@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

↑

@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.

Just a bit of an update... thanks to the information you've shared with me, I was able to argue the case of the oversizing a little better.

They revisited last week (somebody more "technical") and did another heat loss report. This time it came down to around 11 so they've agree to swap the heat pump with the 11.2 Ecodan. Which they did this morning.

I'd argued that this was still oversized but they're adamant that they can only work to the report they've generated. However, they're open to review it as the year progresses if the problems persist.

Thankfully it's been relatively painless to get them to switch this out for another brand new unit. I'm hoping the issues may be more manageable with the slightly smaller unit but not ideal. 

Just a follow up… first full day of the new heat pump… worst COP in weeks. 

COP down from a regular 3.4 to 3.8 to 2.7 yesterday (that’s just heating, not DHW). 

Freaked me out a little as I was hoping to see a mild improvement rather than a worsening.

Will keep monitoring. 

Sorry to hear this, it shouldn't happen!

Did the installers bump up the WC curve again and/or reinstate control by thermostat, undoing all your good work (in which case, I suggest to put it back as it was)?  Is it worth posting a plot or two?

Did you ever measure the temperatures at the buffer tank BTW?  Its just possible they could have made the efficiency loss here worse (another reason to hate buffer tanks - as if there weren't enough already!) 

In the end heat pump efficiency is very simple, COP goes up as OAT goes up, and as FT goes down.  So anything which causes FT to go up will reduce COP.

@jamespa Thankfully they didn’t touch the settings. They checked the pressure on the tank but that’s all.

The only work they did was on the external unit.

We’d been using around 13k per day for heating for the week proceeding and this rose to 18 yesterday.

We woke up to frost yesterday morning for the first time in awhile so maybe there was some over compensating going on for that. 

I’ve “reset” some settings like the quiet mode and the pump speed so I can assess if they’re appropriate for the new pump.

I’ve attached the graph from the last 24 hours but I’ll keep monitoring over the next few days. Ignore the hot water… I’d inadvertently turned off the eco setting and we used a lot of water yesterday. 

Posted by: @sandman1600

↑

We’d been using around 13k per day for heating for the week proceeding and this rose to 18 yesterday.

We woke up to frost yesterday morning for the first time in awhile so maybe there was some over compensating going on for that. 

Its actually very cold at night just at present, mine is runnoing at upper 30s flow temperature and going into defrost mode overnight!

Posted by: @sandman1600

↑

I’ve attached the graph from the last 24 hours but I’ll keep monitoring over the next few days. Ignore the hot water… I’d inadvertently turned off the eco setting and we used a lot of water yesterday. 

Looks like your flow temp is between 35 and 30 and you have got some periods of pretty continuous heating, which is good although even then it still seems to cycle, albeit only briefly.

Maybe a few more days will help clarify if there are further tweaks to be made.  The switch from 14kW-11kW will most likely have most effect in the OAT 6-10 range, which is where the OAT is much of the time, but not currently!  I do hope this works out!

@jamespa It does appear to be going into defrost more frequently than the previous pump at similar temperatures. Would that account for the higher energy usage? Hopefully not the shape of things to come.

Probably just a knee jerk reaction from me but it's just alarming to see COP that low. That's almost as bad as before I'd started adjusting the weather curve.

I'll keep an eye on it over the next few days and see if things even out again.

Posted by: @sandman1600

↑

@jamespa It does appear to be going into defrost more frequently than the previous pump at similar temperatures. Would that account for the higher energy usage? 

It might actually account for more defrosts because the compressor is working harder.  On the positive side it should cycle less at mild temperatures!

I think we are probably well overanalysing one day, the weather is very odd at present!