JohnMo

I believe the program just apportions flow based on loop length and then assumes a fixed dT across the loops. Think all my loops expect one were pretty similar in length. The short loop is in an ensuite, and that never overheats. You can play with outputs in loop cad by having different manifolds, set the flow temp and manipulate dT and loop lengths to get the room output (or room temp) you want. It's quite a while since I used it though. This is how mine is sitting, it's 10 degs outside, full cloud cover so no solar gain and heating came on from 00.30 to 6.00 this morning. Heat pump circulation pump runs 24/7, when not heating or cooling the circulation rate is at minimum set speed. So this where flow rates settle to at min speed, all flow meters are open I just do cooling exactly the same way as heating, just set the flow temp to maintain between a high of 19 and low of 14 through the floor pipes. It generally only cools when the sun's out, it sorts it's self out, I have no input as it doesn't need any. I don't zone anything in heating or cooling there is no point. It add complexity for zero net gain. Heat pump is very happy with fully open circuit. Heat pump running last night, putting out just under 6kW, for 5.5 hrs without stopping, getting a CoP of 5 while heating. House doesn't overheat, floor will keep the house temp stable, until tonight and it will do the same again. (Big spike is DHW heating).

Have you checked the calibration? Your immersion should also have an overheat trip in addition to any thermostat to prevent this from occurring. Something is not correct in your set up.

If a new install you can balance by design, so room loop sizing. Or can tweak room flow down on any overheating rooms only. Yes drive the whole heating system direct from heat pump circulation pump. You just don't need mixing valves and additional pumps. One thermostat/controller by heat pump manufacturer. System is then ASHP Diverter valve (to cylinder or heating) DHW cylinder UFH manifold and loops I would Address airtightness And look at ventilation system. When cold the house air is naturally dry, so if house is heated nearly no risk of mould or condensation, but over ventilation can cause health issues as the air can get too dry. Sustained cold use an enthalpy MVHR or a demand based (CO2 and humidity) ventilation like MEV or dMEV with suitable controls

If don't sort the drainage issue, you will just transfer the issue from sodden grass, to sodden fake grass and have a bad back and lighter bank balance.

G3 regs cover it all.

Generally 24/7 on weather compensation is best. Setback are likely to be cost neutral, so no point. Other ways to run are batch charging the floor on cheap rate. But I would get used to the house for a month or two before messing with things.

Expanding on what @ProDave says. We are in similar area, but closer to the coast so get a bit more defrosting. Slightly higher heat loss due to form factor of the house. I have a 6kW heat pump, but run at a slightly elevated flow temp - the heat pump doesn't really modulate, this afternoon it was 10 degs and with excess solar the heat pump ran for 1.5hrs, putting out 5.5kW the whole time. Decent screed thickness is the buffer, no house overheating either. A 9-12kW unit, will ideally have around 180 to 200L of water. Or a huge lump of concrete in the floor to dump heat too. Choice a buffer if you really don't want, but you just don't need it. I would really eliminate the buffer. Just use a fully open system, a suitably sized volumiser, fully open UFH flow meters, and then let the heat pump circulation pump modulate as it wants. Let it run slightly lower dT (fully open system will do this for you). Then make sure you have a decent screed thickness.

Pretty similar to what I did. I don't have any PIR where you do. The noggin between the rafters is actually 140mm PIR closing that space. I have 256mm deep rafter and have under drawn with battens, to increase to 350mm. Over rafters I have sarking boards (45 Deg roof) and marine ply (12 Deg roof). All have breather membrane above. Full fill with Icynene open cell. Also have a vapour closed membrane below. All seems pretty quiet and keeps the heat in. Did interstitial condensation calculations and no issues.

Aircon?

As above yes in England and Wales, think it seems to have disappeared in Scotland unless you are on government assistance. So has to be MCS install. This can be a self installed if you do it via an MCS umbrella scheme. But has to be a "self builder", not a housing estate building company.

I was meaning take learning on board about the whole build of the extension and the issues you can have if you don't keep an eye out. Not sure it makes any difference. But if clipping UFH pipes direct to insulation I would do 100mm on top.

My rebar was limited to lintels at doorways and window plus vertically at doorways and a wind post rebar midway along back and end walls. So not much at all. Lintel detail The wind post

I would call your local college that does plumbing.

No sure if you have the manual but I've attached it. If you have a multi meter check for a change in continuity on the potential free contracts when it's calling or not calling for heat. User_Manual_Roth_Touchline_SL_Controller_8ch__Master__L12__20240402_UK.pdf

Also notice you have a slab, insulation and then screed. Could that be changed to have the slab above the insulation? So no screed? UFH pipes in slab?

Is the signal for the boiler coming from the wiring centre or is the wiring centre signal making a 2 port actuator open, then the 2 port actuator sending a signal to the boiler? If direct from wiring centre to boiler the potential free contracts should go from open to closed circuit. That should occur at the same time as the pump starts. If not 1. On controller go to fitters menu 2. Go pot free contact 3. Check the Global Control is ticked 4. Check the delay time is something like 2 minutes. I see you have a WiFi connection for the wiring centre, have you logged in to that, to see what it is reporting that should show the status of all the contacts. This is mine, same thing with a different manufacturer label. Pot free contracts are off as there is no call for heat, but shows on when a call for heat. Have you changed any settings?

You could make the low point as deep as you want, then pump up on the main sewage pipe. Think they just run on level control once it gets to a given level the ball cock switches on pump and moves it forward.

Use compacfoam. Attach compacfoam to opening adhesive and screw, into concrete. Then attach window to compacfoam.

Very much doubt it from what I've seen. Think they just cut and paste the last job into your floorplan. Go to boffins corner, there is a heat loss calculation spreadsheet in there. Use that it's pretty good. If an MCS (grant) install the install company does the heat loss calculation. But you should always do your own calculations anyway. You will need an as built SAP anyway. Not for the grant but for building sign off, assuming new build. Couple of different factors for heat pump size. Obviously heat loss, cylinder heating. If a low energy loss house with a decent depth of screed, you can run the heat pump in batch charge mode during cheap electric price periods or run weather compensation. Cheap rate batch charge is ideal for a bigger heat pump, because you need to cram in 24 hrs of heat in a shorter time period. So works like a storage heater.

I used primer on parge coat and airtight mastic/sealant https://passivehousesystems.co.uk/product/gerband-6300-primer/ https://passivehousesystems.co.uk/product/gerband-fortax-6400-airtight-sealant/ Once set almost impossible to remove, and forms better to the irregular surface.

Read this post take learning from everything. https://forum.buildhub.org.uk/topic/42827-cold-extension-insulation-issues/

Double post

UFH more insulation is way better, stick in as much as you can.

Difficult to tell is there holes in it? If not, get a scraper to remove the worst. The hand wire brush. Then use a rust stabiliser, such as https://www.toolstation.com/painting-decorating/paint/c705?type=Rust+Converter Then prime and to coat. Rust often looks way worse, than in reality. It expands about 6x the volume of the base material.

Well done photos?