JohnMo

If you add the same make of inverter and they can operate as master and slave, so the master inverter has all control you should retain G98 certification. But this will be dependent on makes and models etc. The master is what allow stuff to to flow to the grid, slave just does as it's told.

Are you on the correct tariff would be my first question. If you have a battery maybe, without suspect not. Do you have a timer for DHW or is that for CH as well?

As you have thermostats and a buffer For now just go a fixed flow temp to see what temp you need to get FF output correct, start at 35. If your downstairs thermostat(s) switch of the HP, set all but one to the temp you want, leave one calling for heat (maybe turn down the flow to that loop).

What temperature were you flowing at and how long did you run the system? The divide by 40 is just a simplified version of the mass flow calculation. What do you mean by "got was the same flow and return temp on the manifold" were both the flow and return exactly the same going in and coming out i.e. 25 in and 25 out?

At those spacing and if well insulated 25 to 28 will be all you need to flow at. So do have mixer and pump as you are running an oil boiler? Not sure what you mean? 80m loop divided by 40 is 2L/min. You set the flow meter for that loop at 2 etc. Start with the loops at the calculated flow rate. If all open loop you then have to tune the system. If rooms are to warm decrease the loop flow rate, if to cool increase flow rate.

This took 4 weeks, two of us never done it before (working 8 hrs days in Dec in NE Scotland), 70m perimeter, up to 3.5m high at front. Then there was a break over Christmas and new year and roof materials arrived 12th Jan First week of Feb roof on. If you exclude the Christmas break, 9 weeks, not sure where the savings of 3 months come from.

No idea, no idea if the house size etc and how many people live there. MVHR you really need an airtightness of around 3 or your wasting money. Worse than that I would only do dMEV or MEV based humidity control. After moving away from intermittent fans, that are noisy and perform rubbish, wouldn't go back to them. A well thought out MEV or dMEV will yield lowest lowest ventilation heat loss and best control of humidity, with almost no running costs. So humidity controlled inlets in dry rooms, humidity driven fan from a MEV or dMEV system, all doors under to allow ventilation cross flow with doors closed.

I would Self install the PV Pay octopus to do self installation export, which is £250. Then not worry if you are using or exporting. You need an electrical.install certificate, as well as G98 etc, you may need a structural certificate depending how you mount the panels. Keep it all simple.

This does as well https://coolenergyshop.com/products/cool-energy-solar-power-diverter?srsltid=AfmBOoqhPEOh5honAnsCyw0EYcVA8sIS74Zkpt_ENnLL5pn1vo1dzyRM Most don't do two outputs Spending big on diverters is a waste of money, so buy carefully. Pay back can be huge, especially if spending £400 on an Eddi. If you are thinking about a battery, a diverter make no sense and can it other ways more efficiently.

We had similar for our roof, no materials would be delivered until full material payment had been received. We agreed I would pay when the lorry arrived and before it entered the gates. A bit of back and forth, this was agreed upon by all parties. It all worked that, lorry arrived, I did a bank transfer, they check and released the lorry to be unloaded. Push back, say install balance will not be given until install has started and agree a payment schedule. Buying things is a two way street, you have just as much control, you have the money. If you don't get what you want move to another supplier, their loss, generally a few tens of thousands. My windows and doors were all paid on completion of works, except a 10% deposit.

You always have a reserve you can't touch, called depth of charge, my battery is rated at 13.5kWh and has a depth of charge of 100%, so in reality the actual battery is slightly bigger than that. So you need to look at the data sheet and see what you are actually getting, Bit like heat pumps, battery rating isn't always black and white

You have to idiot proof builds, and do what's done locally to great extent or be prepared to do it yourself. Would be my view. Around us it's timber frame kit and brickwork, which I didn't want, so went wood crete ICF and did myself. No walls collapsed or any blow outs encountered.

With enthalpy heat exchanger it should be at a steady 50% humidity. So are you sure it's plumbed correctly - i.e. all extracts to extract manifold and to correct nozzle on MVHR? Go around each terminal 2 sheets of toilet roll. Extracts should hold up the toilet roll, supplies will not. Confirm every one is correct and report back.

Do a factory reset on them first, then a recalibrate on them (it may just be a recalibrate you can do - haven't seen the instructions for a long time). That may kick them into life.

Only up to either a CT clamp on export says no or the inverters limits (e.g. G98) say no Not sure that would work, as the heater would be hot all the time, so hot 24/7 - is that want you want? You have a heat pump, assume that is set up to do the heating/DHW. I would heat DHW to the highest setting the heat pump allows at lunch time each day and also boost central heating around the DHW heating to coincide with peak generation - around 11am to 2pm for both jobs each day. Anything else dump to immersion. Don't get carried away with array size, otherwise you just have a job managing it and it will be wasted. Look at it as a back filler for your normal loads only. Anything above G98 limits can be full time job in summer.