Dillsue

My bill says my 15p outgoing rate is guaranteed til June. Maybe the March drop is for new customers??

As to whether you can ultimately heat your stone cottage, have a read of this posted in another thread https://interactive.carbonbrief.org/factcheck/heatpumps/index.html Note the stone church heated with a heat pump

If I've understood your upstairs build you got stud wall rooms built within the original stone walls? If that's correct, then that's your "tent".....the stud wall rooms upstairs. If that's what you've got and the gap between the stud walls and stone walls is open and ventilated to the loft/roof then you'll be loosing heat up that gap....25mm PIR between the studs isn't alot of insulation particularly if it's loose fitted and not sealed into position tight against the plaster board. If you're thinking of sealing this gap to the loft then I would get a professional condensation risk analysis done before you seal it up. At the moment most of the water vapour permeating through the stud walls is vented so has little opportunity to condense on the inside of the stone wall. If you block the gap at the top then you'll loose this ventilation and most of the water vapour will remain in the gap between the stud wall and the stone wall. If this condenses then you could have water running down the inside of the stone wall and running into the joists/ceiling/room below. If you want to insulate the stud wall rooms upstairs then you'll likely need to strip the plasterboard, insulate fully between the studs(70-100mm?), insulate over the studs with another 25/50mm PIR, tape the joints and then plasterboard. Cross posted with @ProDave

For a like for like swap it's no different than a boiler swap and maybe easier as there's no flue?? I don't know what the current installer skills are but when the current install base of HPs start to get to the end of life I'd like to think(hope!) there'd be alot more HP savvy plumbers around. Getting back to the "should you be worried about replacing a failed HP" question, I think the answer is no but be prepared to pay a bit of a premium to get it done by someone who knows what they are doing.

That's a very valid view but it would likely be very easy money for any plumber/spark that had done a bit of skills updating ie understood heat pumps

If you got an expensive installer supplied HP now and it fails there's nothing to stop you changing to different manufacturer if and when it fails. For most situations you likely need to change the controller but there's a fair chance the existing controller cabling would work. A replacement for our 7kw LG Therma V is £2700, so not the end of the world if it throws the towel in.

It likely will, but needs time. As others have suggested you'll need to leave the heating on continuously for probably weeks to warm up what's likely to be 10s or 100s of tons of stone. Only then will you start to feel warm without the walls soaking up all the heat. A quick Google search suggests your walls U values are unlikely to be significantly different to an uninsulated brick cavity wall but the advantage the cavity wall has is that it doesn't have the thermal capacitance of your walls so will heat up quicker. When we first switched our heat pump on last September it took a few days of continuous heating before the temperature stabilised. That's in a reasonably well insulated house but we have alot of masonry inside the the house including an old brick chimney breast. It all takes time to warm up.

Just check the max power available on the back up output will power the "heavier" loads. The Solis unit I'm thinking of using is a max of 5kw on the back up output so would need a bit of manual load management if we wanted to cook in the winter with the HP running

I think the grid disconnect is done within the inverter as it is with G98/G99 so no external changeover/disconnect needed?? In the second schematic the "on grid home load" will be dead in a power cut and the "load" will be live, fed from the batteries or generator. If the inverter can't sense a stable grid connection it keeps itself disconnected with internal contactors?? In terms of installers knowing the requirements for backup power supplies/generators, are they actually covered in the wiring regs to the level that the regs spell it out in black and white and every spark would know exactly how to wire things?? I've always understood it to be a bit of a grey area??

But if there's a neutral to earth bond, as in the schematic, anything the inverter might apply to the neutral line will run to earth?? I appreciate UK regulation/best practice may want the inverter neutral disconnecting from the grid as well as line, but if the neutral is earthed surely there's no hazard??

Wanting to understand more about this, what is the risk to the DNOs linesmen if the neutral is bonded to earth but the DNOs neutral isn't disconnected from the customers inverter, seemingly as in the schematic??

Its not electrically the same if the DNOs cable gets disconnected/severed which seems to be your concern?? Outside of your house isn't under your control but inside your house is, so you can ensure that link remains. If your inverter meets UK grid regs then there will be people at Growatt that understand the UK grid so should be able to clarify things for you??

As both the house loads and the inverter are connected to both the earthbar and neutral bars in the schematic, I take it that Growatt expect that E-N link to be within your house. It's a poor schematic if that's depicting the link to be out in the grid somewhere. Probably worth asking Growatt for clarification??

No idea about long term longevity but ours has been in since 2020 and works fine. The mat we used is self adhesive so sticks to the tanking membrane and then gets buried in a layer of tile adhesive. Tiles are laid in another layer of adhesive sat over the heating mat. Straight forward to install.

Maybe Im unique in the world as we removed a wall together with 2.5metres of kitchen units. If we'd followed the mantra of only piping open spaces, we'd have a big cold patch where the old units were.