Hi there, just to clarify. On the Grant pre-plumb cylinder, the thermostat dial is the hot water thermostat - a simple on/off device to call for hot water generation. This would normally be in series with an external hot water timer, but can also be wired direct into the Chofu controller, and the Chofu hot water timed program function can be used. Above that is your Legionella immersion time controller, this simply energises the immersion heater on a timed schedule, independent of the ASHP controls. it's not a time controller for your ASHP hot water production. You could possibly use it for that, it's just a timer at the end of the day.

Looks like your system has had the external room thermostat/timer and hot water timer controls removed. That's no bad thing, but it has been done rather badly based on the photos! Redundant wiring should have been removed. The Chofu controller is rather complicated, and getting your head round the programming of it is an art! It's a very capable controller however. Don't worry about the built in room thermostat sensor being in an airing cupboard. Just set the temperature to something like 30degC and run the system with weather compensation enabled on the controller. That way you don't really need a room thermostat. Switching the ASHP on and off with a room thermostat isn't a good idea, it will consume more power that way and lose efficiency as it will never run for long enough to modulate down to a stable control. Inverter controlled compressors are designed to run for long periods, not be cycled on and off numerous times.

@allyfish have you tried a chofu using the chofu controller?  Does it work better than grants setup? I'd be interested to know how you (or anyone who's done it) got on.

If @marvin s system is setup that way already it may well be worth trying it as is 1st rather than rewiring back to grant norm.

@mab I have external clunky digital room stat. I think Grant only use these because few if any of their installers understand how to program the Chofu controller. It’s a very capable controller however, and pairing it up to a non-smart on/off room thermostat is a backward step. 

@allyfish Thank you for that. This is all very confusing lads, but with all you advice I feel more confident in figuring this together.

Suppose I leave the chofu in there, how can I set a target temperature, like 20-21 degrees day and night?

Is there a document that explains how to set up weather compensation?

@marvin91 there’s a separate post for weather compensation on Grant Aerona. It’s quite simple. You’ve got the Grant installation manual? It’s a redacted version of Chofu’s manual. The Chofu manual covers everything the unit can do.

@allyfish Is this the post you are referring to? https://renewableheatinghub.co.uk/forums/postid/22249

I checked and I had weather compensation enable and all the setting like your guide except 2105 which was set to 23, I left it like this, however I'm still quite confused.

Is my heat pump supposed to run non stop now? Regardless of the program?

I can't seem to able to edit my posts, anyway.

I tried lowering Tm1 (2102) to 35 today, and the house took forever to heat, expected, but kept rising in temperature all the way to almost 21 at night with 5 outside, definitely not expected. Must mean the insulation is decent or the heat emitters are properly sized.

Right now I just set it to 30 to see how the house feels tomorrow, but I shut of the underfloor heating to our bedroom in case this water temperature is still pumping too much heat inside.

These setting I'm trying right are pink, red was still too hot. I saw other peoples setting online and my 2102 looks unusually low 🤔

I also changed the DHW supply temp from 55 to 60 hoping it will warm the 210l tank quicker and reduce the off period for cooling the home.

Can anyone confirm that in weather compensation mode, you can't set a target house temperature, or program an on/off schedule? This is my understanding of it so far..