Dillsue

I assume the "50 degrees/auto adaptive" is 50 degree DHW set point and auto adaptive for the space heating(CH)?? So if it's not tripped for a few days, is the DHW up to 50 degrees and reheating as you use it ie things are normal??

The immersion may or may not be faulty but theres likely another fault as the HP should be able to heat the DHW way beyond 20 degrees without needing the immersion....there's no point in having the heat pump if it's going to default to using the immersion!

The 3 modes you mention can all likely run without a stat but your installer may still have put one or more in. Have you got multiple stats suggesting you've got zoned heating? This is unlikely to be relevant whilst you are trying to get the DHW sorted. In your first post you say the system started with this issue soon after installation?? Did the installer not sort it out then, or try to??

OP says "it doesn't heat the hot water either" so I assumed it's CH and DHW??

Can you alter the hot water schedule to come on during the day when you can watch what happens when it tries to heat the hot water? There Should be little need for space heating just now so I would focus on getting the hot water up and running. Do you know what the "stats" do as you shouldnt need them if running in auto adaptive mode. If you're really under pressure to get this sorted I would contact any local Heat Geek engineers and see if they have Ecodan experience and can help out.

Pretty sure that's not the case....desirable maybe but far from essential. I sold my late parents house a few years ago and the buyer(likely buyers solicitor) asked for gas cert, elec cert, this cert,that cert and all manner of things. I just said none were available and the sale went through.

Pump anti seize is enabled. I can't remember exactly how often it operates but the couple of minutes a week rings a bell.

After installing our HP a year ago and it running well throughout last winter, I thought Id run it up over the weekend after a few months shutdown. It started and ran fine and I shut it down after a few hours then went to check the magnetic filter. Found the filter with quite a bit of flaky debris stuck to the magnetic tube. I don't know whether the debris accumulated through last winter or came from the weekends test run. There was more debris this time than any of the half dozen filter cleans/checks I did in the weeks after installation last year, so glad I checked it. After getting only minimal debris in the weeks after install, I was quite surprised how much flaky grot was in there! Our system is 90% rads so maybe we're going to see more debris than an UFH system, even with inhibitor, but worth checking any filters in your system to make sure they're clear.

A quirk of the Vailant design/software then??

I think our low uplift for extensive PV may be down to the Assessor, but maybe a limitation of RDSAP not accepting the uncertified 8.5kw of self installed PV?? Another quirk was that the Assessor reckoned we may have scored slightly better with a modern gas boiler over the ASHP we have. His explanation for this was that as we are heating with radiators the system assumes a 55 degree flow temp which hits the HP efficiency. He reckoned there was no way to alter this to our actual flow temp of 30ish. I think the new HEM would likely give us a way better score than RDSAP but that will have to wait for another day

We've recently had our EPC redone and 12.5kw of panels added a few points but surprised me how little it added. We've got a 4kw MCS system and 8.5kw self installed and the Assessor seemed to struggle with getting those entered into the system. He ended up putting the panels in by area as a percentage of roof area covered. I got the impression that assessor wasn't too familiar with anything non standard so maybe there are alternative ways that may get a better score??

When taling about ROI, you mention 5kw(kwh) of batteries in several posts so I assumed you're planning on getting some

16kwh Fogstar batteries are £1850, I paid £700 earlier this year for a 5kw solis hybrid inverter, a grand for panel mounts, £500 for inverter connection/sundries. Your return is looking much better and you've got treble the battery capacity to play with.

One advantage for the DNOs if inverters are regulated by power rather than current is that if they wind the voltage up everyone's inverters throttle the current so easier on the grid. Our original solaredge SE4000-16, limited the current rather than power and with our base voltage being 25x vac I've seen up to 4.2kw going to export. When that died the warranty replacement was a newer HD Wave version that limits by power so sadly always capped at 3.68kw

16A is without consent via G98. We have 32A via a G99 but I think others have posted higher limits.