JohnMo

The glazing is normally the worst performing aspect a good quality windows. You are after two things airtightness and no thermal bridges. So is insulation spray foam better or equal performance (U value) compared to the window frame - yes/no. Is the spray foam airtight if installed correctly - yes/no

You may want to try purge ventilation, as interim measure. You basically open all windows and interior doors fully, everyday for 3 minutes. Then close them. Most the building heat is within the building fabric, air contains very little heat energy.

The trouble is if you hardwire, you pay the electrician to install wires, you end up paying £50 per thermostat (could be way more or a little less), then you get talked into auto balancing actuators at another £25+ each. So an 8 room house you end up spending several hundred on need less stuff, plus labour to install it... Leave the contractors they will also insist on mixers and pump. I have so much needless stuff I have removed it's unreal.

With an ASHP with no radiators you can, if you wish drive the UFH from the circulation pump within the heat pump. So no mixer or pump needed on the manifold. I would state to the UFH contractor you are going to run weather compensation. I would install an electric towel rail in bathrooms and use that to dry towels when heating is off and drive the room temp up if needed. If you want the bathrooms warmer than the rest of the house on the same flow temp and on a single zone, just put more pipe (closer pipe centres) in the bathrooms. Given a choice of heat pumps, I would just go Panasonic.

We are using the same, our first one failed after 2 years, but it was operating in a near sand pit, so no surprise really. But they are pretty cheap to get new and all parts are available to repair.

Or a big one

You need proper ventilation strategy, air through trickle vents, but what's driving the throughput? Ideally you have cross ventilation - air comes in via dry rooms and out wet rooms. Under doors need a 5 to 10mm gap so ventilation doesn't stop with doors closed. dMEV fans in wet room would provide the motive force for air movement. Then you need to make sure the house is heated, this drives the humidity levels downwards.

Need proper ventilation and filtration then. Business-esque prices also.

May as well write it correctly, your posts look much more informed, and generally could be taken as way more factual. At least you show you know what you are talking about. kW. (k represents 1000, W represents Watts (as in a surname)

Keep all the noise outside and just install a monobloc. Then a cylinder with a 3m²+ coil. Then all you need in house is cylinder, diverter valve and simple UFH manifold. Plus the heat pump controller. Depending on screed thickness just simple batch charging against a thermostat works fine. Or weather compensation. But batch charging via the thermostat gets away from cycling and the floor acts as a huge buffer. Keep it simple.

How thermostats do you have? Are they all timed at different times?

The only issue I have with AI, you get the good stuff and the rubbish, all given you, youras if it's all correct. The not so rigorous reader (not doing correct due diligence) could easily go off on a tangent and do exactly the wrong thing. Sometimes it's just easier, less time consuming, and you get the correct information first time, to just read the rules and regulations directly.

The trouble with a lined tank, is any area where the lining is flawed or has a defect (crack, pin hole etc), the base material of the cylinder is subject to huge galvanic corrosion rates. That is the principle reason for the anode and why they have to be replaced periodically. I would not have a lined cylinder to save a few pounds, copper or a decent stainless steel, would be the only choice.

Airtightness and ventilation heat loss are very different. An airtightness test of 0.6 will give an infiltration ventilation rate of nearly zero. So all the ventilation comes from the MVHR, which in turn recovers around 90% of ventilation heat loss. Scottish regs require 0.5ACH or thereabouts. So your ventilation heat loss is 0.5 x 10%, 0.05 ACH. NOT 0.6. So ventilation heat loss drops by over 90% from your calculations. Based on what you have said your -10 heat loss is going to be less than 2kW. Low heat losses drive more issues as you struggle to get heat into the house without lots of cycling of heat source. UFH is way more forgiving for oversized heat sources, it's cheap to install and acts as a big buffer for heat just like an oversized storage heater. Would rethink your floor design - go concrete, 200mm PIR insulation or 300mm EPS, the UFH on 200mm centres, then concrete over the lot. No screed needed

With a 5 to 6kW heat pump, on a cold day AND if you need to heat cylinder twice, will take about 1.5 to 2 hours in total. Sometimes more sometimes way less. So the easy way to size for is assume you can only heat the house for 22 out of 24 hrs. 4kW x (24/22), 4.4kW heat source is required. So 5kW is fine, as long as the heat output is that at your design temperature. The above assumes 210 to 250L cylinder with 3m² coil for heating. We are on the Aberdeen side of Elgin, we have had full days at -9 to -10. Local weather stations are few and far between, our local one (Kinloss) and our temps can be 5 degs difference sometimes. Roof U value, just do the same as the wall (1/r1...) and then apply the heat loss to the total area of the vaulted ceiling. How are you doing ventilation, this changes this figure hugely?