Ok it’s been a few weeks since I posted and things have moved on so I thought I’d update back with what else I have learnt and changed over that time.

no progress on the flow rate valve windows, thy remain dirty so the flow rate valves are still left wide open.   From what I have read and speaking to Grant Tech too much flow rate (remember both pumps are set to min) is far less likely to cause a problem than too little flow rate.  I think that’s in keeping with comments above too.   So for now this remains on the list for another day.

I’ve spoke to Grant Tech 3-4 times over the last few weeks each time to tune and resolve one issue or another.

1) One of the key things is that the slave pump started running at 5C outdoor air temp regardless of the slave engagement settings.

now this might not sound like a big problem at first but given I have master and slave on open loops and plumbed in parallel it meant that when the salve pump was running it was pumping out equal volumes of return flow temperature water back into the outbound flow and in effect halving the temperature the master thought it was heating the loop up to.    So for instance the master would be sending out at 39C, returning at 35C but the slave was then pumping through 35C and thus diluting the total outbound flow at 37C.   This explained why I was seeing a dip in room / floor temps during 5C and 3.5C when the slave actually engaged its compressor.

It turns out this was due to frost protection kicking in, specifically the one enabled by DIP switch 1 which is factory set to UP which is ON.    Turning this DIP switch off on the slave and also changing params 43-00 = 0 and 43-10 = 0 (which also dsiables frost protection on room and outside air temps) solved this problem completely!

2) once resolved this enabled me to focus more clearly on the impact of the defrost doom loop.  One night in particular was bad when the outside temp dropped from 5C to -1C in just 3-4 hours.   This drop would have resulted in my WC curve adding around 3C to the target loop temp and combined with the falling outside temp was simply too much too quickly for them to catch up.   
Again after speaking with Grant Tech the advise chopping down the WC curve increase.  They suggested simply reducing the top end but I decided to tackle both ends, raising the bottom end a little too.   In total I changed my WC curve change from 12c to just 7c.

old settings were 30c@23c and 42c@-3c

new settings are 33.5C@23c and 40.5C@-3c.

a couple of days later and everything was much more stable.  Small gradual changes are much more likey not to cause the defrost doom loop than sudden rapid increases.

3) then I noticed that the slave was still being slightly affected whenever the master went into a defrost cycle, whereas it didn’t happen the other way around.  Specifically what was happening was the very briefly the master would interrupt the power to the slave at the start and end of the defrost cycles, but this meant the slave then went into a 3min compressor shutdown followed by a 3min compressor start up, all in all meaning I was loosing 12mths of slave running time at the critical moment when I needed it most I keep running.    I sorted these issue myself by reading the Chofu manual on backup heater (param group 46-xx settings).

Specifically I found that Grant had advised 46-05 =2 seconds and it should have been 0 seconds.   This paramater is the backup heater startup delay in seconds, so everytime there was a change in status for the backup heater (the slave) the power would be interrupted for 2 seconds causing a compressor cycle!

Next was setting 46-23 =24C default.    Grant gave no advise on this setting just as they had not given any advise on setting 46-20 to enable the slave during defrost.  Turns out 46-23 is the min loop return temp to engage the backup heater during defrost.   So I set 46-24 to 41C meaning that it I’ll always engage the slave as my loop temp would now always be below 41c.

The overall results have born out to be quite significant.   No more periods of the floors mysteriously getting colder and then suddenly warming up again.   It’s now a much smoother translation and constantly gentle.

looking at power use on my battery / inverter I can also see that the spikes are way less dramatic during cold spells.  Previously power would average 1-4kw over any given 5 min period, whereas now it’s between 2-3kw.  By ensuring the slave runs continuously through the master defrosts (unless of course lightnight strikes twice) then the impact on the loop temp is far less dramatic.    The fact I only have half a house completed and plumbed in means my total water volume is lower than the design so it makes sense that I was much more susceptible to defrosts than I should be once the house is finished, hence fine tuning this was critical.   It’s clearly a good thing to do regardless Id say though.

thats it for now - I’ll keep monitoring and tinkering as necessary over the next few weeks of cold weather.

Thanks again to the excellent advice from both on here on this forum and from Grant telephone Tech support.😊

@adam awesome trouble-shooting I’m seriously impressed. Congratulations so far!

This may not be a helpful post, as you have already purchased the equipment, but I have a couple of questions:

1. Assuming you have just installed the heat pumps, why did you install the Aerona 3 with the antiquated controller, rather than Aerona R290 with the new Smart Controller?  
2. I am surprised that you need two heat pumps.  I am living in a 5 bedroom detached house built in 1968.  The floor areas is 350m2 spread across ground and first floor.  The house has reasonable, but not top notch insulation - e.g. 30% of the loft insulation is below current building regulations.  It is rated EPC C.  We installed a Grant Aerona R290 15.5kW air source heat pump in August 2025.  It has coped comfortably through the winter with temperatures as low as -4.8C.