Hi Andy,
I registered for the firmware update on Thursday night but haven't actually received the update yet. So I don't actually have any measured effects to report. (I'll PM my MAC address anyway - because I have authorised the monitoring even though I don't have the update yet)
However I know exactly what hot water priority would do without even testing it - my system has a "slow" hot water cylinder that takes about 30 minutes to reheat from fully used and as a result I've designed the system to allow simultaneous hot water reheat and heating to radiators, so I'm naturally quite concerned about a software update that takes that ability of simultaneous heating and hot water reheat away from me and does not even give me an option to turn off priority to restore original behaviour.
An example of where this would be a problem would be if we started to run a bath and then a few minutes later turned up the bathroom radiator - a fairly common situation.
The measured cylinder temperature would drop to the reheat trigger point after running the bath for just a few minutes, (long before the bath was finished running) and as soon as that happened the heating zone valve would close and no heat would be available for radiators, so that turned up radiator which didn't get turned up until after the zone valve closed would remain completely cold for over 30 minutes that it took for the bath to finish running and the reheat to continue to completion.
As well as that all the radiators in the house would largely cool down in that 30+ minutes causing the rooms to undershoot significantly in the winter, probably followed by an overshoot as the HR92's tried to over compensate while they were denied hot water.
I'd not be happy with that and I suspect I'll get complaints from my other half about the radiator in the bathroom not heating up with a bath is run.
Hot water priority is fine for a system with a cylinder reheat time <10 minutes, and if the owner/installer decides that priority is right for them and their system, but it seems very short sighted to force it on everyone, including systems where it's not appropriate without any way to turn it off.
It also violates the "principle of least surprise", where a (later general release) software update that an end user didn't ask for, didn't know was coming, and can't refuse makes a fairly major change to the behaviour of their heating system, which when they discover it, can't be changed back as it's impossible to roll back the firmware or turn off the feature.
In my opinion the hot water priority feature needs to have an on/off configuration setting in the installer menu and needs to default to off during the firmware update process (and perhaps during a factory reset) to not violate the principle of least surprise... It can't be that hard to add an on off preference setting surely ?
Anyway hopefully the engineers working on this beta release can take this as constructive critisism. I'm still looking forward to testing the beta release, in particular the load scaling feature.