JohnMo

Do the easy Opentherm first see how it goes. Do you have any zone valves to open, if so you will need them open to get heat around. The Opentherm bit just modulates boiler output and flow temp. Doesn't deal with stuff in the system.

You can build any house on ground screws with correct design. But why? You would hope the house you build would be there in a 100 years, ground screws will have long disappeared by then. So not sure I would bother.

If you have radiators then Opentherm is pretty easy to implement. Set desired temps on the thermostat and it will do the rest. Not sure about Opentherm and UFH, that may be better WC. But both options would reduce flow temp and keep house more comfortable.

As I said you have missed the point - WC curve is set up with all zones opens. House gets to a steady state. Now YOU start closing zones off, so the zones cool down. Additional heat transfer occurs as heat moves from heated room to unheated. Radiators in heated room no longer give off enough energy to keep heated rooms at set temp. YOU then adjust WC upwards to compensate. CoP hit. The WC will never likely to be correct out the box, you can never do a full accurate heat loss calc on a retrofit, even a new build there are so many assumptions, that it is near but never exact. So not sure where the left alone comes from - early you said your house was stuffy and hot when running WC, so curve obviously not correct.

But I believe you miss the whole point - those under heated rooms are pulling heat from the warmer ones. That in effect makes a bigger demand on the heated rooms, so you need to run the system flow temperature higher to compensate. That reduces CoP. Recovery of under heated rooms will be very long if you ever need heat also. The only way that can be effective id running the flow temperature higher than required and a further hit on CoP.

The bulb heats mercury I believe, so a different type of bulb

Trouble with any devise that makes a room colder when no one is present, just stays that way in a low temperature heating system. If you have flow temps anywhere near correct, you have either long left the room or fallen asleep waiting under the blanket you covered yourself with, to keep warm, before the room reheats. Energy usage will be lower, your house is just colder also.

But don't do that if there is a accumulator between pump and UV filter. From here https://www.knowyourh2o.com/indoor-4/uv-disinfection#google_vignette UV systems are designed for continuous operation and should be shut down only if treatment is not needed for several days. A few minutes for lamp warm-up is needed before the system is used again following shut-down. In addition, the plumbing system of the house should be thoroughly flushed following a period of no use.

We installed a combi originally, one of the highest flow rates you can get, with preheat cylinder to make flow better. Now have UVC - water flow rates not massively different time to tap big difference. Given a choice between the two - UVC every day. Just set the pressure in your expansion vessel to match the water pressure. A 4 pipe just installs the diverter valve inside the boiler casing instead of outside. So not really needed, just simplifies pipes a little. Size the boiler to the heat demand, no need to go big boiler. Heat loss calculation is required - plumber, scratching head and saying a number isn't good enough. Then do not accept S or Y plan, you need priory demand hot water (PDHW) then you can have high temperature for DHW heating and low flow temps for the radiators either Opentherm or weather compensation.

Our doesn't seem to suffer even down to prolonged -9. Are your thermostats in the air flow from the MVHR, so MVHR off then read room temp instead of air from MVHR. UFH and that quick change of temp isn't realistic.

Bit of a p*ss take on pricing, but the big one would last you about 50 years.

That's half my December generation - shows the difference of about about 500 miles makes. Less PV and more heating, loose, loose for me, win, win for you.

Northeast Scotland today 0.28kWh. Pretty much nothing.

So you don't open a tap or flush a loo in the night? Nothing uses water? If you do use any water - you introduce anything UV kills off. Do you want that? My answer was no. So run 24/7/365 and replace every 365 days.

Are you sure you are on the correct scale. G3 or G4 sounds more like what you are talking about, not "F", I got mine (G3) from RS Components. Stock number 8279510 Filter media roll white 1mx 2m x 10mm G3. It cost about 3x what you paid and I would order again.

Do real, you won't regret it, concrete and composite looks good for a short period, slate looks good for decades.

A, decide now where to install, ideally decide on what stove you may install. B, Also look at possible routes for air feed. Ideally add and fill with insulation and make airtight. C, Decide flue route, you need to provision for non flammable structural around the flue (check building regs for dimensions). D, Fill area where flue will route with rock wool Saying all the above, either install now or pay twice or three time, to do work, then undo work and then make good to install later.

I would do as big as you can, we added more solar, battery, ASHP all after build, so spare space in CUbis good. The electrician is certifying, and you let him specify make models.

Been thinking some more. Run the boiler and heat pump on the same WC curve. Can easily flick between the two heat sources, via a time switch. Or if the heat pump is smaller than needed, the return temp would and dT would not allow the flow temp to reach it's target, so then the boiler should take over if enabled.

This took about 10 seconds to find https://coolenergyshop.com/collections/air-source-heat-pumps/products/proinverter-air-source-heat-pump-6kw

You can do this, but I would not. I would do a normal UVC and external ASHP like @ProDave mentioned. I paid £800 (incl VAT) for my 210L HP cylinder. Plus £1300 for a 6kW heat pump. Does all the house, summer house and DHW. Cylinder came with diverter valve, one direction to cylinder other straight into UFH manifold. You are going to be getting through way more water than you seem to calculate. Very much doubt it would be that cheap for 6 adults even at £0.08 per kWh We use around 6kWh for 2 of us, so you could be around 18kWh with 6 adults

Can do if you design correctly. But more to the point, store so you don't need to blend down. Install a bigger cylinder? But just install a heat pump, you will be paying a fortune just for DHW. Secondary return heats the manifold also. You take the secondary loop to the furthest wet room, I then flows back to cylinder via manifold. Manifold is hot all user get hot water quicker, even you hot water appliances. Without it the hot water appliances will just be getting Luke warm water at best.

Your UVC has a valve that reduces mains water to 3 bar as well as doing other things. It also provides a balanced cold water flow outlet, so mixing taps function correctly. If you take water before this and use on a mixing tap, you may not get any hot water flow, depending on pressure before the cylinder.

And defrost makes a big dent energy output per kWh input. One degree change in outside temp can be the difference between CoP of 5 and 3 due to defrosting.

It does, so does the controller any thing in the system relating to heat pump. Each electric item should be metered. We take a big dent in SCoP because batch charge. The main circulation pump runs 24/7, as I use it to borrow heat from house to keep a summer house warmish. CoP of 5 doing heating and daily CoP about 3.0. but heated mostly via cheap electric. Our heat pump has only one pump, and a diverter, diverter is powered only when doing DHW.