@FJJ Over the last few weeks I've commissioned a switch from an external Honeywell Lyric controller to using the ASHP's own controller.  I need to write this up properly but here's a short summary:

My system is S-plan, there's a 2-port zone valve for the heating, a 2-port valve for the hot water and a pump.  I've wired it as follows (all relays are mains):

• The 'Pump1' output (Terminal 48) from the heatpump drives a relay which switches the heating zone valve
• The '3 way valve' output (Terminal 51 - NOT the 3 way mixing valve output terminals 4/5/6) from the heatpump goes live when in hot water mode.  This drives a relay which switches the hot water valve
• I have additionally wired the 'Pump2' output (Terminal 49) via a relay ready for a second zone valve - this is currently unused
• The contacts on the valves are used to energise the pump in the usual S-plan way
• I've also fitted thermistors (10K NTC, B=3435K) to both the hot water cylinder (Terminal 7/8) and the buffer cylinder (Terminal 11/12)
• I've wired Terminal 46 (electric heater) to a relay which switches the immersion heater (actually via a second contactor) in the hot water cylinder.  I have this metered so I can see consumption.
• For my personal setup I've added an 'emergency boost' timer for the immersion (in case of ASHP breakdown) and a couple of spare relays for perhaps one day being on Octopus Agile and wanting to switch the immersion remotely.

All of this is in a single 4-way DIN rail box and a small PCB, rather than the 7 boxes Grant have scattered around.

I have it wired on Modbus so I can see parameters changing - I previously had a Home Assistant setup that graphed the data, but that's currently out of action so I'm just watching raw values change (script).

According to my notes (which I need to double check as there was a bit of trial and error), I've set these settings:

• 5119 from 1 to 0 (disable remote contact for hot water)
• 5120 from 1 to 0 (disable remote contact to turn on ASHP)
• After this the room controller buttons now start working according to the Chofu manual
• 5107 = 1 (enable DHW tank temperature probe)
• 5111 = 0 (do NOT use the buffer tank temp sensor) - this is important because the buffer tank cycle involves heating the water up to a fixed (by default) 60 degrees - the house was roasting!  The buffer tank sensor works even when it's not used for setting the set temperature - I just plugged it in and readings started appearing, without any config changes
• 4100 = 1 (use water set point) - this uses the weather compensation settings
• 4220 = 2 (additional water pump as main but disabled for DHW) - this enables the Pump1 output for use as the heating zone valve
  
• 4200 = 1 (main water pump controlled by buffer tank temp). This is not a listed combination of settings in the manual, but it seems to work. It means the main water pump turns off when the unit isn't heating the buffer tank - this is quieter than having it running always or doing sniffing cycles
• 5146 = 0 (use electric heater terminal for water heater)
• 3140/41/42 for legionella cycle (once a week)

With that I have full control over the temperature using the settings on the room controller, and the ASHP is only running when the room temperature falls below threshold.  The hot water modes (Comfort and Eco) work, the legionella cycle is bringing up the hot water to temp with a combination of ASHP and immersion (it says it's currently topping out at 50C, I need to check the thermistor is making good thermal contact as water feels hotter than that).

A few todos: I set a timer programme to run the hot water for an hour, which it does.  When the hot water is up to temp it stops, but it remains in hot water priority mode - if you have a bath and drain off the hot water it'll then heat it again.  I need to work out why it isn't falling back to hot water off mode (which can be done with a button press).  I have not tried the heating timer, since we just run it 24/7 and setback the temps manually.

In answer to specific questions:

@FJJ The controller allows switching into low noise mode (which never seemed to make a difference on the Lyric setup, I haven't tried it on the new setup) and low tariff mode, which can be set via timers.  It's possible to enable dry contacts to switch them.  However I think better is to drive it via Modbus: you might want to control things to a finer degree than just engage those modes - eg heat the water hotter when you have cheap electricity than you normally would.

@GGW In the default Grant setup the ASHP is in weather compensation mode and it's looking at flow and return water temperatures.  So it senses how much heat it's delivering (to the buffer, in my case) and can see whether it needs to increase or decrease its output.  (They don't install a probe in the buffer so it has to sniff the buffer temp by pumping water around and measuring it in the outdoor unit.)  This means it can modulate its output down according to load: my 13kW is usually about 3.4kW electrical input but it can progressively drop down to about 1.8kW when it's warmer outside. 

The way Grant wire it there's no data from the third party room thermostat as to the target temps: it just has on/off, although the ASHP can sense how much heat is being delivered - so room temps and your radiator/UFH balancing and TRVs set how much heat is delivered, and the ASHP adjusts to provide that heat. So it can adjust to some degree.

When wired according to my setup it can now see the room and buffer temp, but I'm not sure it's changing its modulation based on that.  ie it'll still run until the room temp reaches target and then switch off - it won't control its modulation based on room temp, but is sensing heat delivery as before. It is possible it could do that with a different setting, and I'm not plotting the data at the moment so I'm not watching modulation closely.

However it's still a lot better than the Lyric where TPI was cycling it every 20 minutes even if minimal heat was needed - it was accurate for indoor temp control, but the cycling made the outdoor unit sound like a jet engine.  It's now noticeably quieter. So the main goal has been achieved, and now I have wiring for a second zone I can look at installing a fan coil on it for cooling.

@phil-l, what decision did you ultimately make? 

In the end, our installer was able to source the Ecodan ASHP and completed the installation just a week before the end of March. A close shave.

My principal concern was about the installer’s level of experience with the Grant versus the Ecodan. They were open about having done only a handful of Grant installations versus hundreds of Ecodans.

Happy that we got it in before the RHI scheme change deadline. Less happy that the price of electricity then trebled. Heigh ho!

Posted by: @prunus

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A family member is very noise sensitive so the contingency plan is to move it to the bottom of the garden.....

Make sure you do all your resistance calculations.  I put mine behind a wall in the garden, so needed 20x2 (flow/return) meters of underground pipe.  I think I undersized it a bit, 32mm OD when I should have gone for 40mm.   That does limit me a bit in terms of how low I can go with the dT when at full load, which for me is around 15kW