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Octopus Cosy Heat Pump Owners & Discussion Thread

 
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Air Source Heat Pumps (ASHPs)
Last Post by swwils 15 minutes ago
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 KevH
(@kevh)
New Member Member
Joined: 8 months ago
Posts: 2
19/11/2025 5:37 pm  

@andrewj @jamespa I just want to thank you both for the discussion that you have shared over the past few days, it has been so informative.

I have found myself in a very similar situation, and although I understand the basic principle of running low and slow, in practice it is difficult to achieve given the variables that are both under my control and not.  I think that the experiences that have been shared bear this out to be a common problem across Coy users, if not across ASHP users per se.

I have been tracking every move and have now managed, I think, to balance the rads (and yes, upstairs was getting all the warmth!) and my WC curve is currently set at 34/55 with a target temp of 20c, which is being met easily but with some with some cycling.  So, my latest settings are set at 19:00 on the 18th, so my aim is to tweak those later this evening to 34/54 and to see what happens tomorrow. Wish me luck!

I must add that I was amazed to learn that the Cosy installation instructions are freely available online, I assumed that they were tightly guarded, and at some point I might just log in to installers part of the hub for a quick look around...but not until I am happy with WC curve.  On that note, I too have drafted what I think the various possible curve might look like at various AOTs and explaining it to my wife has proved to be very helpful in making sure that I do actually grasp it.  

Thanks again to you both.  

 



   
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 JamesPa
(@jamespa)
Illustrious Member Moderator
Joined: 3 years ago
Posts: 3452
19/11/2025 5:46 pm  

Posted by: @kevh
↑

I have found myself in a very similar situation, and although I understand the basic principle of running low and slow, in practice it is difficult to achieve given the variables that are both under my control and not.  I think that the experiences that have been shared bear this out to be a common problem across Coy users, if not across ASHP users per se

Cosy if the only heat pump I know (and I have looked at the manuals to more than a dozen) which imposes such a high minimum flow temperature @ the cold end.  Its no good for UFH for sure, so presumably they fit Daikins in properties with UFH (or simply dont accept such properties, or possibly run on fixed FT?).   I have no idea why, and no idea whether the settings in the installer menu can override, because the installation manual doesn't say what the limits are.

Posted by: @kevh
↑

Wish me luck!

Good luck!

Posted by: @kevh
↑

which is being met easily but with some with some cycling

In current UK conditions it should be defrosting occasionally (which can seem like cycling, but you can easily tell the difference from the cloud of steam), but not cycling in the normal sense.  If its doing the latter other than in the height of the day it suggests that its well oversized (or that its bouncing off the thermostat so there is scope to reduce the FT further)

 

Here is what my Vaillant (running on pure WC) has been doing today - no cycling just 3 defrosts - the plot is of flow temperature

image

 

Here is the OAT, which, now I look at it, has been a rather good day to adjust heat pumps!

image

This post was modified 1 hour ago 5 times by JamesPa

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.


   
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 swwils
(@swwils)
Eminent Member Member
Joined: 2 months ago
Posts: 15
19/11/2025 6:39 pm  

@jamespa I would guess it's a deliberate choice due the compressor selection, it's minimum stable compression ratio is probably close when at 34C.

Any lower than this drives down the discharge pressure and R290 is already high volatility (low condensing pressure at mild temps).

As that ratio drops oil return becomes... Unreliable. Lol. It's relying on the velocity of the refrigerant to get oil back to the compressor.

I am not sure if emnerson have a oil injection tech like Panasonic?

In any case it's a scroll compressor so modulation at low end is effected because it can’t reduce swept volume anywhere near as much as a rotary.

Given the cold weather performance I am seeing it's a very good trade off.

 


This post was modified 14 minutes ago by swwils

   
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