Having just had a heat pump install comprising a 6.5kW Grant Aerona R290 paired with a 250 litre Newark super cylinder, this is my first post and I wanted to start by saying it’s great to have the chance to join the OEM community.
Over the past couple of weeks I’ve been experimenting with my hot water strategies. In wanting to have a single re-heat each day that’ll satisfy our daily demand, I’ve settled for 50ºC as a set point and have been running re-heats during the early afternoon on “silent level 2” having heard this is a good way to enhance the efficiency.
The cycle starts off with a fabulous COP of c. 6 but I’ve noticed that there is a sudden drop in the heat output when the flow temp gets to around 50ºC (see below) and I’m just curious as to what’s happening. I’m wondering if the throttling of the heat pump output on silent level 2 (40hz compressor speed and the 400 rpm fan speed) means that the heat pump is struggling to get the store temp above 45ºC? I’m going to try the same runs on silent level 1 next week to see what difference that makes.
Maybe try increasing the Temperature Correction setting? This is the target DT between the flow and return. I’ve got mine set to 5. It looks like the flow temperature is not increasing high enough for the cylinder set-point to be reached.
Thanks for the reply and sorry for the slow reply. Hoping to become a more active member of the OEM community now that our daughter has started going to school nursery and I’ve got a few more hours to spare.
Thanks for the suggestion. I’ve checked the controller and it’s already set to 5ºC. I’ve encountered some more “challenges” recently so I’ll post that to the existing thread on HW performance…
Just to say the HW issues are continuing and if anything getting more systemic. As we’re getting into the heating season, I thought I’d start trialling my “winter mode”, which is to run the DHW cycle overnight during the cheap overnight Intelligent Octopus Go window to a higher store temperature 50ºC in the hope that this will provide sufficient hot water for the following day.
I’m now seeing a similar decline in heat output during the course of the DHW run even when the heat pump is running at full power (i.e. not in silent mode) resulting in extended run times and deteriorating COPs. Here’s the run from last night:
I’m going to try increasing the DHW correction to 10ºC to see if that helps but think I’ll also get in touch with Grant next week. Thinking a software update and opportunity to review the settings might be in order.
The dT between flow and return temperatures looks to be stable and the gradient is also pretty straight, so the drop in heat output could be due to the flow rate reducing or a measurement error.
How is temperature and heat output being measured? Can you show flow rate on the graph?
For comparison, our 9kW Grant HP290 was doing a similar DHW run at the same time. Here are the readings. Note: these are the internal Grant sensors, not independent heat / electric meters. The heat pump was in Silent Mode Level 2, heating a 170L tank to 52°C.
There was a consistent dT of 4-5 °C between flow and return, and a 9°C temperature correction between flow T and target T to overcome our small cylinder coil. Reported heat output remained constant throughout the run, between 7 and 8 kW, with small reductions at the start and end (NB: absolute values may be inaccurate). Electricity demand rose during the run, as the flow temperature increased. There is a small drop in power input/output at 2.05 am, when the flow T rose above the target T.
Looking at the metrics from the heat pump, the compressor frequency, fan speed and flow rate were consistent throughout the run. Silent Mode 2 limited the compressor to 40 Hz and the fan speed to 400 rpm. The compressor frequency reduced to 35 Hz at 2.05 am, aligning with the reduction in power in/out when Flow T exceeded Target T.
This looks very different to your records. If there are other metrics from the DHW run that you’d like to compare against, let me know and I can look them up. Otherwise I suspect that a call to Grant might be the way forward. It could be sensible to get an engineer booked soon, before they get busy.
Many thanks for the reply. The temperature and heat output are being measured using the OEM kit and here’s last night’s run which includes the flow rate…
Many thanks for the reply. As I’ve indicated in my reply to Tim below, this was how my DHW cycles were performing before I went on holiday for a week and switched the DHW mode to off, and then back on as we were travelling home. So no change in any of the settings - just the tank sitting idle for a week and then not performing anywhere near as well as it was before we went away. All very puzzling!
I followed your lead in opting for a 250 litre Newark “super cylinder” which has a 6 m2 coil. As mentioned in my reply to @Timbones , it was performing fantastically (and as expected) until the end of August with COPs of 4.5+.
It’s only since I switched the DHW mode off and went away to Scotland for a week that the performance has suddenly deteriorated. So the only thing that’s changed is that I switched it off for a week and then turned it back on again with the same settings…
I’ve checked and it seems to be pretty securely attached. I’ve got the IMS lead engineer coming on the 8th October to fit the DHW sensor so I’ll get him to take a look then.
In the meantime, I might also see if I can get a Grant engineer out to update the firmware and see if that helps. Tried running a DHW hot water tun on silent mode 1 last night and it didn’t get to the target temperature of 50ºC in three hours!
Good to hear from and thanks for the message. Here’s the link to my dashboard - HeatpumpMonitor.org
The DHW is still reaching my set point of 50ºC (rationale here is to heat up using cheap overnight window and use as a thermal store) but last night for example it took 2.5 hours with a COP of little more that 2. Flow rate is around 19l/min.
If you look back to the summer, you’ll see that I was routinely getting a COP of >4.5 for the DHW runs - admittedly with warmer air temps and a lower set point of 45ºC. I think the flow rate was the same too, i.e. I don’t think that has changed.
It was going away for a week, switching the DHW to “off” and then turning it on when I came back that coincided with a marked drop in performance and in particular an inability to sustain a consistent heat output at dT. Just have no idea what could have changed and have the installer coming around next week.
Thanks for the link. I’ve been puzzling over this and can’t get my head around how the heat output could be below the electricity input. Literally negative COP towards the end of the cycle.
As Dan says I feel it could be a monitoring issue unless the hot water is somehow bypassing the heat meter when on DHW, it’s probably breaking the laws of thermodynamics*
The only think giving me pause is that you have noticed that it is taking longer to heat up too and normal CH is unaffected. Its very odd!
