JohnMo

Maybe for the house AND outbuildings. You can easily run both from a single heat pump.

Dust in the duct etc. I had a Dyson once lasted an hour sucking up building dust (motor failed). It's super fine and gets everywhere. A Dyson is small change compared to MVHR. Another dehumidifier?

A HiSense 4.4kW will come down to 1.8kW at 7 degs, so add a 50L volumiser, you will cycle about once per hour. £2100 delivered, plus you can claim the vat back on a new build. So a little over £1800, volumiser £100 to 150. A heat pump cylinder sized at 45L for each bedroom plus 45L, around £1200. MCS as @Nickfromwales says, will want a standard design, so radiators in each room etc. And still charge an arm and leg. If you size you fan coils correctly your flow.temp will never exceed 35 degs for heating, so you should get a scop of 4.5 or better.

Yep, our longest over hang is about 4m. Basically fixed to two wall plates. Whole roof was engineered by posi supplier and approved by structural engineer. Each end of the house has a 6" steel pole anchored to a big block of concrete, as additional hold down

Trouble with mixers is they mix, so quite a lot of return water goes around the loops again, so inlet water flow rate can be low. So low flow rates a 3kW heater, it's possibly boiling the water.

2kW doesn't sound a lot, but that's 48kWh a day at your design day. So would be pulling the cylinder temp down pretty quickly. Then you would be getting a rubbish CoP most the time. Not sure having the heating taken off the top would be that wise, as your DHW would always be cold. Think if you are going thermal store I would go proper thermal store. Have store as a pressure vessel, charge direct from ASHP, so no coils so heat transfer is 100% efficient. So if you store at 50 the heat pump only has to 50 degs flow. Take CH direct from store mid way up and return to bottom. Then have DHW via plate heat exchanger. Size at about 3-400L.

That shouldn't be the issue

What does your piping look like from ASHP to UFH manifold? Length and pipe dia. How long is the plastic pipe leading to the Wilo pump? The isolation valve on your Wilo pump looks to be a reduced bore, which will not help things.

So what is the difference between this and various emulsion paints, say a kitchen or bathroom which is more water resistant?

Brink do a system based around a normal MVHR unit. Basically they dump the supply air in the hall, similar to PIV supply only system. You do need to duct and extract from wet rooms. And depending on room layout you may need internal fans or if routing allows duct to some dry rooms. I read about it first on Passivhaus website. But search for cascade ventilation or cascade ventilation with heat recovery. You main thing you need to do is visualise where and how air moves about the house to make sure all areas are covered.

Our 12 degs roof is covered in the stuff, some off cuts have been sat outside for 5 years now. Not covered allowed to get wet. It's still in pretty good condition, considering.

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Most places that do ventilation. https://www.paulheatrecovery.co.uk/product/filter-cone-125/ They help protect the ducting, the main return filter is there to protect the heat exchanger. But sounds like you will be a while before you are switching on. Don't be tempted to switch the MVHR on early, it maybe one step forward several backwards.

Hope it's better than decorating caulk, which shrinks after a couple of years and leaves gaps everywhere.

Would have thought so, but only up to a point, if you have big gaps everywhere, you may be fighting a losing battle. Have you done a pre plasterboard air test? Then you know where you are and can discuss what is is achievable with them.