JohnMo

Heat loss rate W is the heat loss. You need a boiler that can provide that to heat the house, worst case. Allow some time for DHW heating you are still only needing less than 7kW. But realistically you will be in the 11 to 20kW range of boilers. It's just important that you get one with a good modulation, if it can go down to about 3kW you will have a good stable operation all year, running a simple WC curve. So you just need the following Boiler (that does PDHW and weather compensation) Get a heat pump cylinder with 3m² heating coil Diverter valve. Ideally size radiators for the same temperature as the UFH (you can then run the whole system from the system boiler circulation pump all at the same temperature. (no UFH mixer or pump needed) A controller from the boiler manufacturer. Run all as a single zone, with trvs on the bedroom radiators only. Your system is then heat pump ready also. But will make the most out of the gas boiler.

So you just need a dinky gas boiler. Find a new plumber. They are just sticking a finger in air and having guess. The difference in a good install and poor one is using twice the gas.

Look at as built or as designed space heat required section. Depending on how accurate your sap is, depends how good the figure is. 4th line down - heat loss rate W. Look for the most onerous month. So that is your ball park heat loss rate.

What really matters is modulation range, if it can go down to 2kW it's suitable for just about any house. If it only returns down to 15kW it maybe too big for most properties. Do a heat loss calculation as linked above. Or read your sap report (full version) and it will have max demand hidden there also. I'm currently running a 32kW boiler in a 3.5kW (at -10) heat demand house. But it turns down to 6kW and is running into a thick screed UFH. It doesn't cycle, runs at 37 degs for hours on end, when asked to do so. I am running simulated weather compensation (boiler thinks it's running WC) but delivers a fixed flow temp that modulates power. If your doing a new boiler, set it up correctly - priority demand hot water (PDHW), and weather compensation. I would do a system boiler not a heat only and get one that manages pump flow rate also. Go Intergas, Atag, Viessmann or a new series of Vaillant which modulate better.

Not exactly rubbish - by any stretch. Can't get anything like that here. You may need a little perspective or dip your hand in your pockets.

Not even sure I would leave them in the house, just take them with you. MCS battery install, doesn't allow you export and get paid for it. Panels yes - battery no.

These are much easier, use multi drill bits, through the wood sole plate and carry on through the screed, everything stays aligned perfectly, drop plug in place and just hammer in place. Unscrew if you need to. https://www.screwfix.com/p/easyfix-nylon-hammer-fixings-8mm-x-120mm-10-pack/19157?tc=TA5&gad_source=1&gclid=CjwKCAiAzPy8BhBoEiwAbnM9O8PjSRZDZU-ltLFwyTVeMzEXZsXobLQs9-BlGsCqgey_2u3J-TayVBoCcjQQAvD_BwE&gclsrc=aw.ds

A cascade system for reference h can be done with any ventilation unit https://www.brinkclimatesystems.nl/solutions/ventilation/multi-air-supply-system

Are you sure? If house volume is 130m² x 2.5 (for ventilation purposes) multiply by 0.3 ACH is total 97m³/h. Or am I missing something? I would really investigate cascade systems for retrofit MVHR. This limits the ducts required hugely. But if you are not nearly airtight MVHR is just waste of time, you actually increase ventilation heat losses. Look into demand controlled dMEV or MEV systems.

I would stay with a system boiler, but run unvented - you will have way better performance from DHW. Get a good system boiler, that comes out the box equipped to run priority demand hot water, Atag, Intergas etc. Read this article https://theintergasshop.co.uk/content/189-why-hot-water-priority-pdhw-is-the-reason-s-and-y-plan-should-be-banned Get heat pump cylinder - NOT one sold for a gas boiler, you get a big coil for really quick reheat times. Then run the central heating on weather compensation or Opentherm, room compensation from a single thermostat ideally one from the same manufacturer as the boiler. Video to watch also https://youtu.be/_UQlGryL3JA?si=uRfZQWWvM6UMAhrE Then sit back, knowing the boiler is running efficiently as it can do and is saving you around 30% in gas consumption.

Hager

Bit of a write up here on the Viessmann and Vaillant defrosting https://community.openenergymonitor.org/t/viessmann-defrost/27441/4

We have them in all bedrooms, BUT only used once as they seem to make slight buzzing noise when something was plugged in, which bugged the life out of me, so stop using them.

They are silent on AFVs but say no to glycol and also state water shall be to VDI 2035. Couple of things with VDI 2035, it is pretty pure water, so maybe less in the water to initiate gelling or solidification of the water. I have had drink outside since Christmas (water, beer, wine) We've been down to -10 with our any breakages or freeze up. It's near the house so maybe not so cold, but still pretty cold. Plenty of good marketing going on, to convince everyone to add glycol or AFVs. 10 out 10 for thinking differently with defrost

I would get a full power flush before you start and assess. Do you have system boiler now? Or is another combi?

You really need to look at the compatibility of oak with metals, especially when wet. Oak can be pretty acidic and will eat carbon steel and low grade stainless, in a dry space you can get away with hot dip galvanized, but in a wet environment the steel work could fail well before the wood does. We did a sleeper retainer wall on our last house, and used vertical sleepers to attach the horizontal one too. 10 years later they all looked as good as new. Our current retaining wall was a little different and a lot bigger - no screws used or metal framing. They are concreted in place.

Why do you need steel, at all?

Had plenty of stuff powder coated in the past, great for car wheels, but not had much luck with other things. Hot dip galvanised will last a life time. Your other issue will be the words retaining wall - meaning it's structural. Unfortunately defects in steel cannot be seen when powder coated. Any defects in the coating created a galvanic corrosion cell and the base material disappears very quickly. All hidden by a thin plastic layer. Either that or just plain hot dip galvanized. If it's on show and you want a colour, etch prime and top coat, after galvanising.

I did posi joists, similar detail to I joists, but no bell mouths, way simpler

Burning natural gas produces around 0.185 kilograms of CO2 per kWh. So emissions per kWh changes depending on flow temperature and therefore efficiency. Scotland currently UK Grid over the last 12 months

Our government (past, present and future) are committed to a 4 to 5 year job application, so unlikely to happen. Short term planning wins the day, no-one cares about long term (or those in power, to change things anyway). No upsetting industry, or the voter likely to occur anytime soon. They can't even get energy pricing sorted, so don't hold your breath about bigger stuff happening.

You may have to educate us on breezeway. A quick Google search shows a software company?

I just bought longer timber (4.8 and 5.4) cut to suit. Found very little waste by the time you make noggins etc.

I just approached the joiner (carpenters make furniture), they did all the other leg work, I just made sure it was done and reviewed everything so I was also happy.

Your drawings state by specialist contractor. So are only indicative. My roof worked like this Approach company to build roof, they used specialist roofing fabricators, specialist company design and provide drawings Contractor approved or modified as appropriate. Full drawing and calculation package sent to structural engineer for approval. Again any modification made to enable structural approval. This aspect becomes part of the overall structural certification. Same will be true for you pozi joists.