NMEA

You should at least give it a chance by including a bathroom towel radiator. Then take great care in the length of time it runs for so as to prevent cycling as much as possible. Main problem you will encounter is with cold water it will rip thermo from the coolant and the heater will run normally but as it increases and removes few therms from the coolant so the returning temperature will increase quickly due to the above mentioned factor excasobated by the low coolant volume. So in short, keep the running time as short as possible and expect more frequent decokes.

You are assuming low voltage AC, "conduit wire" or single core multi strand is more often used in extra low voltage DC installations, particularly switches, where the load is one of closely grouped set and has its return via a close by bus bar and single heavy cable back to negative serving a number of devices. In fact this is how I often lay out certain applications on new fit outs and refits. This is particularly useful on nav stations which are rarely found on narowboats but there are many instances where it is useful.

No, conduit wire is single core but on a boat would still be multi strand.

Depends, if you are for instance using three core then it would be fine for ISO BS EN 13297 and RCD but not if you were using conduit wire, it's the sheath that makes the difference

Perfectly normal on many seagoing vessels with twin or even triple coil calorifier.

It's what doesn't happen, internal corrosion of the passages which incidentally is a cause of leaks. I doubt most people change the coolant or at least top up the inhibitor every couple of years.

I would at least get some inhibitor in there as soon as possible.

The error codes should be printed on the control box, is the flashing light you are looking at on the switch or on the control box? It's the on inside the control box that is the error code not the one on the switch. If you pm me an email address I will send you a copy of the factory manual.

Doesn't really matter as all fittings are available in 1" or 25mm

This is what we use, they work well enough and are very easy to service as they will inevitably need to be http://www.aquafax.co.uk/html/product_specification.asp?ID=8924

19mm does seem rather small for sanitation use even as a breather, we would noally use 1 1/2" but 19mm NRVs are available.

Personally if it's a new install I would call back the installer and have an NRV fitted as close to the pan as practical. Minor though noticeable back flow is a perennial problem when there is any head of effluent above the pan but it is easily solved and the reason we always fit an NRV in such scenarios so we are not called back to rectify it.

Joker valves get calcified and don't close fully which allows smells back up. A little acid treatment to remove the calcified deposits works well.

The joker valves are not really up to coping with any head, we normally install an extra NRV if the tank is above the lavatory pan.

Is the lavvy below the holding tank?

I certainly would not the 12v rating is for insulation regardless of amps. You can get a 12v switched 240v relay. I stock them use them often.

In rely to the header question, then Fusion are in my opinion the most suited to use o a boat.

Because the use of solder in low voltage AC is forbidden by all the marine relevant regs, and for very good reasons like solder creep, potential hard spots etc. etc.

The important stuff like tensile strength, stranding, insulation voltage rating is specified as is appropriate chemical resistance for specified locations, would have been simpler to have just required "cable to ISO????" but it must be remembered none of these documents are written for amateur use.

Here you go, available in retail quantity here https://www.edwardes.co.uk/en/categories/2-5mm-3183tq-butyl-rubber-heat-resistant--24-amps-maximum- Low voltage AC is obviously less affected by voltage drop caused by corrosion resistance than extra low voltage DC but I still think it is the right stuff to use on a boat.

Split is much easier to use for a continual long run, but if you go down tht route be sure to get the insertion tool, its only a couple of quid and makes life so much easier. https://www.amazon.co.uk/Opening-6-15mm-Conduit-Sleeving-Applicator/dp/B01I5DNPM8/ref=sr_1_1?ie=UTF8&qid=1490948765&sr=8-1&keywords=split+conduit+tool

Indeed.

Depends, many Webasto pro's including me have used closed systems for some time instead of a header tank, in which case there will be a PRV, usually as part of the filling loop / expansion vessel assembly, oranges are not the only fruit.

Indeed I do, I have some left but it is not easily available, the simplest way to buy it is direct from Oceanflex (made in UK) but that is in considerable bulk, some heat resistant cable intended for immersion element supply is tinned but thats a lottery.

Multi stranded cable (of the correct spec) is mandatory under NMEA, BMEA, RCD and ISOs for low voltage AC and extra low voltage DC, tinned is not mandatory but certainly best practise, the time I spend chasing green copper back to bright on both salt water and inland boats is enough to convince me of that, my stores contain miles of cable and its all tinned, the price premium is not so great when compared to good quality plain stranded copper. The major difficulty is sourcing European insulation colour three core for low voltage AC, but again none of the ISOs or RCD make it mandatory to use those colours, much better if you do though for obvious reasons, US colour is easily available but could confuse. An important point to remember is that these ISO docs are not stand alone, for instance in the extra low voltage ISO there are a further four Normative references to other ISO docs and a further fifteen in the bibliography, costs me a bloody fortune.