JohnMo

Even without boost, you really shouldn't get condensation with MVHR. We rarely have boost on at all, without issue. % speeds don't really mean much, as far as flow rates are concerned. What were the actual flow rates? Should be on the commissioning certificate. Also have you checked your filters?

Well done.

Why don't you post your system design (with make model) and the symptoms you are getting. Could be quite straightforward?

Wonder MCS will try to grab that also.

Scottish building regs are different - extracts below "Guidance is given to minimise the risk of explosion due to malfunction of an unvented hot water vessel by: ensuring that such installations are carried out by appropriately qualified personnel" And "be carried out by a person with appropriate training and practical experience. This might include current membership of a registration scheme operated by a recognised professional body. the installer should be a competent person" Our rules basically say should be a competent person with suitable training and experience. This might be demonstrated by membership of a recognised scheme. I have the experience and training certificate to demonstrate competency. If there's problems it is no different to anything else, the person that signs on the dotted line is the responsible person, member of a sign off scheme or otherwise.

I spoke to our building control and followed up with email and got this reply "Thanks for your email, unless the install is part of a building warrant application we wouldn’t need any details/registering."

Not sure of your heat load for the UFH, but what about something like this. Set at a fixed temperature. https://eco-outlet.co.uk/products/activair-3kw-indoor-air-source-heat-pump

So they have gas already, why not just extend the existing heating system to do the UFH? Most boiler are massively oversized, so should have plenty of capacity spare. Even if the the garage is remote from the house, it should still be possible and will be way cheaper to run than an electric boiler.

Not sure I would pin myself to passivhaus, but instead follow the principles. A2A you still need a heater for the DHW, so I would stick to a normal ASHP. MVHR several ways to do it, the normal way with a duct and terminal in every room, or cascade, generally duct to each wet room and a couple of supplies only that suit the layout. You will have a demand activated fan through a wall or two for awkward layout rooms that do not lend themselves to being cascaded. An example of said fan https://www.ventilationland.co.uk/en_GB/p/brink-indoor-mixfan-co2-gestuurd-tot-70-m3-h/17927/?gad_source=1&gclid=CjwKCAjwkuqvBhAQEiwA65XxQNHSp3Am0ssSPnBCSOd5avKVIvBbL4-5yDgamc6Up--18SUdWGZUpBoCV34QAvD_BwE Get yourself a thermal camera and see where all the cold bridges are, so you can focus on fixing.

If look closely at that and Hitachi monobloc there is little of difference, make you wonder if they are the same machine rebadged

What actually happens to the house UFH is a little different to what you may expect, the floor settles to about 23/24 over the day, following heating it up overnight (I leave the UFH pump stays on 24/7 to distribute the heat evenly), i trip the ASHP off via thermostat. When the heat pump starts against a 24 Deg floor, it's trying to managing a dT of 5/6, so the output temp of ASHP is actually pegged at 30, with lots of concrete to soak the heat away, the temperature takes hours to rise, a sub zero day it gets to about 33 by the time the house is up to temperature. Repeat the next day. The heat pump doesn't modulate just runs at a high sustained load but at a low flow temp. In theory on a 7 deg day I should get an average CoP of 5. Whether I do, not sure, I just look at the most cost effective way. Doing most of it overnight at 13p kWh is as cheap as gas even with a CoP of 2.

I do something similar, one loop does bathroom, hall and kitchen diner., nothing in utility except all the loops travelling through it. Again no zones, or actuators on manifold.

8.5kW is a big heat pump for 4kW duty. A lot of loops - We are 192m2 with 7 loops. Our heat load is slightly lower but not much and we are on 300mm centres. With 15 loops you may struggle to get enough flow out the heat pump circulation pump, you have to do the calcs. If the calcs work then no issues. You need to do a pressure drop calculation for the pipe between the UFH and heat pump, plus your longest UFH loop, that will tell you where you sit on the pump curve and your max flow available. We a bigger heat pump, you could just charge the floor (like a storage heater) use a 0.1 hysterisis thermostat so you don't under or over shoot temperature set point too much. We just run the heat pump at 35 degs and do most the heating on E7. Messed with WC but didn't see much advantage in a low energy house. The shorter run times actually use less electricity.

Depending on time between course and card issue, mine took 6 weeks to arrive.

Or do the course

Better or worse? Just looked at a cylinder datasheet and the usable volume (heated) for a 210L HP cylinder is 189L for a normal shape cylinder and 183L for a slimline. A 250L is 228L and 300L is 268L.

I think if I remember correctly a PHE with 1m2 surface area is equivalent to a coil with around 3m2. Then all you need is a small pump suitable for domestic water (not a CH pump). Tee into the cold feed in and hot feed out of the cylinder.

These work great, designed for the job. When you install, one turn in, half back, a further turn in etc. until fully screwed in. https://www.screwfix.com/p/dewalt-aerated-concrete-anchor-nylon-60mm-50-pack/454HT?kpid=454HT&cm_mmc=Google-_-Datafeed-_-Screws%2C Nails and Fixings?kpid=KINASEKPID&cm_mmc=Google-_-TOKEN1-_-TOKEN2&gad_source=1&gclid=CjwKCAjwte-vBhBFEiwAQSv_xSPJizrRNcYEjQuP7ZUJI4VyVx8cKj8RcKlyx5vZJCAQXNT_FOFBqRoC80MQAvD_BwE&gclsrc=aw.ds

I use 25mm if available, but mostly 19mm.

You would possibly have to pay another plumber to inspect and certify another's work - good luck with that. Or get building control to sign it off - at the plumbers expense But bottom line is the plumber should know better and not be installing, or should have lined another or building control up to certify. I suspect there are lots of un-certified cylinders in the UK.

There is two of us and we have a 210L HP cylinder (with 3m2 internal coil) heated to 50 - never had it running out. If more than two of you you may need bigger, or we had consecutive showers straight after washing up, that may be a different story.

Don't look that cheap, unless you are getting a good deal. Plumbed for 2 zones, good if you need two zones, bit of a waste for a single zone.

Our cylinder is in an upstairs plant room, the pipes go along in the ceiling and down into the utility to the hot manifold and then back up and down to the wet rooms, never seen an issue.

You really need to complete a heat loss calculations, we have 0.14 walls, 0.09 floor and between 0.11 and 0.14 roof, and pretty airtight - we defiantly need heating in NE Scotland. Your form factor makes a huge difference to heat load, ours is rubbish (long thin building and all vaulted ceilings), but suits the location and views. No matter what you decide, put UFH pipes in all down stairs rooms. We put ours in a 300mm centres - very small cost and nothing lost. You could connect to a heat pump or heat on a cheap rate via a Willis heater or two.

Yes choose one system or the other. Not all dMEV units are equal. The Greenwood mentioned above was my second make, the first was quite noisy. Always seem to be plenty of Greenwood units on eBay for good prices. I looked when I was designing my house quite liked the Duco system https://www.duco.eu/uk-ie/products/mechanical-ventilation