Dillsue

If you can't get consent for 8kw then you wont(shouldn't)get approval as your inverter eemingly doesn't have G100 certification to limit export below 8kw. Anything with more than 3.68kw rating and your DNO should always be your first port of call. Your MCS installer shouldn't power up, commission and certify for you without DNO consent. I'd speak to your DNO ASAP and hope you can blag 8kw.

AFAIK you need the system designed and commissioned by an MCS certified outfit to get the grant.

Up until the last few days we'd had 1 day of rain in the last 2 months.....witness the wildfires we've been seeing in the media. The nearest reservoir to us is only half full as we come out of winter. Might be worth being a bit conservative with something we really don't want to run short of??

Just a heads up for what they may offer. Our detailed heat loss cals show a heat loss of 6.5kw at -2. This winter when it was zero outside, we maintained house temp around 19/20 degrees with 4kw of fan heaters running so I'm quite confident the calculated loss is fairly representative at 6.5kw Seeing the Coolenergy offer of a free kit of parts with me sorting the installation I made an enquiry and fell off my chair when they quoted for an 18kw unit. The 18kw unit won't even modulate down to my max heat loss so will spend its life short cycling and putting a significant dent in my bank account. When I shared my detailed calcs with them they stopped talking to me! By all means go down the Coolenergy route but get your heat loss worked out by someone else and don't accept a massively oversized heat pump

Maybe wait till you've agreed an export limit before you decide on the kit you'll use??

Assuming your roof is in fair condition and doesn't need reroofing anyway, there's lots of videos on YouTube showing roof hook installation on slate roofs without a complete strip and relay. I'd be tempted to use an experienced slate roofer to fit the hooks rather than a PV installer, unless they employ a good roofer themselves. Let the PV installer fit rails, panels, wiring etc

Totally agree on the MCS pricing but just giving you the heads up on export constraints. I beleive Octopus also want to see building control sign off so there's another cost to add to the Octopus fee.

Octopus do accept self installed systems but there's hoops to jump through and £ to pay! Have a search on here for discussion but I think consensus seems to be it's typically not worthwhile to bag an export tariff that can be withdrawn at any time.

You mention profit from export. Currently you generally need an MCS certified installer to get paid for export so you may want to get your installer involved in the design/spec to get their buy in.

You don't need to hunt for the rafters as there will be screws/nails in the battens which hold the batten onto the rafters. These will show you the position of the rafters for you to fit the roof hooks. I'd check your battens and slates have been fitted correctly as there's usually space between the top of the felt and underside of the battens to allow ventilation/water run off. Im no roofer but AFAIK the felt usually sags between rafters to form this space or if there's boards below the felt, counter battens are used. If that needs changing get it done before you fit the panels and get the roof hooks fitted while the slates are being laid.

If you're running weather compensation on the heat pump and it's been set up correctly, why do you need Wiser TRVs connected to the heat pump?? The WC should take care of individual room temps via balanced flow through the rads. Connecting the Wiser system to the boiler will control the house temp when the HP tells the boiler to run. If there's a valid reason for the HP to have a signal from the Wiser system then install a relay, as Sharpener suggested. With a 2 pole relay in place the Wiser system will switch the relay and separate contacts from the relay will separately switch the HP and boiler enable/call for heat signals. What has your installer said about the boiler overheating the house??

If you're not sure on the config you want/need, get yourself a secondhand inverter off ebay to get the panels up and running. No pressure to make uninformed decisions and fair chance youll be able to sell it again when you know what you want.

A concept often missed in our quick/cheap fix world

Shouldn't be a problem with the right sized pipes. Remember that your not pumping up to the loft but circulating up there and back down again. The flow up to the loft will be working against gravity but the return back down to ground floor will be working with gravity and the two gravitational "resistances" cancel each other out. All your circulation pump needs to overcome is the restistance of the pipe work and fittings

I know for a fact that at least one DNO has accepted self installed and N/A for qualifications on both G98 and G99 commissioning docs. G98 is a notification and I truly belive any DNO will appreciate receiving them regardless of the installers details

That's good going. I played email ping pong for a few days, then they asked for the detailed calcs which I sent them and they never replied.

Collecting parts at the moment. Once it warms up will swap/fit new radiators and then get the heat pump fitted. We're having a hybrid system with WBS and LPG boiler as backups so aiming to have a slightly undersized HP with the WBS topping the heat up on the very few freezing days we have.

My experience with Cool Energy is they massively oversized our heat pump suggesting a 16kw pump for a 6.5kw heat loss. When I gave them the detailed heat loss calcs for comment, they never replied. There's a recent thread on here where they offered a 10kw pump for a 3kw heat loss. It's likely that sort of oversizing and attendendant high running costs are leading to the bad publicity you're reading.

I think the dilemma is whether you can put self installed which Roger wants to do

It's a G98 notification so export is already happening and an "installer" has already done the work. In theory the only legitimate answers are Building Control or a part P spark but some DNOs do accept self installed;)

The industry(power distribution) is all for best practice and asking for installers details/qualifications sort of drives that but reality is your DNO ain't really interested in what goes on the other side of their cutout/fuse. Thats BC's/part P's domain. For a G98 the system is live so the testing/certification horse has already bolted. For G99 you could be submitting months in advance and not know who your installer will be so quite legit to put N/A or N/K.

It was the cool energy quote at 10kw that I commented on. I had a similar daft quote from them

Possibly less than 75m2 with the loft.

Would need at least a 300mm slab over 75m2 to gobble up 57 ton so not your average build

I'm guessing a HP heating 57 tons of concrete isn't gonna be found in your typical British home??