I have an update on our heating: The installer (British Gas) in early April exchanged our 7kW Vaillant aerotherm+ with the 5kW version! I am very grateful for them to acknowledge that the 7kW unit was oversized* and they showed the goodwill to get us a new heat pump!
Of course it is very new and now spring, but we did have a few cold nights. I think the difference to before is already quite clear, I have put in a graph below for comparison. Our heating COP with the new unit so far is 4.7 (3.5 for DHW, 4.6 all included) and I am very happy indeed.
A lot of credit to the British Gas heat pump team! I think, when I got the install when they just started, at least here in our area, and I have the impression that they improved over the last 2 years. Replacing a heat-pump is very expensive, and I think they will be much more cautious to size correctly, to avoid them costly complaints (like mine) in the future. I went with BG as a big company after a rather bad after-sales experience from a small company that installed our solar panels. BG have a formal complaints process, and while it took me some nerves to navigate through, I think I got very good after-sales support. it paid out in the end and I don’t regret having chosen them. The general installation of the new radiators was done well from the start anyway, and with their size we can use low flow temperatures (heat curve 0.35) and the house gets nice and warm throughout. It seems the main issue was really that the unit was just oversized. The 4-port buffer likely throttled the efficiency even further. The smaller ASHP with the lower flow rate might have matched the circulation pump rate after the buffer better, so the buffer would not have affected the efficiency as much I guess, but now with the volumiser set-up we certainly save energy for the pump at the least.
*I did not understand previously why the 7kW unit was so inefficient, but I think this is simply because it doesn’t run efficiently with a dT of only around 1K. Lowering the flow rate by throttling some valves had apparently improved the overall efficiency considerably, but the best we got with that seemed to head for a SCOP of around 3 at most. I think the ASHP needs to have the nominal flow rate to work optimally (heat transfer from refrigerant to water?), while our house demanded a much lower flow rate than the 1205l/h of the 7kW unit. The lower flow rate (around 900 l/h I think was a compromise between the optimum of our house and the ASHP unit, so not really great. I am pretty sure that the 7kW unit was not cycling much at all, which is why I struggled to understand the bad efficiency. It seems that the controls just kept it going and that the low dT (due to high flow rate) and long run times at comparably high electricity consumption was to blame. The 5kW unit now runs at lower consumption for the same compressor speeds. I think it is much quieter, which might be that the compressor is smaller but also that the flow rate is much lower.
So to solve our problem unfortunately, the only solution was the expensive route of a new, smaller ASHP unit. Luckily, BG did step up and got that done for us. The previous, 2 year old 7kW unit apparently now has a new job in a training centre, so it seems to be a happy ending. Below is a graph showing the daily COP (heating only, daily average indoor T minus outdoor T), green before pipe change i.e. with 4-port buffer, red: after buffer changed to volumiser, yellow: ASHP unit swap from 7kW to 5kW unit. I think the majority of days are around 9K difference outdoor to indoor temperature, so I think the SCOP could be around 4.
This video from John Cantor shows (about 20 mins in) curves of compressor efficiency vs modulation ratio. Its easy to see from this that an oversized heat pump has the potential to underperform. Additionally an oversized heat pump will cycle more which may further degrade the COP.
I have heard several people say that low deltaT is a further cause for underperformance but none of them have yet come up with a mechanism why that would be the case, other than any degradation in the control loop resulting from noise on the measurements. That doesn’t mean that this is not a cause, but until someone proposes a mechanism I personally will continue to treat the assertion with ‘healthy’ scepticism.
Vaillant is really good at dealing with oversizing. My 10 kW aroTherm is 2x oversized at -10°C and even more at higher temperatures. Cycling is still not terrible (1x/hour) and COP is essentially as claimed in the datasheet. From the same datasheets, you can see that the lower the modulation the higher COP is. My heat pump never runs continuously and I still get a heating SCOP of > 4.5.
Hi, I don’t know exactly, why, but the data clearly show that the previous heat pump was running less efficient than the new one. It wa inefficient s from the start, so I think age of the unit was not the cause. Cycling was also not the case.
No fault was found in the service by Vaillant. I don’t know what else remains an explanation, other than that the larger 7kW ASHP was running a bit outside of is optimum conditions. The larger compressor has a larger minimum consumption it seems.
Yesterday I checked and the dT was also only 2K, but consumption was also only 0.4kW so resulting in a COP of 5 at 860l/h if I remember correctly. Low dT simply means less heat transferred at the same flow rate, so if you say dT is not relevant, than it’s all about consumption. I don’t know why the unit consumed more energy for the same heat, but it did if the values shown were correct.
It seems that the app is now showing more sensible looking values where it was rounding a lot before, so I cannot exclude this as an issue, however our energy bill seemed to agree well with the app in general.
Anyone help with this? Last couple of days I’ve been getting error towards the end of my DHW heating cycle, mainly just the afternoon heating rather than the evening DHW run.
My 12kw unit often seems to run with a positive integral. Am I alone in this ?
My understanding is that it should switch off when the integral rises to 0 and on again at -60.
For those who are still running their HP’s I’d be grateful if you could take a look at the integral on the interface if it’s Active and let me know if you see a positive value.
Same for me. The compressor switches off at 0°min but the flow temperature will stay above its target for quite some time afterwards, so the integral accumulates beyond 0°min. This is normal and expected behaviour.
Hi. Sorry I should have been clearer. I’m referring to the integral being > 0 AND the pump showing as Active in the interface and consuming electricity.
And it’s not just that sometimes it doesn’t stop at 0. Here’s what’s just happened…
11:32 -60 Prerun (as expected)
11:33 -58 (as expected with target of 28.5 and actual of 30.8)
Then what I’d expect for a while then
11:41 -42 T30.3 A28.5 Active
11:42 -41 T45 A27.6 Overrun
So it’s switching off though integral hasn’t reached 0. Note then when it’s not running the target seems to always be 45.
Am I right in thinking the way to monitor the integral and temps as you do is with the ebusd adapter. Is all I need the adapter which can connect via Wi-Fi to home assistant running in an old pc ?
I had been wondering about this but there doesn’t seem to be a readily available integration for ebusd and HA that includes the heat pump specific variables. Have I missed something?
Im sure I could configure something if I put my mind to it but a ready made solution would be preferable.
One specific config version caused issues with Home Assistant, but that is fixed. No idea about cascade, but you can run ebusd without connecting to HA and just see what it detects and work your way up from there.
Thanks. I will take a look. I currently monitor via the MyVaillant integration with HA which is plug and play but omits some nice to have variables and relies on the internet.
I think, when I got the install when they just started, at least here in our area, and I have the impression that they improved over the last 2 years. Replacing a heat-pump is very expensive, and I think they will be much more cautious to size correctly, to avoid them costly complaints (like mine) in the future. I went with BG as a big company after a rather bad after-sales experience from a small company that installed our solar panels. BG have a formal complaints process, and while it took me some nerves to navigate through, I think I got very good after-sales support. it paid out in the end and I don’t regret having chosen them. The general installation of the new radiators was done well from the start anyway, and with their size we can use low flow temperatures (heat curve 0.35) and the house gets nice and warm throughout. It seems the main issue was really that the unit was just oversized. The 4-port buffer likely throttled the efficiency even further. The smaller ASHP with the lower flow rate might have matched the circulation pump rate after the buffer better, so the buffer would not have affected the efficiency as much I guess, but now with the volumiser set-up we certainly save energy for the pump at the least.
