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If the boiler is still firing up doing the tests you run through in post #1 and the circ pump runs but no firing when you select either CH or HW then its a boiler service/heating engineer you require. Can you use a multimeter?

Do you get a good flow if you turn the temperature setting to fully cold and is the cold water mains supplied.

Is that boiler a system boiler with a internal circulation pump or a heat only boiler with a external pump? and is it a sealed system or a a gravity system with a small feed and expansion cistern in the attic?.

4.9K ohms maybe?, my Triton solenoid resistance is ~ 3.6K ohms.

That's right but sometimes a high head external circ pump can satisfy a greater number of rads than a lower head internal one without having to install a LLH and a extra circ pump, also most system boilers have a internal bypass that limits the head to a max of 4M or so.

They both provide CH only but a system boiler has a internal circulation pump and a heat only boiler has a external pump. Some manufacturers like Vaillant label heat only pumps starting with a 4, a system boiler with a 6 and a combi boiler with a 8, a 30kw heat only boiler will be a "430", a...

The circ pump apparently is starting, I thought the Viessman Vitodens 100W would be a system boiler and not a heat only?

If you are saying that the boiler doesn't fire up either when the roomstat is turned up then looks like a boiler fault, is the circ pump starting as well if you request CH only??

Your cylinder is 580mm diameter so that length element, 14ins, 356mm say 300mm effective length is heating ~ 130L, Ok as a back up which I presume is what tou will be using it (them) for, I think the UK gas cost is capped at ~ 7.5P/kwh so assuming a minimum boiler efficiency of 80% means that...

Possibly because its one of the standard length elements, did you measure the actual length of this one and the other one?.

Heated to 80C only with the solar (themal) panels, it increases the cylinder HW capacity "thermally".

Yes, I think using both immersions should give more uniform heating but can't see it helping much with heating the water below the bottom of the elements, the problem with top mounted immersions on any type of cylinder is that the heat emitted per unit length of the element is practically the...

Where are those two immersion stats located?????, The immersion heating element will only heat directly as far down as the bottom of the element, eventually in will slowly heat up the remaining layers of water below it due mainly to conduction or whatever so the cylinder stat will rarely if...

Yes, thats the whole point of them I think, I've seen a schematic for a Drayton MPV (MoMo) and it showed 4 limit switches in the actuator which switches the power off once any desired position is achieved. Here is the patent for the MoMo type...

This is what you should have, with top mounted Immersion as standard, don't see blank for the second immersion. It gives a reheat time of 35 minutes using the indirect coil, this means the coil is rated at 24.5kw but this would probably be at a 80C boiler flow temperature which you are unlikely...

This is from Sunvic, be nice the the internal limit switches etc. It also states that the Sunvic SDMV 2304 MoMO 22mm Mid Posn VLV has spring return so how does it hold position without any power? This is the wiring diagram for the mid position valve (have deleted the others).

You have a heating coil in the cylinder as well?, can you post a link to that cylinder, you generally don't have both immersion and boiler heating on together, the immersion is normally be a back up for the boiler (normally cheaper) heating or maybe just used during the summer months. If your...

A old (very) electrican friend of mine did this many years ago, he has passed away but the 3 port is still the original.

Another interesting link. https://electrical-engineering-portal.com/methods-braking-motor

How long was the cylinder heating on for?, with one or two elements?. Is the Thermowatt the normal standard supply and how is the other element wired in the system/. The element(s) is easy to check out, a 3kw will have a resistance of either 19.2ohms (240V) or 17.6ohms (230V).