Nickfromwales

Normal plywood. Tanking with a liquid tanking solution or membrane ( if you're rich ) in known wet / splash areas is a no-brainer, so sense always prevails. I've used Ditra etc where it's been a dodgy screed ( laid by other and I've not laid it ) and brilliant results. I've only ever used Ditra once over chipboard, and that was in communal flats where the chipboard floors were all non-mechanically fixed ( zero screws whatsoever ) for acoustic control. Every single other job has been plywood, without a second thought.

Good luck monitoring and facilitating that!!! By the time the suns out and the excess is generated, and the HP switches on, and then gets up to premium temp, and is then up and running long enough to be heating DHW effectively, the sun and any "excess" ( aka the requisite surplus thereafter base loads are satisfied ) will have disappeared, and the whole lot will have switched off again. The HP and ancillary equipment will take a battering with this setup. Excess to immersion is the better choice AFAIC, and zero moving parts, zero LAG, and nigh-on 100% proportional ( so anything after 100W gets used by the immersion ) whereas those pockets of excess would NOT provoke the ASHP to fire at all. Add to this the extended longevity of the heatpump from NOT doing such a crazy duty cycle, christ knows how many times a day, and it's just not a great idea IMHO. Nope.👎

I doubt you'll have enough solar during the winter to provide space heating tbh. A2A A/C ( which will heat in winter and provide cooling in summer ) would be my choice. Very cheap to run, and only about £2k to buy for a reputable make, but you can get much cheaper.

£1500 is the price of a decent MVHR unit !? Let alone a whole installed system. The requirement for MVHR is not a choice, it is mandated by the number of air changes per hour demonstrated by your blower test result. Have you had one done?

You'll want 5 screws per chipboard deck, per joist, minimum. If not, add more before ply'ing.

I've NEVER once used backer-board when tiling over timber floors, ever. Never had a single crack / failure / issue and I've been tiling for north of 25 years. Plywood, glued and screwed, is absolutely bombproof AFAIC, and I actually dislike backer board because it doesn't conform to undulations like plywood does. If the sub-floor is "wonky" you also have to set the backer-board down into flexible tile adhesive, and screw it, so I'm not a fan. 6mm ply or even 4mm will suffice, and I only ever use thicker if I'm trying to match adjoining floor thresholds etc.

Email building control and ask for a breakdown of their responsibilities, and also contact the Citizens Advice Bureau as they can offer free impartial, honest ( sometimes brutally honest ) advice to you so that you do not trip yourself up during this process. I doubt they would become pedantic about anything else as that would see them leaping onto their own sword ( as why didn't they identify that during a previous visit ? ). I'd just ask the cowboy builder to supply all the materials for the re-build, and say that's all you want from them and they can leave without any further responsibilities, as, to be frank, this is now just about damage ( cost ) limitation and pretty much nothing else. The builder may already have a CCJ, so won't give two hoots about getting another.

You have WAY too much spare time on your hands! PMSL.

Ok, then does the combi not fire DHW to all outlets via the buffer? Eg when any hot water tap is run, the combi diverts to DHW mode?

Too much off the ears.

Ah, so not a Combi boiler, just a combi-nation of components. Gotcha Now I recall your setup, sorry. Been a looooooong day.

I would, but I'm washing my hair....for a few hours. It's on my to-do list though, and I'll not forget!

Exactly what I came up with. You literally JUST beat me to posting that.

You've described @Dave Jones to a tee Which was one of the main reasons why I have mentioned gas tbh. 22.5 kW of microgeneration ruffles the feathers, then add EV charging, and THEN mention a whopper of an ASHP and they may well shit the bed. Prob be a good idea to choose a PV system where you can volunteer to accept zero export. Wipes out toying with Octopus etc though. Cracking idea, and perhaps could be further simplified by just doing that from a huge UVC stored at 40oC?

That will be sized, most probably, to the electrical requirements of the house, with most of the excess feeding DHW or topping up the battery. Can you fit more PV?

If you say @SteamyTea three times, he will appear.

Diminishing hot water? When the combi registers DHW flow the pre-heat circuit shuts down and the volume of heated water starts to diminish rapidly.

Sounds like a huge expenditure on equipment with limited service life. Probably a high-temp split ASHP would be better CoP-wise? How much PV / battery do you have? Or have you not bought and installed anything yet?

The ASHP won't have a CoP of 3 or 4 when driving DHW at 65oc, more like 2 if you're lucky, and that's if it's not winter and the air is freezing / damp and the HP freezes over from charging this HUGE volume of hot water. This needs a serious re-think in honesty! Winter PV will do next to zilch too, unless you're installing north of 12kWp on 3ph. Do you have gas available? I think you'd be much better off with gas for this scenario.

You wouldn't mix them, but it is very dependant on your consumption patterns / peak DHW demand at any one given time. Excess solar from 9kW will not leave much to go into hot water, as I am assuming this is a very big / multiple occupant dwelling (?) to need that much DHW? I'd look at a pair of 400L cylinders, with the second for DHW and the first for DHW pre-heat. Putting pre-heated water into the 400L DHW tank will probably double its useful capacity. Can we have some more information about house size / occupants / number of bathrooms etc?

Do you have a drawing of what you have detailed?

I use Vortex, and bulk buy when stocks are dwindling. I just look for the cheapest online supplier that will delivery FOC at a certain purchase point. https://www.bolts.co.uk/5mm-x-50mm-woodscrew-vortex-pozidrive-countersunk-ybzp-pack-of-200-p-WSPCPVZY05050/?keyword=&matchtype=&device=c&campaign=GS_|_Woodscrews_-_Vortex&gclid=CjwKCAiA3pugBhAwEiwAWFzwdaRPlnMfus-nZpbAc-HwOkn0V6C05tOOHnt8FGZACZNDSlFB3fVl0RoCGOoQAvD_BwE

which brings about the question of The less the need for heating, the higher the likelihood of needing cooling. Have you done a room by room PHPP analysis?

You could use an "in screed" heater wire, if you want utter simplicity. The caveat is that an ASHP could never be retro-fitted, which leaves you completely reliant on some off-peak tariff or other. I am not sure of what heat pads you mean, but you defo do not want to heat the immediate upper surface ( as that will take much longer to heat the entire mass of the slab ).

Yup, on the Brink stuff, but a pretty generic function. You were probably just giddy from the good news 👎