That’s interesting, maybe.
I have been looking at your performance as a reference for how my heat pump is performing.
Maybe I am chasing something that isn’t the same as I am reporting?
My LWT and return used to calculate the COP is measure inside the house.
Your LWT is not what is actually entering the house?
Have I got that wrong?
If so your COP is lower and much more similar to mine?
I’m intrigued too! Maybe I’ll be able to eek a little more efficiency out of the unit 
Also, I did a 3.5 hr off period earlier, followed by a startup with the curve offset of “0C”. As expected, there was a full ramp up just like yours.
Thanks Stephen,
It’s all helping to establish if my heat pump is behaving as it should, or at least like others.
I feel a bit exposed with no support from Daikin or Octopus, I just want to make sure it is working properly.
Yep, ESPAltherma is sending measurements from the outdoor unit where the sensors are located inside.
That said, return water temp is also measured there too, so possibly balancing the descrepency out - power is a function of dT and flow rate after all, so if the dT is the same, it may not matter that much “where” it is measured if you see what I mean.
You may However be running a slightly higher flow temperature than me per measured values, which of course impacts CoP.
Lots and lots of variables the deeper you look!
It’s why it would be interesting for someone to have both heat meter and Espaltherma to see what differences appear.
Hi Stephen,
Yes, I always have to remind myself that the flow and return temperatures may not be exactly what is leaving and entering the heat pump as my measure are about 6 metres of pipe run from the heat pump.
No offence intended, but I would say that it looks like you are measuring the LWT before the last point it is recorded in the heat pump and I would assume taking the return temperature at the first point of measurement back to the heat pump.?
Your inlet water temperature above is 30.5c which is where it comes back to the heat pump and the actual LWT is 35.5c, it is those that give you the true COP. The 36.3c before the BUH and the 0.8c drop in temperature after the BUH is lost to the outside world and is not delivered heat to the home.
If I measured my LWT before the BUH my COP would be massively higher.
I don’t think that is really recording the heat delivered to your house, the LWT is overstated by 0.8c which makes quite a difference. The water entering your house must be at the temperature after the BUH?
It is of no concern to me other than when comparing performance to work out whether my heat pump is working correctly.
Now I understand why I cannot get anywhere near your performance.
I’ll go to bed happier tonight 
Hi John,
Using the LWT before the BUH overstates the heat produced when the BUH is not in use and understates it when the BUH is in use.
The water leaves the heat pump after the BUH so that must be where you take the reading to calculate the heat produced as that is the temperature of the water going to the emitters
Anything else is misleading.
I have just spent three days trying to emulate a COP on a similar heat pump that is based on temperatures taken deep inside the heat pump.
It’s no wonder I was struggling to match it!
Matt - if it’s me your referring too, I am measuring the flow and return temps into the house - so after both heatpump and back up heater. My heat outputs are correct or slightly under-stated with the exception of Dec 2022 when I still had the back up enabled. The odd 5 mins of back up operation do not show in the heat output though
I’m not sure that’s correct given the BUH is bypassed as not enabled, so temperature there is likely not accurate. Losing almost a whole degree C here is just very unlikely- so doesn’t pass the sanity test.
Also, the dT most of the time as set at 5C is what is delivered when using the pre BUH figure and return figure, otherwise it would be a dT of ~4C a fair amount of time.
That said, I’m not going to push the accuracy of my readings in general as I’ve stated a number of times I’m not using a MID heat meter, im using the data direct from the unit. For me it’s about relative performance and the data from the unit is good enough for that.
I’ll leave things here I think.
No Colin,
It is not your heat pump, i know where you are measuring and I can rely on your data, it is much closer to mine a lot of the time and I always have to bear in mind your house is much larger than mine so comparison isn’t always meaningful.
Don’t get offended though, I do still look at your data
No, it is Stephen’s in Cambridgeshire, still an 11kW but much closer house size to mine.
Stephen has been really helpful but I now realise he is using the LWT before the BUH to calculate the heat output, the temperature is 0.8c higher than the temperature after the BUH where it then enters the house.
The COP is overstated by some way and when I compensate for that our COP’s are actually very similar.
That makes me happy.
I am on the right path with it.
Hi Stephen,
I don’t know for sure but I would assume that the water passes through the BUH whatever, disabling it just stops the heater consuming electricity, I can’t imagine there is some sort of diverter valve? It’s just a direct heater as far as I am aware?
My dT is set at 8c, I am currently seeing 5.9c, I’m not sure that can be relied upon.
But in any case your return temperature is 30.5c and flow after BUH is 35.5c, a dT of exactly 5.0c as you asked for and expect, it is the 36.3c that outlies your dT of 5c and it must be the wrong figure to use. Also looking at your data it shows a constant dT of 5.8c, a constant difference of 0.8c which is the difference between before and after the BUH.
Maybe take a measurement on the pipework coming into the house?
It doesn’t really matter to me what you use, it’s just useful to know what I am comparing to when trying to optimise my performance.
It’s all academic for my non-mid metered system, but the following is interesting.
A dT of 5.8C was showing on leaving water pre BUH and return temp. I.e. the data I post to Emoncms.
Just taking a looking the primary flow and return directly as they come into the house, with the FLIR camera I have and some black tape over the pipe (to get around the emissivity issue with copper), and the dT hovered around 5.8C +/- 0.2C……
What is also odd is the Daikin controller shows the higher and lower figure as well (rounded up to nearest 1C), just the opposite way around to ESPAltherma - which is an arbitrary label that could have been incorrectly applied to each sensor.
I should also reenforce running hours for BUH are zero, and not switched on at the breaker.
The schematic for the BUH does indeed show it as inline, so no extra energy coming from there, yet showing higher on the Daikin unit by up to 1C:
So this could all just be a labelling issue in ESPAltherma.
I get that a MID heat meter is the optimal solution here, but I don’t have that so I will make do with the same data the unit is actually operating on.
I’m not convinced I have an issue therefore unless others can take a look on their setup and get to a different conclusion.
I think it is the same way round, LWT is what is coming out of the heat pump into the house, 36c, and inlet plate heat exchanger is the return, 31c
I just checked mine
Per MMI
LWT 37c
Before BUH 38c
Return 31c
Per my heat meter
LWT 37.1c
Return 31.2c
It is the LWT that is coming into my house, not the temperature before the BUH.
Our data is not comparable because we are measuring different things to arrive at the heat produced.
The flow temperature you are using is inside the heat pump before it has passed through a component part of the heat pump.
If I adjust your figures then our COPs are comparable which is what I would expect.
In the diagram your 37c is measured between f and g
Your 36c is measured between g and b
It is the measurement between g and b that is going into your house, 36c, not the 37c you are taking from between f and g.
The water cools down as it passes through the BUH, that is why you have 37c before it and 36c after, mine is the same, but it is the after the BUH (between g and b 36c) that goes to your house, not the before…
You are overstating the heat produced.
I think I have to agree that the actual LWT is at R2T, post BUH, and more disturbingly than @Stephen_Crown 's mere 0.8C, mine is consistently 1.8C less! Not that actually matters in respect of the value of the ASHP to me, but I can see that it’s made your efforts a bit more stressed than they need have been, @matt-drummer ! And I’m not too concerned about the ASHP League Table, so that’s ok. I will be changing my monitoring setup - I just wish I’d noticed the diagram with the measuring points before I went with the values I did.
How do I get the BUH bypassed?! 
Stupid thing - that’s a vast amount of heat to lose!
Right. @matt-drummer you have convinced me to change the sensor data sent to Emoncms. 
Hi Stephen and John,
I am glad it came up.
Like John says and I think Stephen agrees, I don’t care about league tables or beating anybody.
I do like to think this is a reliable reference source and it has really helped my with my heat pump.
But the value of that is distorted if not all the information is comparable, or maybe more correctly, it’s worth less if you don’t know what the differences are.
It has caused me a bit of stress for the last few days, all my own fault of course, as it usually is!
I know I am a bit of a pain at times and I sincerely apologise to those that get fed up with me but sometimes a bit of persistence pays off and in a purely selfish way i get the information I need.
Like most of this, I hope it helps other people in the future, who knew a heating system would be this tricky!
Maybe we can just take it out and replace it with a piece of insulated pipe?
I’m not joking!
Maybe it’s a good idea?
Is this a design feature that can be improved upon?
In Stephen’s case a loss of 0.8c through the BUH on a dT of 5c makes quite a difference to efficiency.
A COP of 5.0 becomes a COP of 4.3 quite a difference 14% lower, that’s a lot to give up that could potentially be quite easily recovered.
I have just bought a LLH and Grundfos pump to supply my office and they both had insulating jackets available as optional extras. In the case of the LLH they claim the jacket reduces heat loss by 90%
I wonder whether a similar idea could be employed on our BUHs?
I’m pretty amazed at the temperature difference in my system - 1.8C seems like a lot, given it’s going to be at the flow temp after a relatively short period, and it seems to be insulated to some degree from the manual pictures. I don’t think taking it out is a goer in the short term, you’d need the pipe and auto air bleed valve, which is located on the BUH itself, and obviously isn’t on the non-BUH model. I’ve got a box of 12 cans of expanding foam in the garage, maybe I should just inject it into the back section and fill the damn thing up 
How come yours is only 0.8C, @Stephen_Crown?? That’s what I want to know! Are you sure?!
My MMI was showing a 1c difference but the figures in the MMI are rounded so I don’t know exactly what it is.
What we don’t know yet is how far before the BUH the temperature is measured and where the heat loss actually occurs.
It may not be through the BUH, it could be measured way before the BUH and lost through some pipe work.
I think this is worthy of some investigation, it’s a serious heat loss on such low dTs
I don’t think the non BUH model has an auto air bleed in there, if I recall it has a manual one somewhere else but it would be pretty easy to add an auto air bleed valve.
Don’t think it’ll do much for the warranty though!
In the picture, “p” is the thermistor for LWT before BUH, and “c” is the LWT after BUH. So quite a bit before, arguably.
If there is no BUH, then the U-bend after “p” goes straight up, gets a (manual!) air bleed valve, and then loops back down into the pump, with no “c” thermistor. Interestingly, there’s no insulation shown on any of the pipes, but of course, this is a picture, not reality!
I will add to that investigation that I’ll create an issue and possibly a discussion over on GitHub re: sensor used for CoP calculations.
I have traced back to where I started using this sensor, and it appears in the repository’s main documentation page in the formula here:
It should be noted that this is correct for those models without the backup heater BUH as the 2nd temperature sensor does not exist.
Something still doesn’t sit right: 0.8C (even at > 30L/m!) is a lot of energy to lose in such a short space of time, @John is seeing a whopping 1.8C loss, and measuring the pipes just inside the house tied up closer to the other value, and this was around 9-10C outside, so not exactly extreme cold or anything.
Anyways, I’m sure we’ll get to the bottom of it, but for now I’ve created a new parallel feed and am as of now feeding that into the my heatpump app
