hot water program to never come on?
Do I have to change the boiler to completely off as currently it is on the hotwater/rad sign but even when I just changed it to hot water only, the rad still comes on.

It seems currently the boiler is constantly on and ever so often it will go into full on mode to heat up the cylinder temporarily?

 

[Brambles]

Always best to do the maths first. If I assume that you don’t have any solar panels or solar hot water system:

To heat your tank from cold (10 degrees C) to 50 degrees C requires just under 12KW of energy. As an order of magnitude in the UK, electricity costs 12p/kw and gas 3.5p/ kw. Check your specific tariff. Whilst the efficiency of an electric element in water is far better than that of indirect heating by gas - that is dwarfed by cost difference in the energy sources.

In simple terms it is probably costing you 2.5 to 3 times as much to heat your water with electricity rather than gas.

If you have solar panels and a feed in tariff, then that is a very different calculation.

As to the issue of your gas boiler coming on to deliver hot water (HW) when the Nest controller is switched to HW off - that is either an incorrect installation issue or a faulty control valve.

 

[Chris1012]

Thank you for your response. As a fix, can I simply leave the boiler switched off?

 

[Brambles]

yes you can, but it is probably a more expensive route to get hot water

 

[chris watkins]

I have read on some forums it may be cheaper to run via immersion if kept on 24hrs.

I don't know what forums you have been reading but clearly it wasn't this one.
Anyone spouting crap like that would have quickly been quite rightly shot down in flames.
Turn off the immersion & stick to timed gas fired hot water heating.