JohnMo

Looking at the calc inputs for the rooms, your number of air changes per hours looks very high. 43% impact on heat loss. A property with 7 ach @ 50 pa and with wind loads applied would have an effective air change rate of 0.5. So 0.5 or better would be more appropriate. If you don't have MVHR the efficiency of that box needs to be set to zero. Correcting the above will half your ventilation heat loss.

Could you locate the plenums more centrally this would limit duct lengths. Reduce system pressure drop, less noise as the fan. Just buy the plenums there's normally enough other stuff going on and life's too short. I would use passivhaus flows as a good starting point up them slightly to get br rates if required.

@Jenki Did you end up buying one of these heat pumps?

To add to the above, if you have a prolonged cold period outside, such as central Europe experiences, and MVHR, then as you heat the incoming air with out going air, you can do reduce the relative humidity. A big house with only one person in it with sub zero temps for quite a few days could experience well below normal humidity levels. But not sure it's that likely in the UK. Our house today is sitting at 42% RH and 20 degrees inside, 6 degs outside and 77% RH outside.

You can specific airctete rawlplugs. A very course screw in type. I got mine from Screwfix. They fix stuff well. Screw in half a turn, the back a quarter turn, put them in slowly. Work a treat.

All I would say about Salus Auto balance actuators, is they keep a supply and return DT stable. They do not control or limit the flow temperature. So if you set your flow temp to 50, then return will be 7 degrees lower, at 43. If you set it 40, same again return will be 33. What they are designed to do, is keep the DT constant on each loop they are installed, irrespective of loops being switched on/off on the same manifold and a fixed pump speed.

You would have one nut either side of the white threaded part. Basically lock the first terminal, the other units, move the nuts out of the way, the inner one, screw well out of the way so it doesn't limit adjustment. When adjusted, pop the terminal out of the ceiling and lock the nuts.

I have mine on a internal wall on a double layer of 18mm ply from floor to ceiling. Little or no noise transfer. So you should be ok on a solid brick wall.

Have the cover in position, but wind out as much as possible while still having enough screw engagement to install the lock nut. Good luck

If it's where your are sleeping, generally poor ventilation could / would be the reason. So let's get a bit more info. Where in the room is the grille? What ventilation do you have, continuously running dMEV, intermittent bathroom fans or nothing? Do you have vents in the window, if so are these open or closed?

The more you split the system into zones the bigger the required buffer. If you had a big enough buffer then a couple of immersion heaters in the buffer would get rid of a need for seperate heater. Simplifying layout and piping runs. LLH have to be correctly hydraulically sized to work efficiently. So would steer away from them. Great for commercial applications. Many do not like thermal stores, but you can get them sized for heat pumps, the main difference is the DHW coil size, being huge (about 6.6m2). That way there is about 4 to 5 degrees difference in store and DHW temp. You keep the store at low 50s, supply heating and DHW. Here is a typical drawing showing the hydraulic layout of a system with an additional heat source as well as a heat pump. Pump for the gas boiler if not already within the boiler, follow boilers manual. Note: Hydronics just means a water based heating system. The hydraulics is the use and design of the system for water in the system.

All things are a compromise, which has the bigger hit on efficiency short cycling or heating a thermostat controlled buffer? The thermostat doesn't need setting very much above heating flow temp, hysteresis depends on volume I believe. The non thermostat buffer, doesn't require additional temperature to operate. So win win. That is how Kensa heat pump want the buffer installed. The difference in CoP for a 5 degree uplift in supply temp for a given outdoor temp 0.5. If you heat pump was sized quite closely to max heat demand, a clever option may be to bypass the buffer once it gets to about 4 or 5 degrees outside. Then short cycling should not be an issue anyway. You just need the defrost capacity is maintained in the system.

Any photos? What pump are you using? How many zones, does this occur in a steady state (ones all open) or when zones are opening closing etc What is the flow temp of the UFH and what is the supplied temperature from the boiler? Looking at the manual for a T4 mixer that should be 41 degrees

I started writing, but gave up it was too late and lost the will...

I have a couple of the central heating pumps, you just need to size them for your system. Also have a grundfos all are super quite. Is the noise the pump, or just the water moving through the system?

A buffer is just an extension of the heating system, it give the system more volume. The heat pump has to run longer to get it from temp A to temp B. Once the return temp of water to the heat pump is at a level to hot for the heat pump, it stops its heating cycle. The circulation pump continues running. When the return temp is within limits again the heat pump starts. There are two ways to manage a buffer. No thermostat Within the period of time from heat pump stop to heat pump start, the heating system draws heat from the buffer, the buffer temp rides up and down within a few degrees of it's set point, the same as the heating system. With thermostat Heat pump heats the buffer to a thermostat set point, this thermostat only managed the buffer temp, this is set hotter than heating system flow temp, works the same as a thermal store. Heating system sucks the heat from the buffer. Buffer is recharged by heat pump it doesn't know or care what the heating system is doing. A bigger buffer gives a longer the run time and longer the off time, less cycles more efficient heat pump operation. Heating system 0.1kW to max HP output, it doesn't care. The heating system is a seperate system, you could have a 10kW heat pump driving a 2kW heating system, with a correctly sized buffer. Down side is slightly high flow temp with the thermostat system, some vessels heat loss also.

25% higher that trickle.

A buffer will limit cycling, that's what they do. The bigger the buffer, the longer the run time and off time.

Boost would normally be 25% of trickle rate. Just calculate the internal area, and use below https://www.sensorsone.com/volume-flow-and-area-to-flow-velocity-calculator/

Have you looked into PIV ventilation systems, with an activated carbon filter. One less fan to drive. MVHR great in an air tight house, but an additional cost to run in a non airtight house.

We had a normal reliance mixer on our UFH and it was really hit or miss what temp it gave to floor. Leave the dial where is alter the flow temp going to it, was anyone guess the flow temp to the floor. They generally like a 10 degree hotter difference in water temp in and water temp setting. I've just installed an Ivar mixer, which mush stable and has adjustable internal bypasses, to make it really adjustable. A lot of this internet articles are written with a good theory in mind, but often not tried in a real world situation. I turned my combi flow hot water temp down, monitored gas consumption it went up, turn it back to original setting +5 degrees gas consumption went down. Go figure. I would set everything back how it was, measure the temp of one of the pipes coming from the hot side of the manifold or manifold temp, see what temp it actually is. That will give you a reference, then lower boiler temp a couple of degrees, adjust floor flow temp to match previous temp, ignore the dial markings. Do that step by step trying to keep manifold temp stable.

With posi joists, you cannot notch, so you whole really needs to sit on top of the joists. You need to insulate below the UFH upstairs as well downstairs. By the time you've finish you are likely to loose 40-50mm room height quite easily. If I did it again I would just install rads in the bedrooms and electric UFH in any upstairs bathrooms. Slow heat up time and bedrooms don't really work in my head. Carpets will only add to the heat up times

No I am saying get all the information first, condensation analysis is the key part of any decision.

A useful table and comparison for different ways to calculate flow rates. Notice you have different flows rate totals for supply and extract, these should be the same (the higher of the two rates), so the house pressure is balanced. Extracting more than you supply will lead to smoke from your fire coming into the room each time you open the door, or even the flue leaking into the house. Carbon monoxide leaks can and do kill. You mention only 4 supply, what about the office? If the other two bedrooms are doubles the flow rate needs to be about 11 l/s in those also. Calculate all the flow rates for each room, before you start, then you can do the pressure drop and velocity calcs, to ensure you have enough duct runs to each terminal and size the MVHR unit correctly.

If your doing a self build for yourself to live in, you cannot be construed as being a developer. If you were building one for self plus other properties on the same land that may be a different story. I took a couple of years off work to built our house. Now back at work, did not get paid for the work I did, but obviously paid the contractors used. If I was some how making a profit and sold it soon after it was completed, then it falls under capital gains I believe, not being a company or a contractor.