Here's an example from our today, showing continuous running (with defrost stops) up to this afternoon, where the temperature rises and then there's a 20-30 minute pause every couple of hours.
Ah, many thanks for explaining. That makes sense! I've no idea how often ours defrosts, but there are no problems with either hot water or heating so assume it's happening as it should when required.
@lizzie I realise you didn’t ask for this but, just in case it helps in some way, here are the times our Daikin 8 kW system defrosted over the last week against OAT. Regards, Toodles.
Toodles, he heats his home with cold draughts and cooks his food with magnets.
@lizzie I realise you didn’t ask for this but, just in case it helps in some way, here are the times our Daikin 8 kW system defrosted over the last week against OAT. Regards, Toodles.
Thanks, Toodles. Interesting to see - no defrosts once the temperature reaches 5 C. I am sure ours defrosts still at higher temperatures but we are in a damp part of the world - south-west Scotland.
Thanks for all the faithful contributions - both from yourself and the other regulars!
@lizzie Thank you Lizzie, you are most welcome! As the fly crows, we are little more than a mile (perhaps 2 kms.) south of the River Thames and see plenty of mist but our heatpump regularly reports a temperature approx. 1 degree C higher than local met measurements. (possibly this is that we are in a warm spot or the probe is slightly high in its’ readings - who knows?) Our heat pump runs 24/7 but it does not service our DHW needs as these are provided by a Sunamp Thermino heat battery. Regards, Toodles.
I have a Nibe SMO 20 controller paired with an 8KW F2040 heat pump.
One peculiarity of this Nibe system is how it manages degree minutes and controls compressor frequency. Unlike other manufacturers, the compressor frequency isn’t governed by the leaving water temperature (LWT).
By default, my heat pump tended to select a higher frequency than necessary. As a result, it would produce a higher flow rate than calculated, quickly reducing the degree minutes to zero and then shutting off. This led to frequent cycling, which is something I sought to minimise.
To address this issue, I accessed the installer settings (hold the back arrow for 7 seconds) and selected a custom compressor curve based on outdoor temperature. Fine-tuning this curve took considerable time, and you can only choose the two endpoints, but now my system operates at a lower flow temperature for extended periods. This approach prevents the system from frequently overshooting the calculated flow temperature and reduces cycling. To me, this seems more efficient, as the system maintains a steadier operation without consistently exceeding its targets.
From what I’ve read, the software doesn’t adjust the compressor frequency quickly enough when degree minutes either exceed or recover from certain thresholds.
There are in-depth discussions online (mostly in Swedish) about these limitations in Nibe systems, and while many users have tried to push for software updates, there’s been little success. Adjusting the compressor curve manually has been the most effective solution for me, resolving my issue around excessive cycling.
In respect of cycling in default mode, my unit is using circa 2.5kW of power, flow temp reaching 50C, at an outside temperature of around 1C, it will defrost every 50mins to 1hr.
If I use my custom curve, the power use would be circa 0.8kW, the flow temp reaching 40C which is the calculated flow temp at 1C, and will defrost once every 3hours!
Instinctively this feels like how ours is behaving. And worth exploring for sure. Will have a read of a google translate version of the link thanks very much for sharing.
@luke would love to hear more about how you went about the fine tuning and what settings you ended up at on the custom curve. Was it trial and error? How did you monitor what was happening with each adjustment? Do you have access to the full controller data via Ethernet or not? I can see the regular spikes on my energy consumption that align with the compressor working hard but I don’t yet have the full data of the kind shared above.
I spent several days adjusting it. The easiest part was tweaking the lowest temperature setting (for the warmer end) when the weather was stable and mild. I figured out the right flow temperature based on how the system was running on and off and took the average flow temperature. Then, I gradually lowered the compressor’s temperature setting during mild weather. When it got colder outside and the system couldn't reach the desired flow temperature, I adjusted the lower limit.
If you look in the settings at the compressor graph it will make more sense than what I am saying above.
It’s not perfect, though. Between 1°C and 5°C, it doesn’t work as well because of frequent defrost cycles. But below -4°C, it works better, although it uses more energy, as you'd expect. So, when it’s in the defrost zone, I just let the room temperature drop a bit and use the fire to keep the lounge warm.
In terms of monitoring, I have the unit connected via ethernet to the internet. I then use the Uplink API to connect into Home Assistant, where I can get most values from the unit and I can make some adjustments to other parameters.
I have a Shelly PM that gives me power usage for the heatpump only.
I'm in London UK so 1-5 is pretty typical for a colder winter day, sometimes goes below zero but rarely to -4. Wonder if that means I can get the same improvements.
If I unclick the auto compressor curve the two points are set at -3 and 7.
Is the idea that if curve is shallower (lower than -3 and higher than 7) then that means the controller is less likely to tell the heat pump to sprint?
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