Posted by: @l2jad

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my tank is plumbed 4 pipes was told to do with defrost cycle.

Posted by: @editor

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@razz, unfortunately ‘hydraulic separation’ has become a comfort blanket phrase for poor installers. It’s not inherently evil, but it’s also not a free lunch. A 4-port buffer guarantees mixing on some level, and mixing means lower emitter temperatures, worse efficiency and higher bills. 

What I find genuinely disconcerting is the idea that Octopus appear to be defaulting to a buffer or volumiser as policy. Very few homes ever genuinely need a buffer tank with a heat pump. A volumiser is only required where system volume is genuinely short, which again is not a blanket condition. 

The line about “Cosy 12 is new so we’re playing it safe” is also worrying. If the system needs hydraulic crutches because the model is new, that risk is being pushed straight onto the homeowner in the form of potentially higher bills and poorer comfort. Your experience (higher costs and a colder house) is exactly what you’d expect when a buffer is masking design issues rather than solving them.

This is why blanket approaches in heat pump design are dangerous. Heat pumps reward precision and punish laziness. Buffers (extremely rarely) and volumisers have a place, but when they become default fittings rather than last-resort tools, the system is already compromised.

I think its worth turning this question on its head in the case of Octopus and worth asking: notwithstanding the apparent limitations of their installations, which I agree are undeniable, and the current problems with Cosy 12, why does Octopus seem to deliver good results most of the time.  It definitely appears that it does deliver good results much of the time, so something they are doing must be right.  My suspicion is that they have taken advantage of their vertical integration and ability to do OTA updates to do some sensible things, but I don't know this and don't have any real evidence other than what we see reported here.  

@editor I struggle a bit with the logic of buffers but I think the minimum flow rate needed by the cosy 12 is 26 or 34l/min and there was a fear peoples systems couldn't achieve that. Our system wasn't achieving that

@jamespa, fair point about Octopus.

Posted by: @livinggreen

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@editor I struggle a bit with the logic of buffers but I think the minimum flow rate needed by the cosy 12 is 26 or 34l/min and there was a fear peoples systems couldn't achieve that. Our system wasn't achieving that

And a buffer tank doesn’t even guarantee minimum flow. We had a buffer and still suffered constant flow errors. Our system needed roughly 30 l/min and the heat pump would regularly trip on insufficient flow, so the installer response was simply to keep adding more circulation pumps.

Buffers don’t solve this. Once you introduce multiple pumps running at different speeds (as we’ve discussed ad nauseam on these forums) hydraulic distortion is almost inevitable. At that point you’re no longer fixing the underlying issue, you’re just masking it and hoping the controls can cope.

Somehow, as @jamespa, has mentioned above, Octopus have kept distortion to a minimum (but it's still there).

Posted by: @livinggreen

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@editor I struggle a bit with the logic of buffers but I think the minimum flow rate needed by the cosy 12 is 26 or 34l/min and there was a fear peoples systems couldn't achieve that. Our system wasn't achieving that

Yes but...  If the effect of the buffer is to allow the system to 'get away' with a lower flow rate to the emitters  (and thus higher DT for any given output) then that higher DT will reduce the average FT to the emitters for any given FT from the heat pump.  This means that the FT at the heat pump has to be set higher, reducing COP.  On the plus side if the flow rate from the heat pump is higher than the flow rate to the emitters, at least the hot water from the heat pump is pushed down the tank so there is less mixing of the flow, and thus you aren't suffering from this particular distortion mechanism. 

In summary there will be a penalty due to the first effect but perhaps not too harsh.  For example if the emitters are running at DT10 instead of DT5, then the FT would need to be raised by 2.5C to achieve the same output, corresponding to a performance penalty of around 8%.  

Of course it begs the question, why bother introducing the buffer, why not instead just accept a higher DT at the heat pump given that Octopus have control of this being vertically integrated.  Maybe thats something to do with the dynamics of the heat transfer mechanisms inside. 

@l2jad  regarding 'its to do with defrost' its true that buffers will add additional system volume which helps with defrost, but so will volumisers and volumisers, unlike buffers, dont cause a performance penalty. 

@jamespa I am finding all this chat about buffers and volumisers a bit confusing. Should I be asking TJ why a buffer was installed with my heatpump and not a volumiser which may affect my bills?

@harriworld

Who is TJ?

In answer to your question it depends on what the actual problem that the buffer is trying to solve is!

(a) If the underlying problem is that the emitter system cant sustain sufficient flow with the inbuilt water pump, then possible solutions are

1. just run at higher DT at the heat pump with or without a volumiser
2. install a buffer
3. install a more powerful or supplementary water pump

1 & 2 lead to approximately the same penalty but if all else is equal (1) is the simpler.  3 is penalty free, but viable only if the resultant flow velocities are acceptable.

(b) If the underlying problem is insufficient water volume for defrost or to avoid short cycling then a volumiser alone will suffice, and will not incur a penalty

(c) If there is no underlying problem then a volumiser is unlikely to cause any harm other than the additional install cost 

In cases (b) and (c), where there is no need for a buffer, its quite likely that there is a significant performance penalty if a buffer is unnecessarily added (rationale can be provided on request).

To do a design you need to know what the problem is, or indeed whether there is a problem in the first place!  The issue occurs when components are added 'to be safe' or 'because we always do it that way' or for reasons where an alternative component would be better.  In this case the likelihood that there is a penalty without a benefit is elevated.

Posted by: @jamespa

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Who is TJ?

TJ = Thomas Root, Heat Flexibility & Cosy Heat Pump Program Director, who made an appearance in this podcast episode:

@editor Thanks so much for this link Mars, have started watching it and will finish! Could have done with researching all these options before buying but good to know now (and I may well gave still gone with Octopus)

So I got an email from Octopus today saying they added a software update to my heat pump which should reduce the noise. The Octopus engineer has told me to set it to 21c in the day and 18c at night for the next few days, so he can monitor it. It seems to have reduced the noise from the external unit a fair bit from before, but it's still clearly audible in the house. It also seems to have reduced the buzzing (from pipes?) a lot too. It's still there, but less than before.  

It also looks like the HP is struggling to get up to 21c. The issue now is that 21c in the hallway, where the main temp sensor is, being 21 is no good cos my living room is at about 18c, which is too cold for my mum. It also doesn't eliminate the buzz from the secondary motor and buffer. 

Also, it looks like the radiator balancing is way off. Right now, my bathroom is 22.5c, hallway is 20.8c (target of 21c), and living room is 18.2c. 

How much longer must I put up with this? When do I just say enough is enough and demand they just install a boiler?

Also, what happens next winter? A lot of people seem to want to excuse the performance of these heat pumps saying "it's only really loud in winter", doesn't really sound like a valid excuse to me.

I think once you have a hp installed with the grant you can’t go back to a boiler? 

I must admit i haven’t tried to go above 19 degrees as that's comfortable for us, i expect it won’t be when it’s sub zero outside.  I’m getting a decent cop 4.78 presently.

did they do many radiator changes? Only place i don’t get at a decent temperature are the bathrooms as i got them to leave my towel rads, my choice. 
can you increase your flow temp? I know will probably use more energy but should up the house temperature ? 
guess your going to have to keep chasing octopus to get a resolution,

i’ve had a week from hell but last night was by far the best, will be seeing what happens tonight!! Keep pushing i doubt you’re the only one.