Posted by: @harrisonc

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As others have mentioned you cannot change the flow temps below 30/50 but you can change the outdoor temps for the curve to whatever you like. Normally warm weather is 12 and cold weather is -10 degC. I currently have it at 13/-10 but am going to try -20 to get the flow temp to drop a bit more when its colder outside as I am getting overheating.

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It would be useful if you can post back on here how that goes as I too get overheating at cold weather.  The "start of the fix" has to do what you are doing but not completely there yet: mine has changed from 50c at -3.2c to 50c at -7.0c and I am in contact with an Octopus design team member who is proving very helpful.  I will be changing to 50c at -10.0c to see how that goes.

@andrewj I set the weather compensation curve to Hot Weather being 8 degrees and Cold Weather being -15 degrees last night, with flow temps still at the minimums of 30/50 and had good results. Last 24 hours we have had temps from 10 down to just below 0 and seemed to work well with very little overshooting. House has levelled off nicely at 21 as well.

Red line is flow temp, green line is return.

@harrisonc Thanks - if you've read any of this thread you'll have seen I have pretty much the same problem, somewhat improved recently with adjusting those settings.  Last night was a cold night, first in a while, and some rooms are still overheating although not as bad as they were.  I've had to put the Primary Pod into one of my colder rooms and with a set temp of 19c, other rooms are getting up to between 22c and 23c, although upstairs rooms are limited by TRVs. Temps today ranged from 1.5c at midnight to 9.4c currently and the system has used just over 18kWh of input power.  I have a re-survey being carried out tomorrow and I'm in discussions with the Design Team for optimising the system.  I do think further changes to these settings are required at the Cold Weather end so with a bit more data now available to look at I'm hoping that the Design Team can make some further recommendations.  Ours is a Cosy 9 and I have been previously informed that these can't modular down below a 35c flow temperature so at some point in the WC curve, it will level out.  I don't know if the Cosy 6 is the same.

Good to know someone else has experienced the same "feature" though.  The engineer I had out, and the Design Team, are pleasantly surprised they're not working on a problem of being too cold!!

@andrewj I had to have a recalc done after my survey in October 2024 as I was having issues heating my living room initially, turned out they needed to change another 3 radiators and put in a K3. 

The Cosy 6 seems to be able to go down to a flow temp of 30, possibly less but I cant test as I cant set it to anything less. 

There apparently is a new power PCB for the Cosy 6 that they retrofit at your first annual service, so will be interesting to see what this does.

@harrisonc where did you hear about the new PCB, I’m due a service in the next month so will be checking if this is being rolled out to all Cosy6 installations

@agentgeorge I first heard (read) about it on FB this morning but there were no details about it.

Posted by: @harrisonc

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I set the weather compensation curve to Hot Weather being 8 degrees and Cold Weather being -15 degrees last night, with flow temps still at the minimums of 30/50 and had good results. Last 24 hours we have had temps from 10 down to just below 0 and seemed to work well with very little overshooting. House has levelled off nicely at 21 as well.

This is very interesting and is something I was totally unaware of until now.

If I understand correctly, you have adjusted the ends of the WCC which has the effect of altering the FT at the 'given' temperatures.  

Would you mind explaining how you have managed to do this, as I have no clue and I am beyond curious.

@kevh Yes, so effectively over 8 degC outdoor temp it will always have a flow of 30 degC and below -15 degC outdoor temp it will have a flow of 50 degC, but in between the curve will be a shallower as 1 degC outdoor temp change will equate to 0.87 degC flow temp change.

To do this you have to go into installer mode in the Cosy Hub by pressing the button on top of the hub for 10 seconds till the light is solid blue. Then on a device (suggest a computer, rather than a mobile) connect to the OCTOCOSY (think this is the name) WiFi network that the hub is now broadcasting. Navigate to IP address 192.168.1.1 in a web browser and then you can adjust it under the Setpoints tab. There is a publicly available installer guide here that explains it in more detail, although is slightly out of date.

Posted by: @harrisonc

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@kevh Yes, so effectively over 8 degC outdoor temp it will always have a flow of 30 degC and below -15 degC outdoor temp it will have a flow of 50 degC, but in between the curve will be a shallower as 1 degC outdoor temp change will equate to 0.87 degC flow temp change.

To do this you have to go into installer mode in the Cosy Hub by pressing the button on top of the hub for 10 seconds till the light is solid blue. Then on a device (suggest a computer, rather than a mobile) connect to the OCTOCOSY (think this is the name) WiFi network that the hub is now broadcasting. Navigate to IP address 192.168.1.1 in a web browser and then you can adjust it under the Setpoints tab. There is a publicly available installer guide here that explains it in more detail, although is slightly out of date.

To get onto the web page the first time, the passcode is on a sticker on the Hub.  It ought to be remembered after that.  

Also here is the manual - https://www.manualslib.com/products/Octopus-Cosy-6-14454801.html

@andrewj Forgot that bit as I haven't had to do it in a while. Trying to figure out the Manufacturer password (or a bypass), as Engineer password is rather easy but doesn't offer much.

I had a re-survey completed this morning and the heat loss came down by 1000W.  I'm not totally convinced it shouldn't be more, but notwithstanding, it basically means that 13 out of the 15 are oversized for the flow temps.  Two radiators look like they are "correctly" sized but empirically they aren't: I can't get those rooms above 19.5c when everywhere else in the house is 22.0 - 22.5c and one of them will fall to mid-18c on occasion, even when I had the situation of massive overheating (some rooms at 24c+.)  Hopefully, these two will be upsized to balance to the sizes of the others.  As for the rest, I want to agree to go down the "adjusting flow temps" route before anything changes.   

Currently, having shifted the primary pod to the cooler room with a Set Temp of 19.0c it seems to keep the house comfortable at around 22.0 - 22.5c in other rooms and the Cosy stays on longer.  When it was cold yesterday, many of the rooms were getting up to 23c and that isn't uncomfortable for us although the Cosy was working hard so it's not very efficient: having said that, I have no real historic data to go on, and COP yesterday was 3.61 with an outside temperature ranging from 1.5c at midnight to 3.5c at 08:00 and then climbing through the rest of the day to 10.5c at midnight. Total usage of 21.7kWh with around 16kwH used in the first 12 hours of the day (temp reached somewhere between 7c and 8c at midday.) 

I spoke again with the Design Team and we've adjusted the WC curve to be 50c at -10.0c so I'm now running at 50c to 36c - he warned against dropping it below 36c, as it will keep turning itself off, and as I understand it the Cosy 9 can't modulate below 35c anyway.  In theory, this WC curve gives me a flow temp of 43.6c at 0c.  From Harrison's post above, it does look like it is possible to adjust the -10c point further so I shall be running at the current settings to see how it goes with scope to make further adjustments.  It's due to be cooler next week and cold (1c - 3c) on 26th and 27th.

Anyway, progress.