Recommendation for Immersion Stat for thermal store

[PeteAtRaspberry]

Hello
I have an unvented thermal store, supplied by an oil boiler, wood stove, solar panels (when I can afford to have them installed) and an immersion heater. The immersion is powered from excess electricity from photo-voltaic panels, so it is FREE
The immersion is fitted with (of course) a Becker 7" stat preset to 65C. This is fine, but as the year progresses it cuts out pretty quickly. The oil fired boiler is set to about 70 C, the lowest the plumber said was viable (and to be honest, it would have been better if it was a bit hotter in the middle of winter). So I am balancing the hot water and heating demand to try and keep the store cold in the morning, so the immersion heats it up as much as possible during the day. Otherwise, the boiler heats it to 70C and the immersion does nothing even if its sunny all day.
Obviously I want to get the most heat out of the FREE immersion to save on burning oil. What I think it needs is a stat set to (say) 75 or 80C, to allow the immersion to do more work and so save me money on burning oil. Then there would be less balancing, and fewer cold showers when I get it wrong! Even when I get the solar panels, the immersion would be 'topping up' the solar making it more useful for longer in the year.

Can anyone recommend a 7" stat, including a re-settable overheat cutout, which is adjustable up to this sort of range? Do such things exist? Can you get separate re-settable overheat trips which I could put further up the store (probably on the vent pipe at the top) and use an adjustable 7" stat? Is all this against the required standards anyway?

All help gratefully received.
-Pete

 

[flameport]

If the objective is to have the immersion heater as the priority source of heat, then all that is required is a relay connected to the immersion supply which disables the oil boiler.
You then get:
Immersion on = boiler off
Immersion off = boiler can be on if required.

How exactly is the wood stove connected to this unvented cylinder?

 

[Norcon]

It's probably a sealed thermal store with expansion vessel.
Not an unvented cylinder.
Could still boil and blow though if no heat leak rad fitted!

 

[PeteAtRaspberry]

Sorry, meant to say vented thermal store - my bad.
Wood stove is directly coupled, it sits underneath (floor below). The heat leak is done from a stat mounted on the vent pipe at the top which drives the (downstairs) central heating, and there is (of course) the vent pipe into the cold feed (which won't melt).

Relay isn't going to do it I'm afraid - the power isn't full mains, it's only the 'spare' from the PV panels. The problem I have is that there is more than enough energy to heat the store (the top 2/3 anyway) to 80C, but the immersion turns off at 65C. The oil then gets burned at the end of the day when it's timer comes on, to ensure that we have enough heat for the rads and hot water.

Does this makes sense?

Thanks
Pete

 

[PeteAtRaspberry]

Also, the wood stove has it's own stat to damp the air supply, so it struggles to heat the water over 80C even on full tilt. Not that I would ever leave it going any more than tick-over if I wasn't at home anyway.
Not worried about overheat as things stand - but as I understand it, the immersion heater regs require a separate not auto-reset overheat sensor to prevent boiling if the stat on the immersion fails. Even with only using the spare PV power, a failure here could cause the main overheat to kick in, which seems wrong.
-Pete

 

[flameport]

Sorry, meant to say vented thermal store - my bad.

That's fine - if it was unvented it would be fairly likely to explode if the wood burner overheated it.

Relay isn't going to do it I'm afraid - the power isn't full mains, it's only the 'spare' from the PV panels.

What exactly is that supposed to mean?

You are unlikely to find an immersion thermostat which will go up to 80, as heating water to that temperature would be unsafe in a normal cylinder.

 

[Norcon]

Sunvic VKL 2000 or even higher VK4000.

 

[PeteAtRaspberry]

Norcon - Thanks very much, that looks perfect.

flameport - I have a 'variable' power diverter fitted, which will alter the mains waveform into the immersion to supply with reduced (FREE!!) power.

Thanks everyone for reading / commenting; I have been searching for ages and I really appreciate the time and effort you put in to helping people like me.

Thanks again
-Pete

 

[flameport]

flameport - I have a 'variable' power diverter fitted, which will alter the mains waveform into the immersion to supply with reduced (FREE!!) power.

In that case, connect the relay to the unmodulated output on the controller, most have an output designed for this exact purpose or to operate destratification pumps etc.