JohnMo

No, the layout is specifically for the Fresh R units, but can't see why it can't be adapted to other units

Mine is going to open vented thermal store. Direct to top and bottom of store not via a coil.

Some initial comments Kitchen/diner only needs an extract. Move supply and extract points away from windows and doors. You could remove extract from landing Make use of coanda effect supply and extract valves. These throw the air across a room at ceiling level, may make duct routing easier. You could remove a supply from the snug as all supply air in the lounge and family will travel through the snug prior to being extracted. 8 supply 6 extract. Anything outside the heated envelope has to be insulated. I have attached a layout drawing I got from Fresh R for our house. There are only two supply points and three extract points (200m2 floor area) so some food for thought. They have looked at the way the air would flow through the building and simplified. I didn't go that route, because Brexit was just happening at that time.

If doing ST do a drain back system, look on eBay etc for an Alpha SolarSmart drain back system, comes complete with pump, drain back cylinder and plate heat exchanger. You can take the water back to anywhere and the PHE it is certified for drinking water.

Assuming you are using around 50% the capacity of the brink 400, so around 200m3 flow (suitable for 600m3 house volume), that only 18m3h average flow per inlet and outlet, which sounds wrong.

A house layout may also help, long runs may need to be doubled up. Have worked out flow rates?

You don't really want to sucking all the heat out of the living space. You need to do from wet rooms, this will also assist moving some air through the house

Are you E7 or similar?

Read your deeds also see what they say

One of these? https://www.hygienesuppliesdirect.com/mylek-portable-air-cooler-with-ice-packs

Not surveys or professional fees (except labour on site) are vat free, just the build costs.

On pure financial terms bonkers, most the heating plumping needs to be changed, new cylinder and radiators, possibly the electrical supply to 3 phase. Ecological terms makes some sense, but only if improve overall efficiency of house to displace the other wise needless replacement of lots of copper etc.

No I don't think you have read what was written correctly. This cylinder will only ever be heated by the immersion.

When you convert the boiler it would still heat the cylinder as a system boiler does using a y plan or similar. There is a big difference in gas and electricity prices, I pay 7.33p for gas and 30p for electricity. Your easiest is the second option If you want vented install something like this https://www.toolstation.com/indirect-hot-water-cylinder/p62376 in the the cold water supply to the combi. Install 3kW immersion and use the diverter valve mentioned previously. Job done. Normal cold water passes through cylinder and diverter valve to combi. Combi fails switch on immersion, once up to temp water diverts past combi and hot water to taps.

We were £1200 +VAT in 2020. Depends on building size and complexity.

We are 7.33p incl vat for gas per kWh. The fixed price deals around now are circa that price (10.87p).

I use an Atag Wize thermostat as the main controller - I have A325ECX and Wize is better suited to that. If you are going Atag use the Atag One. My turndown is to 6kW, but I am using a 160l buffer (also used for summer solar water heating upstream of the combi). My heating demand is circa 3 kW on a cold day.

First a boiler cannot produce sand, it can produce other deposits, lime scale and calcium etc. If it is really sand it is likely it's coming in with the mains water, so something as simple as a filter in the line will address that. What does your kettle look like inside as that goes through same process as a boiler, are there any deposits? One way to test, draw of cold water in a clear jug, leave for an hour or so, are there any deposits? Draw of hot water do the same compare results. If you are producing deposits in the hot water that are not there in cold water, you need to treat the water prior to heating, something like a combimate, adds a food safe chemical to the water to stop deposits forming, or a water softener if in a hard water area. Installing a cylinder which is not used often if at all, could be unhealthy as you are really having a place where water can be stagnant - not good for your health. If you are going a cylinder route you really have two choices, convert your combi to a system boiler. Basically cap off the cold and DHW feeds. But install an unvented cylinder so you get decent DHW flow. Then have your immersion ready to use should the need arise. Or install a small cylinder upstream of the combi, so all cold water passes through it. A solar diverter valve could be placed between the cylinder and the combi. Such as this https://www.bes.co.uk/thermostatic-solar-valve-kit-with-bypass-21082/ Then if the immersion is on and the cylinder is above 43degs all water bypasses the combi.

Need a little more information. Have you done heat loss calculations, these need to be completed first. If you have them there is a spreadsheet for calculating them in boffins corner. Are you doing a self install or via a grant through a registered installer?

I have an Atag, they seem to a good boiler. Need for a buffer is down to heat output of boiler compared to heat required for the building. You need to know the heating requirements of the building to make that call. You can get an idea of the heat required from an existing EPC or use the tool on here (boffins corner) to calculate yourself. The worst case for short cycling is when its 10-12 degC outside, as the heating required is very low. Try not to have to many zones and certainly make sure you have no really small ones.

WC vs load compensation. You can have both. Master thermostat, just decreases flow temperature, but it applies to all rooms not just the room the master thermostat is in.

Hire a decent one, you will only be disappointed, they were only shredding small stuff.

Insulation?

Agree no-one seems to know the difference between a screw and bolt, even lots of so called engineers. A wood screw is just a type of fully threaded self tapping screw, but designed for used in wood. By definition a part threaded wood 'screw' is really a self tapping bolt design for use in wood.

10 to 15mm - 10mm is a 1/3 the volume, so instead of 50 seconds for 16m you will be closer to 12 to 15 seconds for you 12m.