Posted by: @jamespa

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...  Houses have a lot of thermal mass.

Is 50C really the minimum low OAT flow temp, if so thats crazy crazy crazy and wont work at all with UFH so sounds wrong. 

Are you sure that there is not an installer setting for min permissible flow temp, which can be adjusted down.  Alternatively can you make the -10 figure even colder, which would have the same effect.  

Interestingly, being a timber framed house, all internal walls are plasterboard.  The internal walls which have an external facing are plasterboard backed by insulation backed by vapour barrier backed by brick or timber cladding.  The plaster board doesn't really retain a great deal of thermal heat in the same way that bricks do but the overall insulation is good.

Octopus give very little info on their apps so I know neither the flow rate nor if there's a way to impact the minimum and maximum cold/warm weather values: 50c-70c and 30c-40c respectively.  Defaults are 57c and 37c.  If there is an installer setting I'd have to go through Octopus to get it changed.  I don't have UFH and I suspect they would install a Daikin unit for those scenarios, but just to be clear that we are talking the same thing, I've posted the Flow Temp adjustment screen in the app.

I think internal temps are slowly dropping but there has been at least one period of time today when the Cosy wasn't running - it is again now, ticking over at around 600w.  

Posted by: @andrewj

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but just to be clear that we are talking the same thing, I've posted the Flow Temp adjustment screen in the app.

we are talking the same thing.  Its crazy if its really like that, but Octopus are, in fairness, pursuing a 'low installation cost, good enough' approach rather than the 'maximise cop at all costs' approach that many erly adopters have defaulted to. Is there an Octopus Cosy user forum where perhaps you could get some more info on this aspect, with no meaningful user manual we are all in the dark!

EDIT this may be the Cosy installation manual, if so then, as I suspected there are some possibly relevant installer settings see section 11.3.5, although its not clear whether there are hard limits on these or how they relate to what the app displays.

Posted by: @jamespa

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we are talking the same thing.  Its crazy if its really like that, but Octopus are, in fairness, pursuing a 'low installation cost, good enough' approach rather than the 'maximise cop at all costs' approach that many erly adopters have defaulted to. Is there an Octopus Cosy user forum where perhaps you could get some more info on this aspect, with no meaningful user manual we are all in the dark!

EDIT this may be the Cosy installation manual, if so then, as I suspected there are some possibly relevant installer settings see section 11.3.5, although its not clear whether there are hard limits on these or how they relate to what the app displays.

There is a Facebook group in which the lead engineer participates but he doesn't get involved in every discussion and his responses are often cryptic and I've never seen him follow up if someone asks for clarification.  Which is a shame really and he definitely doesn't advocate for SCOP chasing and holds the line that running on the Primary Pod thermostat is the right way to go.  In other words, the Cosy isn't specifically intended to run purely on flow temperature although it can do so (and I don't know if there are limitations built in to its firmware.)  It's probably why the default Flow Temp settings are how they are.  and the design specs are to the MCS specifications for room temps.  

That manual correlates to the flow temp settings I pasted above.  Apart from the "Target Flow Temperature" value - I don't know if the Cosy Controller is using that in some way or what it might pertain to.  The instructions in there for installers does align with what people, including me, see in the app when installed.  Adjustments are made from there if necessary.  It's an interesting cautionary note: the Cosy 9 (not sure about the Cosy 6) cannot modulate a flow temperature below 35c so if that is indicated by the WC curve then it starts cycling (although it does that by default based on the Primary Pod sensed temperature anyway).  I currently have it set at 33c so it may be a bit low.  The Senior engineer I just mentioned recommends this is 15c above desired set temp (so, in theory, 37c in my case as I am really aiming for 22c in the living area.)  I may have to tweak that up if it cannot maintain temps at 5c (probably that end of the curve rather than the cold weather end.)  Shame it doesn't go into commissioning approach as that might give more insight into why they want customers to run it the way they do.

The problem with making changes to these "hidden" configs is (a) access; and (b) unknown implications of changing them in terms of the Cosy itself (as opposed to ASHP operation principle in general.) 

Posted by: @jamespa

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Oversizing wont affect internal temperature which depends on the flow temperature (ie the WC curve adjustment) not the capacity of the heat pump (assuming its not undersized of course).  However if your house loss was overestimated (which is quite likely) then your radiators may be bigger than is needed for the flow temperature it was designed to work at, so the house will overheat if run at that flow temperature.  Not a problem, simply drop the flow temperature (WC curve) to fix that, which will also improve efficiency. 

I was thinking about this a bit.  It's probably not a problem if they are all similarly oversized otherwise the flow temperature might be too high for some and maybe not enough for others.  Not helped that they design the system to MCS room temperature standards.  It might explain why the radiator in my study heats the room up way more than, say, the living area (a good 2c-3c) even when the LSV is only open 0.25 turns.  That might be one for a decorator cap!