This may be totally unrelated to your problem but I was observing a similar saw tooth plot on a Mitsu with Melpump. The issue was caused by a leaking 3 way valve allowing water to bypass the heating loop.

I was able to diagnose the problem with temp probes on the relevent pipework

When you say converting the buffer tank, I presume to a volumiser?

Did you remove the secondary pump?

What happens if run in pure weather compensation. 

At 12C OAT you will almost inevitably get cycling, but I might have hoped the cycles would be longer?

Can you remind us what capacity heat pump, what house loss and design oat, size and construction of house?  

@bontwoody Thanks for the suggestion, but DHW is heated at 5:00 am on reduced tariff and I don't think that can be happening because there is no sign of extra heating taking place through leaking 3 way valve, throughout the day. We can top up with solar in the summer but just scrape through in the winter.

@jamespa Thank James. Yes, buffer tank changed to volumiser and second pump removed.

Very similar data at OAT of below 10C .

It is currently running in pure weather compensation, but have had to set at +5 to maintain comfort.

I tried lowering the high temp on the weather compensation thinking this might reduce the cycling, but the house temperature dropped even lower.

Pump speed change didn't seem to do anything

Downstairs rooms (including the master bedroom) hardly move above 19C and we did not experience this last year. 

It is 16kw PUHZ 120 ASHP.

342 sq m upside down house and for what it is worth? (presumably based on data over last month) MelPump say: "Your house needs 35.91 kWh/ sq m per year. That is a very good result ( energy efficient house)" 

I have some suspicions (16kW for 340 sq m is not 'energy efficient!') but before I make any concrete suggestions can you please clear some matters up please

Posted by: @davidalgarve

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"Your house needs 35.91 kWh/ sq m per year. That is a very good result ( energy efficient house)" 

Is that energy to the house or energy to the heat pump.  Presumably you know from other data or your electricity meter

What's the construction of the house and what temperature do you design for in the Algarve (which presumably is where you are)

Posted by: @davidalgarve

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It is currently running in pure weather compensation, but have had to set at +5 to maintain comfort.

What does that mean.  +5 relative to what and at what temperature?

What emitters do you have, how were they sized?

What settings were you running on last year?

Is the volumiser in flow or return (is there any possibility it connects the two)?  Could you please post a picture showing the plumbing?

What is the flow rate in l/min or however the heat pump reports it (it will know)?  Is the water pump you removed the same model, or different to the one that remains (I think Ecodans have external pumps)

Whats the vertical scale on the graph, its hidden unfortunately?

Sorry to ask lots of questions but without some more detail its impossible to diagnose with any certainty.

Posted by: @davidalgarve

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It is 16kw PUHZ 120 ASHP.

342 sq m upside down house and for what it is worth? (presumably based on data over last month) MelPump say: "Your house needs 35.91 kWh/ sq m per year. That is a very good result ( energy efficient house)" 

I haven't been able to locate the outdoor unit model number here or in your signature, but a 120 in the part number suggests it's 12kW rather than 16kW

There is a note on the page of MELPump about your energy data taking some time to build the full set and can be overvalued in the first few months

@f1p 

OK noted

Without further information about the system I cant really comment with any certainty, all I can do is cite generic statements

• Cycling is to be expected at mild OATs and certainly at 11C plus and you cant expect to stop it entirely.  If the heat pump is oversized then the onset of cycling will be at a lower OAT.
• Somewhere in an earlier post and thread you mentioned undersized emitters, which could account for not reaching the desired room temperature. 
• Depending on how you have re-plumbed the volumiser it may or may not be properly engaged so may or may not be properly contributing to the system volume. 
• DT is a bit high for mild temperatures suggesting perhaps inadequate system flow, or it may be that the heat pump modulates water pump speed down to maintain a set DT.

Any of these could be relevant, the information on the system will help determine which and what further might be changed.

Posted by: @davidalgarve

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COP figures (3.0 to 3.6)

@davidalgarve I dream of that sort of COP. I have a Mitsubishi 11.2kw ASHP (currently managed by Havenwise (still in trial period) ) and only achieve a COP of 2.4 Total. 2.4 Heating. 1.6 DHW. 🙄 

@f1p You have better access to information than I have, but I took the information from the Ecodan Data Book, here:

@morgan Hi Morgan. Last year, operating on weather compensation and based on Mitsu figures, I was getting around 3.2 in December & January. When I went onto Havenwise in December, I was getting similar figures  to yours (2.4-2.6), but the house temperature was down. I accept that the latter was partially influenced by the necessary location of PAR 60 wireless controller and (according to Havenwise) its granularity.

I thought that there had to be more to it than that, (particularly after converting the buffer tank), hence deciding to go back to straight weather compensation and gathering information from MelPump.

So far, in January, MelPump are reporting daily COP figures of around 3.5, but I note f1p's caution above.

Unfortunately, now I have the data, my ancient brain can't make any sense of it and I am getting complaints that the house is not warm enough!  

@davidalgarve 

Would you be willing to in MELPump > Detailed Report > Share As URL 

So we can see a detailed and interactive chart please?