@sandman1600  Im glad the analysis helps with understanding at least, that particular plot was a real goldmine.  3sq m surface area for the DHW coil is pretty standard and sufficient.

One thing that occurs is that the buffer (which ideally shouldn't be there) may be contributing to the very short on period during cycling and if so increasing the flow rate from the HP may make things worse if, for example, it results in excess 'recirculation' of the water from the HP.

Ideally you want the flow rate from the HP and the flow rate to the emitters to be balanced ie for it to operate as if the buffer weren't there).  One way to do this is to buy at least 2, and preferably 4, thermometers (£10 the lot from Amazon), calibrate them by clamping them all to a single pipe, noting the differences, then firmlty clamp one to each port of the buffer and insulate.  Adjust pump speeds for minimum temp drop between flow from HP and flow to emitters, subject to their being at least a couple of degree C (preferably around 5 at design OAT) between flow and return on both sides.  This is more difficult to do now its warmer outside, but you can still do it by artificially increasing flow temperature, letting things stabilise, and then making the adjustments (letting things stabilise each time you adjust).

It may be that you can get this sorted out without resorting to the above but if not I would suggest that investing in £10 worth of thermometers is worthwhile.

I cant find an eco mode for heating (I believe there is for DHW) but the NR modes may well suppress the initial spike.

@jamespa Thanks for this.

The COP for yesterday was 3.57 (heating 3.79 water 0.87) so the DHW eco setting might be making a difference but I'll keep monitoring. There was a lot of cycling overnight but the OAT was 9 or 10 degrees for most of the night/early morning.

I'll try adjusting quiet mode on the heat pump to see if that has any further impact. The thermometer process sounds like a good investment. And the pump speed adjustments would be changes I'd make on the Mitsubishi control panel?

I've attached the graphs for the last couple of days but I'm assuming a lot of this could be down to the milder weather.

It looks like Eco mode may have tamed the DHW curve a bit (see below), but a COP of .87 is less than 1 and you would be better off heating on the immersion!  Suggest to monitor over a few days. 

Heating COP of 3.9 isnt great for this time of year, but with a buffer tank and oversized heat pump is probably not unexpected.  The impact of the latter will be particularly severe at this time of year although the impact of the former somewhat less.  If you can get some thermometers on the ports of the buffer tank that should tell us what is going on and whether there is any scope for optimisation.  However to do the measurement reliably you need (a) first to calibrate the thermometers by strapping them to the same pipe and noting differences and (b) to whack up the flow temperature so that there is a decent DT across the radiators.  Not sure where pump speed is changed, it depends on make and model(s).  The secondary pump speed has to be changed on the pump itself unless the installer has wired a PWM signal from the controller, which I doubt.

Normal

eco

@jamespa Thanks James.

The DHW COP is the worst we've had yet strangely. And there were no big drains on the water yesterday. It's like it's heating less frequently but having to use more energy to heat when it does kick in. I'll give it a few more days to see what it performs like.

Posted by: @sandman1600

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@jamespa Thanks James.

The DHW COP is the worst we've had yet strangely. And there were no big drains on the water yesterday. It's like it's heating less frequently but having to use more energy to heat when it does kick in. I'll give it a few more days to see what it performs like.

 

Sensible. 

After you have done that perhaps consider what is triggering reheat.  It looks like its DHW tank temperature alone not a timer plus DHW tank temperature.  I suspect that, when it reheats, there is still a lot of hot water at the top of the tank which could be drawn off and that only the sensor (typically two thirds down the tank) is in cool water.  Could you get away with heating once per day around midday, when its likely to be warmest outside, rather than every time the temperature two thirds of the way down a likely stratified tank drops below 42 or whatever the figure is.