@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.  

Posted by: @kevh

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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

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Wish me luck!

Good luck!

Posted by: @kevh

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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

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

@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.

@jamespa I have cosy6 and UFH, the maximum flow winter temp is set at 50 on mine, checking the UFH manifold stats today showed a steady input of 37, return of 27. No mixing valve or pump on the system.

No idea if the HP has been told I have UFH, I haven’t seen temps near the maximum the App shows WC could use, anyone know why?

@jamespa Yes, I now get that the 50c top end of the wc range is too high, though I I can still try dropping the setting back for 5 nights (55 to 50) and see where that lands me. If it is still too high, I’ll follow your advice and tweak the target temp up…is that right?

Now, it has got me thinking. Looking at my existing curve, at todays AOT of around 2c, my FR should be around 42/43c, a good 9c above yours. Thinking back to a comment by TJ (the Octopus bod who pops up occasionally) the target ought be about ‘FR+15c’, which suggests that at AOT of 2c and a target indoor temp of 20c, then my FR ought to be aound 35c.

Does this sound about right? I have no possible way of achieving that using WC however, although it is possible if I set a static setpoint. But do I really want to do that? No.

Thanks again for your input.

@jamespa just read the manual you linked to. 
very interesting section 3.3.2 restrictive requirements if microbore present. None of this was advised to me and Octopus knew I had rads on 10mm upstairs, and 16mm bore UFH without a blend pump.

This is why the system as initially installed failed to heat the upstairs, and backs up why my insistence to change all radiator feeds to 15mm has worked well for me.

im just looking at room temperatures again now and will be adjusting the lock valves to get a balanced heat for each bedroom

reading section 3.6.1A and B

the instructions have an error in the description of the MCB and RCBO for the cosy6, and the Immersion/hub

The labels on the fuse box are opposite the description in 3.6.1.

They’ve got the type of MCB and RCBO in swapped between the CU diagram and the text below.

@jamespa Yup, 50c is minimum cold weather flow temp.

Section 7 states the Cosy 6 is 5.63kW and can achieve COP of 4.29

Posted by: @swwils

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@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.

 

That's interesting.  We don't know what design trade offs octopus, or for that matter any other heat pump manufacturer, have made. 

Octopus however are particularly interesting, they are vertically integrated from energy supply through heat pump design through installation.  Sofaik nobody else spans that space.

They are therefore in a unique position to deliver an attractive offering or lock you in to a bad one.  Which is it I wonder.

@jamespa the insides are very well done. High quality compressor and inverter. 

Such a large monolithic case is also an achievement in itself. 

Personally I think the case geometry is going to be a cause for future trouble, but I'd love to be proven wrong!

@jamespa I chose Octopus as they will be in business far longer than any individual company, their heat pump uses R290, which is supposed to be state of the art better than R134, will see how the HP manages this second winter that I’ve owned it