Dillsue

That will be a whopping battery to run an ASHP for 24 hours in freezing weather. Not sure it's cost effective to have a battery that size rather than use peak time grid eleccy for the few freezing days a year most people have???

From a quick peak at Google gives the following- 14300 kwh of gas@0.18kg co2/kwh gives 2574kg carbon per year currently 14300 kwh from HP at COP 4 uses 3575 kwh grid eleccy@0.177kg co2/kwh which gives 632kg carbon per year with HP. Google suggests 200-400kg of embodied carbon in boiler manufacturer and install. Assuming HP embodied carbon is the same as a boiler then first year of scrapping boiler and installing HP gives 632+300(boiler)+300(HP)=1232kg co2 for new HP versus 2574kg co2 continuing with gas boiler. Half the emissions in year 1 by switching to HP now. Second and subsequent years is 632kg co2 for HP versus 2574kg co2 continuing with gas boiler. Quarter the emissions in future years after switching to HP. More than happy to have these calcs challenged but seems that moving to a HP beats sticking with a gas boiler from an emissions perspective. OP has significant PV and storage which would likely lower the HP emissions further??

If youve got a near new working system then your primary motivation for change is likely only for environmental reasons?? If your boiler is still running perfectly in 20 years time that would be an awful lot of carbon emitted that you could have avoided if you'd made the change now. I'd bin the boiler now but my measure of ROI isn't just £££. Only you know your measure of ROI.

I'd be wary of tampering(repairing) a revenue grade meter given that a replacement is so cheap. FIT payers have an obligation to monitor compliance with the FIT rules and I'd be surprised if the rules allowed for a certified meter to be repaired and not recertified. A FIT meter is supposed to be inspected every 2 years and if an inspection revealed tampering, you could find your payments stopped or worse still reclaimed back to the previous meter inspection.

50vac or 75vdc for ELV. I assume part P applies above those voltages??

Is that for all DC applications? I get that there's been changes in the last few years over the use of SWA for DC PV and the Ultra PV SWA has been developed to meet the specific requirement but not necessarily for all DC applications??

AFAIK std SWA is rated for DC as the SWA I've seen includes ratings for DC as well as AC. I think the issue with DC PV is that the short circuit current(Isc)is limited by the panels to a value only slightly higher than normal generating current(Imp). This makes it virtually impossible to cost effectively fit fuses or circuit breakers that will detect the difference between Isc and Imp and trip on Isc but not trip on Imp. As you can't then reasonably protect against short circuit between the 2 conductors, double insulation is specified to reduce the chance of a short circuit between the conductors. That's my understanding but happy to hear other takes on it??

With a flow sensor rather than a flow switch I guess yours is a gen 4 HP.... model number ending Ux4? In our earlier gen 3 there's a strainer inside the HP on the return pipe. If thats fitted is it clear? Have you got a magnetic filter in the return line? Ours picks up a bit of black gunk which is clearly magnetic. If youre getting a black coating inside the flow sensor and that coating has a metallic content then that could affect the sensor?? A mag filter is stated as mandatory in some LG documentation so well worth fitting and definitely worth fitting if you've got ferrous components like radiators in the system.

Would the 10% inefficiency represent an oversized heat pump not being able to modulate its output low enough to match a more usual winter heat demand and then start short cycling 24/7?? I appreciate that an oversized heat pump might perform better in cold weather if that's what manufacturers say, but cold weather only makes up a small proportion of the heating season. The rest of the time it's mild and an oversize heat pump could be short cycling for months. Do the manufacturers performance figures cover the situation where demand is less than the minimum modulated output?

We've got a calculated 8kw loss at -2 but installed a 7kw HP on the basis that it very rarely gets to low single figure temps and being undersized means the HP should run more efficiently most of the time. It's dropped to -1 a couple of times in the last month and the HP maintained the temp OK, but it worked hard when it was cold!!! We have back up heating by way of an LPG boiler and a wood burner, but would be happy running a fan heater for a few days if we didn't have the backup and the HP couldn't cope. Remember if you oversize you're stuck with potential inefficiency for the life of the HP

I think you're correct in your views on isolators but if you need a connection in the loft MC4s are the lower risk and in my limited experience far quicker to install and more reliable than screwed terminals, either in a JB or isolator. The only exception to the reliability of terminals would be to use cage clamp terminals where the terminal applies spring pressure continuously on the cable strands so they don't work loose over time. You can certainly get cage clamp terminals but I'm not sure if you can get DC isolators with cage clamp terminations.

This! Only change I'd make is have the MC4s for the last connections on the roof within the loft so they were accessible without the hassle of accessing the roof. If you've got to change either of the last panels in the string then you can easily pull the MC4 through the felt if you've got a tiled roof. I don't know if you can do that with a slate roof so maybe MC4s outside on a slate roof?? If you want to use screw terminals at each end of strings then you'll be cutting off the factory crimped MC4s and then screw terminating them, meaning a JB on the roof or in the loft. This entails 4 screwed connections in the same way as you would for an isolator which apparently is a fire risk? From posts above, MC4s are a lesser fire risk than isolators so those would be a better bet. I think you asked in another topic about crimping which you accepted was doable and from the table above is an approved way of disconnecting a string.

If you have the inverter in a ground floor plant room and that inverter has an integral isolator or there's a free standing DC isolator next to it you've surely met the requirement @Beelbeebub posted without putting a second isolator in the loft. The loft is remote and rarely quickly or easily accessible so the isolator in the loft is going to be unused. If you need to work on the cables in the void, shut the inverter down, switch off the inverters DC isolator so there's no load in the cables then disconnect the MC4s in the loft to isolate the void cables. That's in line with the PV string disconnection method in @Beelbeebubs table. Wait til its dark if your concerned about voltage on the plugs. I really dont see the point in putting an additional isolator tucked away in the loft where it's going to get forgotten about, particularly if it's seen as an unnecessary additional fire risk.

Is that requirement for an isolator as close to the point of entry as possible a mandatory requirement or a recommendation?? On the use of SWA for DC PV I read recently that SWA generally doesn't meet the requirement for DC PV as the regs require double insulation between the conductors which standard SWA doesn't have. I beleive there are some specialist SWA cables available but the common or garden SWA shouldn't be used if you want to meet the regs

Maybe Im missing something but the Isc is likely very close to the Imp so you'd need a very precise fuse/mcb that can differentiate between normal operation and a short circuit. You'll be sizing your PV cable for the Isc so adding fuses/mcbs isn't going protect anything as a short will just take the circuit to Isc unlike a grid powered circuit which can hit very high currents more or less instantaneously and where fuses/mcbs are needed to protect cabling. How you temporarily make the PV cable ends safe depends on whose gonna do the final connection. If it's you and you can connect up when it's dark then put the ends in a JB and move them to the inverter when its dark. If it's an electrician that's likely only going to want to work in the daytime then terminate them in a DC isolator assuming your electrician would be happy terminating in the dead side of an otherwise live isolator....probably best asking if you're using an electrician.