Dillsue

Standard leakage current trip is 30mA so what have you increased it to?

Be conscious that you can have existing leakage current that's below the trip threshold so the protection doesnt trip and it's only a very minor change in the fault, or a small additional fault, that cause the protection to trip. If it only happens with the unit warmer than usual then a breakdown of insulation within cabling/components is quite plausible but difficult to find!! Maybe start with close visual inspection of all the mains cabling and components for any build up of dust/dirt/moisture. If you've got access to working spares then you could try swapping out components but having a spares kit is a bit of a rarity. Therma Vs come up on ebay from time to time for peanuts so that maybe a source but you need to know and trust the reason they are being sold

Don't forget that earth leakage detection and tripping is a secondary safety feature and if Joe bloggs is plugging into system that isn't safe it's not because he doesn't have bi directional earth leakage detection.

The article doesn't really comment on specific safety concerns it just says that PIS doesn't comply with current regs and needs to be safe.

Next to nothing to go wrong if you've done your homework. I guess if your builder is responsible for building regs compliance and sign off then he may be a bit hesitant to hand responsibility to you but if you make it clear he's not responsible for the heating system performance it's hard to see why he'd be concerned. If you put it writing it's a valid change to any contract you have. If he's got a mark up on the heating system then he's potentially a bit out of pocket if the HP installer is out of the picture.

So you need to decide who's taking on responsibility for the systems performance and providing the warranty for that. If you want things done your way then make it clear that you take responsibility. If you want someone to blame if things don't work then you probably need to leave the design and install to the builder/plumber/HP guy. As mentioned above its straight forward to install a HP........if you understand what needs to be done!!

Dont forget everytime you change or add water you're adding to the scaling potential. I'd be tempted to buy a pump that will alow you to return any drained water back into the system rather than adding fresh water.

Given that the changeover switch is wholly cable connected and doesn't need busbar alignment/compatibility with the CU, what hazards would you envisage being on the sparks risk assessment if you used a different manufacturers unit??

I guess the Geya description trumps the symbol on the unit....a 0 printed on the unit would make it clearer. If you are having the changeover switch in a separate enclosure to the CU then having the standard 2 pole main switch in the CU would remove the need for the changeover to have an OFF position?? If you need to pacify your spark by having an OFF position then the Geya or Hager units seem to do that. I'm pretty sure all the units you've posted will fit in standard CUs so it might be neater to have a slightly larger CU and house the changeover switch in that rather than a separate enclosure??

I think that's a 2 position unit? The I and II show the 2 positions

I've got 2 of those changeover switches, one in each of the main house consumer units. They let either of the CUs be fed from the grid or the backup output from the inverter. One is normally fed from the grid, the other normally fed from the inverter backup output. I'm not aware of any regs that prevent that configuration but it's such a niche that I'm not sure those producing the regs will have detailed if and how they should be use??

Ask them for a breakdown so you can see where the money is going

An extra inverter on top of an exiting 4kw unit would need a G99. Might be easier to let your FIT payer know about the extension as thats just a phone call/email??

At 13 degrees your roof will likely be below the level where the panels are self cleaning so you'd probably get best results by setting up an automatic cleaning system that sprays them once a week. You can change out the panels if you want but you'll likely need to extend the mounting rails so factor that in. If the total generation capacity goes over the FIT registered capacity you need to let your FIT payer know and they'll pay you the proportion of generation attributable to the original capacity only. You won't get FIT payments for the extension!

If you want a "ladder" so you can climb the roof, just slide every other tile up in a vertical row. Your tiles look exactly the same as ours and sliding them up and out of the way makes a great way to move around.