Dillsue

Does the HP always start up in heating mode before it switches to cooling so at the start of every cycle they'll be a short period of heating.....circulating pump running to establish flow is always done in heating mode??

Don't think you were napping....... MIs were an option til Simon mentioned connecting to an inverter

If you're looking to hook up panels to the inverters second MPPT dont use micro inverters as they push out AC whereas your inverter is needing a DC input. Optimisers is what you want if you're connecting to the MPPT input. I don't know how much Tigo optimisers are but I beleive you can use solaredge optimisers with any inverter if you turn off the safe shutdown function with a Solaredge optimiser Key. You can pick up SE optimisers off eBay very cheaply if youre prepared to wait. Last time I looked the Key is about £80

Does a mounting system need to be MCS certified? The only thing listed on our MCS cert from a 2015 FIT system is the panels themselves, not even the inverter is listed let alone any mounting.

I think there's lots of inverters that allow 100% over sizing of the DC input compared to the inverter rating....our Solaredge certainly does. If you're looking to max out year round generation then maybe look for inverters that will allow more than 46% oversizing??

Anyone can put a G99 application in...you, designer, installer.

That's my understanding of how the DNOs operate. Our second system, sat in parallel to an older FIT system, had 8.5kw of panels but SPEN were only interested in the additional inverters rating that was much lower than the panels rating.

The inverter is what connects to the grid so DNOs would want to and do regulate inverters and any generator connected to their system. Where have you heard DNOs are interested in panel capacity??

If you've got 2 single phase supplies to the HP then you can use a single 4 pole isolator. I ran our data/control cables through the wall tie wrapped out of sight to the outside of the return pipe insulation. The mains supply came via a surface mounted SWA cable

How is any other cabling being routed in and around the insulation??

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.