JohnMo

Don't, most plumbers are knot well.versed into efficient heating design, and it's their day job. Doing heating well isn't something done in the UK for various reasons. Simple take away if it sounds complex it's often complexity for complexity sake. The most efficient systems are sized well, run low and slow, have limited outside influence. With a boiler or heat pump if you allow 2 hrs for DHW heating per day that will be ample in most cases. So if house needs 5kW to heat it at the design day, that is 5 x 24, so 120kWh. To heat house in 22 hrs you divide 120 by 22, so 5.5kW boiler size would be needed.

Either a dedicated heat pump cylinder - big coil, fast reheat or or low flow temperature for added efficiency. But as mentioned above, looks like a AI cut and paste.

First you don't size a boiler based on house foot print or size, you size based on heat loss. Nearly every boiler installed in the UK is massively oversized. So to size well you need to do heat loss calculation. Second at a more average heating day in the UK it's about 7 degs - so heat loss is approx half your design heating day -3(ISH). So you want a boiler that modulates down to pretty close to the output of the 7 Deg day, then it can just tick away adding only the heat lost. So you are looking to size system based on a once a year eventually if ever. This can get difficult, expensive and maybe make system over kill so be careful what you ask for. Two showers at full performance is easy, three may push things at full flow depending on shower heads. I would do your 300L but insist on a heat pump one with 3m² coil - this gives super fast reheats or you can reheat at a much lower flow rate for added efficiency, in a more normal time. Boiler plumbing - Do not accept anything other than PDHW (priority domestic hot water), this allows one flow temp for cylinder heating and a different one for central heating. And you fully boiler power given to the cylinder. So S and Y plan is out. If your plumber doesn't understand priority domestic, strike from the list automatically.

Not 100% sure, but soot will suck up moisture (hence the ventilation)

So how are room temps now in the house and using house proper? Do you run all the time or do you switch on off at different times of the day?

Adding to what @SimonD says above - no matter what boiler you go with, also steer clear of boiler that will only do S and Y plan and get one that does priority domestic hot water (PDHW). This will allow you to run heating a more low and slow and balls out doing cylinder heating - so two different flow temperatures for heating and DHW. While your at it, if replacing the cylinder do a heat pump cylinder, the large coil in a heat pump cylinder give really quick reheats of the cylinder compared to a normal gas boiler cylinder or do a much lower flow temperature from boiler for best efficiency. A decent system boiler will also give you variable circulation pump speed so boiler can better adapt automatically to changes in flow requirements.

Is an 18kW boiler a good boiler is the first question? Depends on you heat loss, if it's only 6kW it's too big. Min modulation is about that, so is too big. Open vent boiler, do you need that or a pressurised system? Then is a the specific boiler good, should be ok, in mist instances. Modulation of any heat source will vary with output temperature. It's pretty much true with any boiler or heat pump. Flow a different temp you get a different output. Some boilers will have a hard lower limit other won't.

I would just go around the perimeter with a multi saw. If you need say a 10mm expansion gap make an 8mm wooden former to put against the skirting and use it to move around the room to ensure a straight cut through the floor boards. Once cut 8mm plus the cutter thickness, will give approximately 10mm. Change numbers to suit the required expansion gap. Then attach the quarter round trims, use a thin piece of card (cornflakes box) to space the trim away from the trim piece from the floor boards, to allow free movement.

If you need a fan look no further than a Greenwood CV2 or CV3. They are silent at normal setting. You will need a 100mm hole through the wall. Runs on 230v

Do you have a chimney feeding it or are the fireplaces all closed off? If so is the chimney vented from inside or outside at a low level?

2 zone is just a little different from multiple zones. Read into that what you want. Read above

But is your installer saving you from yourself? He is wanting a good performance system as well installers should. If you had asked for a thermostat in every room, he may not of done that either.

Something like this may be better. https://www.recoheat.co.uk/

You condensation is only forming because pipe is not covered. Once insulated the air doesn't get to the cold pipe, so water formation doesn't occur. As said above. You need ready access to a stop cock, hiding under units, isn't the correct thing to do. The actual stop cock should be inside the cabinet.

Tiles? Epoxy? Micro cement?

What is in the hole other than the pipe? Do you have a solid floor or suspended floor?

300mm centres but no pipe under fixed kitchen units, or under beds (whole house designed around views, so unlikely that rooms will be moved much over time) no dedicated loops in hall, utility, so approx 600m pipe buried in 192m² of floor. System only been running for 4 years. Not expecting any maintenance costs, nothing is moved in the system all flow meters except one are fully open on manifold, no actuators on manifold, no zone valves, pumps or mixers to go wrong.

Actually installed a spirodirt filter

Trouble with them is the available flow area is pretty low, especially with any debris collected. As I said earlier I had a good jump in flow removing the element and installing a proper filter. But the improvement in flow is really dependent on where your system has the bottle necks. You got a 5% increase in flow, just by cleaning and that is the issue, any debris in the system the system unseen just slows down. But as said above you need to identify where you actually have bottle necks. 26 to 32mm may save you pressure drop but is the issue else where, if so it will not help you. You need to go through the whole index loop and see what is what. Not just pick an area and point at that, as the issue.

I would take it out, look at how dirty it is, give it a good clean, put back in a rerun, for an hour (check before and after flow rate), then check again, if clean, remove and do a quick run to look at flow rate. All those pipes need external (UV resistant) insulation.

Is it an Olympic sized pool? 3x panels will not touch the sides of 32kW. Which way does the shed roof face if the ridge runs north south you can have panels on both sides of the roof I e and east west array

We are similar sized I have mine down to 0.3ACH, almost never have to boost for anything. It just 24/7 at the same rate. In summer we have windows open when we fancy (bedroom is window open all summer). Lounge will go high co2 but only if we have about 8 to 10 people in there. Again I will open the window, as it starts to get hot in there anyway. Your 3 way valve will actively encourage more more noise as the flow rates increase locally. So may be not the best solution for noise.

Simple way to think of solar and heating - they do not align well. Solar output when there is a heating demand is very low. A battery can be charged by solar, or filled via the grid ideally on cheap rate. Most of the winter it will be filled via the grid. Your heating - a direct electric heating system is likely to bankrupt you, will cost at least 2x gas more likely 4x without a huge battery. Either stay with gas or do a heat pump hot air system (no grants) or dump the hot air system altogether and go wet ASHP (grants available) or replace whole system with A2A heat pump and even storage heaters will be better.

We did double sided tape and stables (makes initial fixing easy), then taped all joints and taped over all staples. We then battened for a service void, with 50mm battens nailed in place through the membrane and double sided tape. Only things going through membrane are MVHR, all wiring was attached to battens

Not actually come across any GSHP that do cooling out the box, Nebe have a super expensive add on kit for it, but not seen anything else. I suspect all this comes about as they as they not reversible - so instead of adding the functionality to the GSHP box, they add it externally via a load complication. Not really thinking inside the box they are literally thinking outside the box.