JohnMo

I would install trickle vents that open and close automatically based on humidity. If the area is ventilated enough the trickle vents should close. They are pretty cheap, require no power. Example https://www.bpdstore.co.uk/glidevale-energy-saver-humidity-sensitive-trickle-ventilator/p/182 Q1, set the flow of the fans for the room the are in, the self opening/closing trickle vents will look after the rest. Q2 yes Q3 no Q4 yes

@IanR do you have a buffer? Do you have any real numbers to share? Everything mentioned above is good, when you have all zones open and are running in a steady state. Cannot see how it's true when the input and output of the buffer isn't balanced with respect flow or pressure. All the flow diverters in the world will not stop mixing, when the system is not balanced.

Better take my batteries out. +1 DeWalt. We got the red Lazer one, great inside rubbish outside. Green one would be much better.

The test was completed in steady state conditions there were no transitions to different temperatures; as far as the simulator house was concerned. So even if a load compensation was involved, it would do nothing. The graph of output temperature was very stable. There has been other sources of information on buffers, well prior to the Brendon test. Just go on to heat geek , as an example, they's been saying avoid a buffer and or LLH for ages. Even just having a second pump could be cost £50+ a year in running costs.

Look for a summer house, most are designed to take advantage of permitted develop. Even a u value of about 0.2 you only need about 6 to 700W to heat to 21. My sister has one and with two of them in it, and a few candles, it really warm.

They probably aren't designed to mix when heat pump is delivering XX l/min and system is absorbing the same XX l/min. The mixing will occur when heat pump is still delivering XX l/min, but 1 or more zones have closed and the heating system is only taking X l/min. Or at any time where ASHP pump and secondary pump flow rates differ even slightly, then flow and return will take a direct flow path through the buffer as it becomes the path of least resistance.

Yes - to keep it the point

You are correct.

We have a roof over 240m2 all vaulted and have 350mm of icycrene spray foam, behind an airtight vapour control layer. Heat loss is pretty much as predicted. Agree it is expensive, but compared to spending weeks doing insulation and ripping my hands apart stuffing between the metal webs of posi rafters - a bargain, job done in two days.

Are you talking loops or zones? 5 loops as a single zone is easy, just remove the actuators, they stay open loops as a single zone. If you want to keep the zones, you need a volumiser to stop short cycling and allow defrost, when you only have a zone open. Grant do a couple of nicely designed ones. A volumiser doesn't need an additional pump, nor does it does it effect efficiency of the heat pump.

Plumb it the same way as a two port buffer, with over sized tees to give hydraulic separation. You need these on the inlet and outlet. You will need check valves as well

My view and there are others that will say no. Your heat pump is not doing cylinder heating, so there is no need to have a mixer to protect the floor from 55 deg flow. So use pump that comes with the ASHP and no mixer or pump on the CV UFH manifold.

Here you go, controller manual and technical details. If you search google for Maxa i-32V5, the user/install instructions come up in English. You need the installer password, which is 195 Controller-Manual-i-32V5.pdf Technical-Bulletin-i-32V5-1.pdf

Most efficient way to add volume is either a 2 port buffer, that sits in parallel to heating system, or a volumiser, it sits in either the flow or return piping and flow passes through it - it is connected to only flow or return, not both. Neither add hydraulic separation, so only a single pump is required. One zone - no wiring centre needed, single thermostat (normally your heat pump controller), no buffer/volumiser, simple system is all that's required for heating only, couple of isolation valves (use a filter ball valve in the return). Get your rooms where you need them by balancing loop flows.

You need to meet minimum manufacturer heating system, which is normally around 30 to 40 L, which you should exceed, as long as you don't zone and leave it as an open system. Min volume, is the smallest zone plus interconnected piping. My install will be open on a single zone no buffer.

@NickfromwalesIt was a one off on eBay, there is a 9kW still there. Item number 266070280966 The seller got them direct from Italy, as the postage bits were still on the packaging. I paid £300 less than the original asking price on eBay.

If you have a buffer you may well get a temperature drop across the buffer so will have to compensate. Increase your second floor flow rate, that will increase the mean flow temperature to increase energy input to the room.

Just compared @ReedRichards figures with the Brendon test, and interesting that the flow temp in test two is 38 to 40 and the flow temp in test 3 is 35. Almost exactly the same figures. But more importantly the use of a buffer is causing the heat pump to run hotter than it would without a buffer. For your bathrooms it may be worth increasing the loop flow rates a little so they are putting more heat in to the floor. Higher flow rate would give a higher mean flow temp, without increasing the actual flow temp.

Your EPC should have an energy usage figure on it, that will give you a start

What is your flow temp setting on the boiler?

So cost update as I been shopping, cost includes a 2 zone UFH manifold and 100m of 16m pipe. All pricing include vat and delivery. New Maxa 6kW heat pump (same as Viessmann, Vitocal-100 normally £4k) £1300 28mm Fill flush manifold and filter ball valve £78 Computherm thermostat £50 Manifold and UFH pipe £208 Wiring centre, expansion kit, 2x auto vent valves £86 AVM feet and flex hoses £147 Viessmann 1" Flow and return isolation valves, electric meter and isolation switch £158 28mm 3 way valve £59 Total £2086. Only have add inter connection piping. Hope this give people an idea of what the costs are or can be to do it yourself. Will do all plumbing myself, will need an electrician for a day.

Test 3 has less water in the system, so would be more susceptible to short cycling. So wouldn't that make test 3 perform less well or what's the point of a buffer. But in test 3 CoP gets better. More water may have improved CoP further. Question If you have a buffer why not record your own findings and post them. It's better for us to see real life installation, rather than experiments. So what is the a temperature change across the buffer or is there no change in flow or return?

No. To many variables. The house side of things stays the same, same energy loss on each test, same flow into the simulated emitter. He sets up up test 1 fixed flow to match a low temp as it enters the heating emitters equal to design flow temp at min outdoor temp. Test 2 same test but with WC engaged so at the test temp of 7 degs outside, the flow temp is lower, but with a slight uplift due to buffer. The flow temp into emitter is always the same temp. Test 3 same as 2 but no buffer, flow temp in to emmiters the same as previous tests. So test compares the cop at the same weather conditions and heat going into the house. Only two variables fixed flow temp and or a buffer with weather comp.

Heat pump efficiency is directly linked to flow temp, the lower you can get the more efficient. What are your flow temps? Can you fit bigger radiators?

Because we historically had a few days a year when Aircon was needed, historically they were also expensive to run, unlike now. Toshiba make a water heating module for A2A. But generally for commercial sized units.