Posted by: @agentgeorge

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@andrewj have you tried setting it at 2 and 1 and recording what temperature the room gets to.

I did that in all my upstairs rooms with radiators and have successfully managed to control them at 18C and 19C, not much solar gain affected the readings this month 

I haven’t tried that but may do so.  I (my wife) wouldn’t want it below 21c when we go to bed so that’s my target.  Last night, I managed 21.8c so I shall be trying another small turn on the cap later today.

@jamespa The flow temp of my system is set at 36C on the Cosy Hub, the temp gauges on my UFH manifold show a flow of 37C and return of 27C, effective DT10 this gives a comfortable room temperature of 21C thru the downstairs. Upstairs is still radiators all but bathroom have TRV and each room is set at 18 or 19C by adjusting the TRV 1/2 a point until the room doesn't overheat. The bathroom has a towel rail that is adjusted with dual lock valves to give a quiet flow and temp of 21C.

it has taken all year with the system to gradually adjust it for both comfort and peak efficiency; I am now regularly getting a COP 4.5, only in the cold winter months with -6C days being the worst with a COP 3.6

The Heat Pump FT has to be adjusted to the house, there is no one temp suits all.

Think of the energy transfer in a motor car. If you have a huge engine, you get to cruising speed quickly and have to keep throttling back to stop speeding. Conversely if you have an underpowered engine, you have to rev it harder to get anywhere and keep your foot down to maintain cruising speed.

Posted by: @agentgeorge

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@jamespa The flow temp of my system is set at 36C on the Cosy Hub, the temp gauges on my UFH manifold show a flow of 37C and return of 27C, effective DT10 this gives a comfortable room temperature of 21C thru the downstairs. Upstairs is still radiators all but bathroom have TRV and each room is set at 18 or 19C by adjusting the TRV 1/2 a point until the room doesn't overheat. The bathroom has a towel rail that is adjusted with dual lock valves to give a quiet flow and temp of 21C.

it has taken all year with the system to gradually adjust it for both comfort and peak efficiency; I am now regularly getting a COP 4.5, only in the cold winter months with -6C days being the worst with a COP 3.6

The Heat Pump FT has to be adjusted to the house, there is no one temp suits all.

Think of the energy transfer in a motor car. If you have a huge engine, you get to cruising speed quickly and have to keep throttling back to stop speeding. Conversely if you have an underpowered engine, you have to rev it harder to get anywhere and keep your foot down to maintain cruising speed.

All understood and agreed. 

DT10 is quite a high DT however and in a UFH only scenario it would likely be more efficient to drop the flow temperature to (say) 34 and operate at DT 4 with higher flow rate (same average temperature thus same output)

Posted by: @agentgeorge

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Think of the energy transfer in a motor car. If you have a huge engine, you get to cruising speed quickly and have to keep throttling back to stop speeding. Conversely if you have an underpowered engine, you have to rev it harder to get anywhere and keep your foot down to maintain cruising speed.

Throttling back to a constant throttle setting is one thing, however if the way to get to cruising speed is alternatively to run it at alternately at full throttle and half throttle that is likely to make it less efficient.

Posted by: @agentgeorge

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The Heat Pump FT has to be adjusted to the house, there is no one temp suits all.

And that's the point.  There are definitely houses (typically low loss houses with UFH throughout) where the design FT is <35, even into the high 20s.  To limit the min FT to 35 therefore excludes these houses from the Cosy market.  I doubt that matters to Octopus because that is unlikely to be a significant proportion of their target.

Posted by: @andrewj

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That question @editor asked TJ came from me but as you say,  the response didn’t really answer the question and none of the other experts said anything about the topic unfortunately. 

It's the elephant in the room.  TJ dismissed the idea of running 'low and slow' out of hand, which effectively means that the system is designed as a gas boiler replacement utilising a modicum of weather compensation.  

For most average Octopus customers this is probably a good thing provided that they achieve hot radiators at reasonable cost with no fiddling.

I can honestly say that my house is certainly more comfortable with the Cosy, however I have tweaked and re tweaked both radiators and the WC to get here, and I am still looking for the promised performance.   I don't regret choosing a Cosy but am not totally sure that, knowing what I do now, I would have chosen it. 

@kevh Indeed, when is a heat pump actually more of a boiler?

@kevh hopefully they will release an advanced controls mode. 👍 Or a "quiet mode" that does what you want.

Posted by: @kevh

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I can honestly say that my house is certainly more comfortable with the Cosy, however I have tweaked and re tweaked both radiators and the WC to get here, and I am still looking for the promised performance.   I don't regret choosing a Cosy but am not totally sure that, knowing what I do now, I would have chosen it. 

Mostly my thoughts although I'm on the fence whether I regret it or not.  My other quote was £12k more from Heat Geek so in some respects it's a no-brainer.  If I'd gone for a third quote, would it have been closer to Octopus' price?  Doubt it, I think Octopus were buying the business - I needed 13 radiators replacing so it was never going to be as cheap from an independent given the base price of a HP and tank would have cost as much.  So....I don't think I would have chosen differently.  It's definitely more comfortable than gas heating and once my bedroom is habitable for me 🙂 then I'll be mostly happy.

Posted by: @swwils

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@kevh hopefully they will release an advanced controls mode. 👍 Or a "quiet mode" that does what you want.

If the hardware limits what can be done then I don't see what an advanced controls mode could actually do.  Currently, it is possible to run as "always on" but only if the necessary flow temperature WC curve lies within the technical limits of the unit.  Do you have any thoughts on what might be possible?

@andrewj mine seems to select a totally different control strategy f you put the temp up to an unachievable temp, as soon as it can't maintain it it settles right down. So they could easily just have a mode that does this all the way through instead of the massive ramps.

@swwils I tried that, but couldn’t get the flow temp down low enough to stop overheating.

@andrewj The heat pump survey should identify the heat loss of the building.

As your house is overheating, this points to the survey being wrong and your heat pump is oversized and your emitters are oversized

Octopus wanted to install an 8kW unit for my house, saying the 6kW one was not enough. Id already had a professional survey done that showed I could manage on 6kW. I had a long argument with them and finally won to get the 6kW unit. I also had 3 bedroom emitters resized, 2 incorrectly calculated, which I fixed by swapping them between rooms.

Instead of throttling back the oversized radiator, Id try reducing the size of the radiator in the bedroom that’s overheating. You can find charts that show the heat output of a radiator at lower flow temperatures so you can select one with less output. It worked for me in the one bedroom that continuously maxed out at 21C, and now settles at 19C.

@agentgeorge They redid the survey and determined the heat loss was 1kW less than original.  This also then identified that most of the radiators were oversized and they redid a radiator schedule.  This unfortunately specified a new total heat output 1kW higher than originally and didn’t suggest replacement for one that is supposedly 337% and one 240% more output than is required.  I’ve lost faith with them on that side and I’m not convinced the heat loss is right: I wouldn’t be confident they wouldn’t undersize replacements!  Notwithstanding, reducing the size of the radiators seems crazy, in general, but maybe not so when it appears the Cosy is limited in its flow temp range.  Anyway, I haven’t pushed this because my wife is happy with the temps we achieve and if they did undersize replacements it would be a bigger problem; we seem to get reasonably good COPs (high 3s, and into the 4s; low 3s when it’s been very cold - < 0c.  Also, I’m sick of decorating!!

I just want to reduce the temperature in the bedroom and then I’ll be cool with what we have, if you will excuse the pun.  Currently, it’s 21.5c so getting better: I’m beginning to think that actually a good chunk of heat is coming from rising air from downstairs being held by 300mm of roof insulation.