JohnMo

Have a read of the manual, it shows various schemes. Go the link I left and scroll down

Fixed speed, little or no modulation. But ideal for batch charging the floor as the OP does. Would I bother connecting to DHW? You may be better setting to 35 to 40 degs.

It's not required, there is no outside unit. Nor is any planning permission.

So if you have 7 hrs of cheap electric, that's about 2kW per HR to charge the floor, plus an allowance for DHW heating. I would do it my self. A very basic heat pump set flow temp. Your floor is a buffer, charge as you do now, so keep it simple. This lives in the house so you need two holes in the wall. It's a fixed flow temp, super simple, which is all you really need. Don't bother with the grant. https://www.theheatpumpwarehouse.co.uk/shop/heat-pumps/trianco-activair-indoor-heatpump-3kw/

Then the other side of that, is a heat pump sized for the heat loss, doesn't work for the high heat demand periods - so as way of an example, its -3 outside, you have a 4kW heat loss, you have a 4 kW heat pump. The heat pump needs to run 24/7 to keep up. If you have an 8kW heat pump it needs to run 12 hrs. Or you accept for most the heating season you batch charge during cheap electric rates, and when cold you burn expensive electricity as well.

As I posted above - direct cut and paste from ofgem website, new builds don't need an EPC, your installer obviously didn't bother reading the rules.

As I said that is NOT correct for a new build. OP is a new self build - EPS is not required No its up to £7500. So if it costs £4000 your grant is only £4000, not £7500

Thicker screed does not improve UFH/house temp performance. It makes the UFH slow and very unresponsive. So depending how you assess the question, you will get different answers. Thick screed is a thermal store, so to change its temperature using a low flow temp takes an age, using high temps takes a while also. But once up to temp it stays there for an age. Not good if you want to move temperature similar to radiators - that will not occur, if you add to much heat you live with it for the rest of the day or open the windows. You can do WC, bounce off a thermostat and use it as a thermal store. You do not need a buffer as the floor is the buffer. Mine is currently being batch charged as a thermal store, topped up if needed. The energy is being used to heat the house, but circulation water continues to circlulate and I keep a summer house also heated by fan coil from the circulation water (currently at 24 degs) carrying heat from the floor. Summer house is currently 18 degs (-2 outside) heating went off at 6:30am and house at 21.2. Having a high flow temp and or a thermostat with a hysteresis of more that 0.1 will lead to big swings in house temperature. But: once you have the heat source in control - as well as your head, you can do things with a thick screed that are not really done well with a thin screed. You can stuff plenty of kWh's of energy in, at will and not affect house temp for many hours. I have added 20kWh's of heat from PV excess. It just delays the heat pump switching on later in the day. You can charge the floor on cheap rate energy and have stable house temps the rest of the day. You switch the heating on at midnight the house is the right temp for sleeping as the floor energy is depleting. Having the heating on overnight isn't an issue, as the floor takes a while to charge up, so it is about right for getting up in the morning. I set my thermostat high to start to the heat cycle, then after a few hours drop the temp to 0.5 degs below my target temp, two hours after the heating cycle is complete the house is at 21. If you want simple, but slightly less flexibility, go 65mm. 100mm and beyond is a bit of learning curve. Zones are an utter waste of time also with thick screeds, as its like trying to stop a two mile long train quickly, it isn't going to happen

No - that is NOT correct for a new build. Because you have to meet minimum building regs anyway. Plus you don't have to await a the building being complete, before installing the heating. OFGEM states on their web page - important bit in bold and underlined "EPC eligibility An energy performance certificate (EPC) gives information about a property’s energy use, with recommendations on how to reduce energy demand and save money. It’s required every time you buy, sell or rent a property. Before we approve any grant application, we check that a property has a valid EPC that was issued within the last 10 years. You can check your property’s EPC report We do not check EPC reports for eligible self-builds." Just go to ofgem website and read the rules

Couple of things I have found with home assistant, If I have a power cut, home assistant, stays shutdown, until I manually repower it. If a sensor looses WiFi, automations can do strange things. So you may need to implement binary sensors so if a fault occurs the automation does something sensible. You need a robust WiFi system throughout the area you have relays and sensors

Definitely not new, but space age stuff to many (including the installer), where a boiler is installed ignoring all these features, because plumbers just wants to install S or Y plan with 70 Deg flow temps.

I did, it must have auto corrected

Their efficiency comes mostly from, the free steam they generate from the flue gas, and this takes awhile to stabilise and be usable in the steam turbine. So a CCGT, if started and stops frequently is only as good as and CCGT. Note: CCGT, is Combined Cycle Gas Turbine. A gas turbine, with a free power turbine in the gas stream diving a generator. In addition steam is made from the heat in the exhaust gas, which in turn drives a Steam turbine to give additional input kW driving force for the generator.

The generation emissions factor (2024) for grid electricity is 0.20705 kg CO2e per kWh. (source: https://www.itpenergised.com/new-2024-uk-grid-emissions-factors/#:~:text=This lag is standard and,CO2e per kWh.) A gas boiler Co2 emissions according to Energy Saving Trust, work out at 0.225kg CO₂ per kWh. So lower for resistive heating, but not by much - 0.02 kg CO2e. So if you are doing resistive heating for great environment credits, not sure there is much in it. The only way a heat pump is better is because you have a 3 or 4:1 output ratio compared to input.

You don't need a 25-80-130, if the one you had failed is only a 25-70-130. UFH doesn't need much head but you may need plenty of flow depending on how many looks you have. Size is broken down something like 25 is the nominal size 50, 60 70 , 80 etc is metres of head developed at a closed discharge. 130 is distance between flange faces. You took out a 70m head pump installed a 60m head pump and are now proposing an 80m head pump. Just get a 70m pump.

Couple of concerns pop up If your pump is rusted you have a big issue with circulation water. It should be corrosion free environment. So if your on glycol it likely it's started to breakdown, if fresh water with chemicals that needs attention... Plenty of pumps out there, just a matter of getting flow and head rates matched. Or look to delete the pump mixer altogether.

Just had a read of the manual and notice it states with LLH you have to run at fixed speed, but there are plenty of speeds to choose from. So flow rates could be lower. Have you adjusted?

So that gives a mean flow temp through the radiators of 48.3 degs. If you flow at 52.2 and have a return of 43.5, you heat output will drop by a big chunk, as the mean flow temp drops to 43.7. So not sure that will work Big gains for not condensing to condensing with the return below 54 degs. Small margins after that. You only have 4kW of water flow going to the radiator and 7kW to the UFH, so the return from the UFH will dominate the return temp. Think I would be more interested in getting rid the LLH

I'm the same have a few Shelly Plus relays, with add on for temperature measuring and H&Ts. You can track these in the Shelly App, but the detail drops to historically record in hourly steps - so pretty rubbish. However in they automatically integrate into Home Assistant and the logging is every few seconds. The only switching I do with a small relay like a Shelly is a circulation pump on the heating system (max 33W) and zero volt switching. Have a smart solid state relay for the immersion, but that about 40 times the size of a Shelly. Once you get your head around automations in Home Assistant its quite easy to do basic stuff.

I shoulder season mine would circulate but just blast a bit of heat when needed. But ran a single thermostat in the hall as a temp limit. But if you are running weather comp on the correct curve, there should be no need to fire up the stove, because the house should be toasty enough already. Big fan of WC, but starting to migrate back to batch charging the floor, as using 30p electric when the battery runs dry over the last couple of cold weeks, started to get more expensive than it needed too.

Radiators or UFH? UFH as it's close to room temperature, will stop outputting heat, so self compensates to a decent amount. Radiators maybe less so, or not at all. If you had a trv on the radiators in or close to the stove, they would modulate flow down in response to room temperature increase.

Looking at the images you have a heat meter. Press the black button until it displays kW. How many kW are you inputting to the floor?

That's just a load of rubbish - ignore. If your loops are less that 40m long it maybe ok anything else bad advise.

Not sure where you have been reading. You would normally set the flow by loop pipe length. The flow rate for an underfloor heating (UFH) loop is calculated by dividing the loop length by 40. For example, a loop that is 100 meters long has a flow rate of 2.5 liters per minute (l/min). You then set the flow to get the dT between flow and return within acceptable limits. Low temp systems are normally between 4 and 6, high temperature 10 degs is better. I would start by setting to 2.5L/min for each loop and see what happens room by room. If the flow will not get to 2.5 l/min then you have air in the system, this will have to be removed. If room ends up too hot reduce the flow a little in that room only. That's pretty normal depending on floor build up. That is very cool. You will not feel that in the floor.

I would expect a world of issues, without a positive break between the two systems. Assume you also have particulate filters upstream of the UV down to 5 micron, to stop UV filter contamination and effectiveness. Think the only safe way is a 'break to make' transfer, so isolating the spring water first with a removable pipe, which then transfers to mains supply. Mains supply will need a double check valve also upstream of the connection point. I would add a flushing point also to both systems, to get rid of the water which may be stale etc. But overall it's a daft idea.