JohnMo

First you need to really understand your heating load at design temperature, so for -3 degs. Your heat load at 7 degs will be half that and at 10 under half. So say it's a 6kW design load, you want to choose a heat pump that does close to or a little over (allow for DHW heating and defrosting) at -3 at your design flow flow temperature. Manufacturer nameplate rating are pretty meaningless. So you need to find the technical data. A few snippets from a 6kW Hiaer R290. So -2 full load it puts out 6.4kW at 30 Deg flow temp and at 45 it's 5.66kW, so if you have radiators it's too small but ok for UFH. It's min output at 25 flow and 12 degs outside, it's min modulation is 1.7kW. pretty much would tick away all day every day. ScoP should be about 5.0 if UFH and an open system on WC, so heat pump doesn't cycle.

Or big enough for a cosy tariff and charge 3x

Could also be old fuel. Unless new dump the fuel and our in new

May have a winter summer switch, to change the carb settings. In winter it could be running really lean.

3 phase (still no idea why anyone needs it in a domestic house) allows 16A per phase export on G98. So about 3.6kW X3. G98 you tell DNO after you start generating. But how you manage the phases, to make best use of the solar is something I have looked into. As most the house will only make use of a single phase.

Sounds about right, plus you delete a standing charge, so win win.

PV, battery and inverter can be confusing. So looking at battery first and how it powers the house. You need an inverter big enough so it passes enough kW to house so you don't draw too much from the grid. So for example if you have a 3kW inverter, every time you put the kettle on you could be drawing peak electricity in to cover base load, Same when cooking your tea. We have a 6kW and that suits our life style well enough. Then there's the question of DC or AC coupled battery. DC coupled generally will only supply a dedicated circuit in a power cut, AC can/will provide whole house. Then there's the panels and there relationship with the inverter, you can over clock (way more kWp than the kW rating of inverter), 90% of the year you loose nothing, a good summer day when you are producing bucket loads of energy the inverter will clip this back. So loads to think about. First MCS install or self install? G98 export limit (3.76kW) or unlimited or something in-between? Do you have power cuts?

There are plenty of really good performance ASHP out there now. They don't cost a fortune either. Around £2500 is about all you need to spend, get through the grant while they are still about, or self install. You already have the cylinder, so a small amount of plumbing changes, a bit of electrical work, job done. Tell your gas supplier to take meter away.

If you are doing MVHR you really need an airtightness better than 3. Otherwise there's a good chance you are wasting energy not saving it. If you are not aiming for level of airtightness there are better or more effective ways to ventilate. There is no reason to have any vents in windows or anywhere else and still be fully compliant with building regs if you are using MVHR.

I migrated from boiler to ASHP, mainly to do cooling, then ran it hybrid in winter, with gas boiler. Reason, heating with ASHP in mild weather is so much cheaper than gas. Did the sums after a year in hybrid mode and realised the boiler was actually costing me over a £100 each year, just having it connected. It also saved nothing in energy terms. Now full time ASHP and no gas, have left the gas boiler in place, and if required I could always have the meter reinstated. But my ASHP is oversized, so am actually looking to replace with a smaller one.

Big assumption there, it certainly may not be airtight - unless it was tested to show it is.

I'm in the process, of really considering replacing my 6kW ASHP, that doesn't modulate well with a 4kW one, that does modulate well. Will then either repurpose the 6kW ASHP, as a hot tub heater (via a plate exchanger), or sell it on to someone with a suitable heat loss.

Should of added if low temp system and/or you do cooling you also add biocide and chemistry should match your inhibitor.

If it's not a building of historical interest (listed) and you are doing insulation internally and externally, not sure why anyone needs to be involved. Photograph everything good for future reference. Only thing other than the above, if the wall is currently breathable keep it that way, so add mineral wool wood fibre etc not PIR. I would be adding another 150mm on top of that.

I used a blunt fine tooth wood handsaw. But also a plunge saw or bench makes light work of it. You can also get specialised insulation saws.

But you also need to buy the correct grade for appropriate compression strength. Not all are equal. Not sure I get that, why is it a pain? I have cut PIR and EPS,both are equally a pain or easy.

And don't trust your builder to make engineering decisions - he isn't qualified

Are you sure your maths is correct? Have another coffee and retry? You would need to use your mean UFH flow temperature, not room temperature for the heat loss downwards. That would be around 24 to 25 averaged across the heating season

Don't you have a design from either the structural engineer or architect or both? You normally have a stepped strip foundation, but this is normally detailed on your drawings.

Only thing I would add, watch modulation levels especially at or around 7 to 10 degs. If you have a unit that doesn't modulate well at this temperature your overall SCoP will suffer. Even healthy cycling has a direct impact on achievable CoP, due to the impact of standby energy usage. The lower the overall heat loss the bigger the impact on CoP. So if you have a design heat loss of say 3kW at -3, you want a heat pump that at 7 degs has a min output of 1.5kW or lower, so it can tick away all day if it wants.

What will achieve?

You don't but the high number is an instantaneous air flow rate for intermittent fans. Or maybe a house leakage rate at 50Pa, which isn't the actual air movement through a building. You can have good controlled ventilation without MVHR. MVHR is the icing on the cake, when you have done, wall, floor and roof insulation to death, triple glazed and made house airtight. Most houses don't need it, don't benefit from it. But they do benefit from a well thought out and appropriate ventilation system.

You seem dead set on two units, so you have a few options. Do ground floor in isolation with ASHP, but unlikely to get any grants, as you haven't covered upstairs heating demand. Add upstairs later when and if permitted development rules change to two units and they also allow cooling under permitted development. Get your noise survey stuff done and press on with planning permission, sounds expensive and they may still say no. Or change to something different, talk to other installers, there are plenty of water based ducted fan coil systems out there,to allow one outside unit.

You can already get it it's called epoxy resin. But plastic sheet is way cheaper. Can remember my dad doing a house up in the 70s, it had some pumped under the ground floor to just below floor level, black but I think that was bitumen based.

Unless your actually pretty much airtight I read wouldn't. What are your current fans in wetrooms? You just need to promote a cross flow ventilation from dry rooms to wet. It doesn't need to be howling gail just a gentle flow. You also need to make sure you can close doors by have a small ventilation gap at the bottom of all doors. What I have written many times Greenwood CV2 or CV3 fans in all wetrooms including kitchen. They run all the time at a very low rate, they are almost silent. They automatically visit on rising humidity. Automatic trickle vents that respond to humidity in all dry rooms. Seal all trickle vents in wet rooms. Undercut all doors. You then have a system that automatically responds to where people are in the building minimising ventilation. Easy to retro fit, pretty cheap implement, zero maintenance.