Posted by: @lfranklin

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Hi

We have had our ASHP for 10 months now. And it cycles like crazy?

currently on Water pump 3 setting. Compensation set to 30/44 

these are what I can see on monitoring tool and on controller:

 

@lfranklin From your consumption graph, I am not sure if this is just for your heat pump or the house, but the cycles appear to be approximately 5 per hour. It is generally considered that less than 6 cycles per hour is not harmful, although it might be less efficient. John Cantor has a YouTube video about cycling and gives an example of a 5kW Samsung heat pump cycling every 9 minutes, which he says is not harmful and only reduces COP by 3%.

So your cycling may not be such an issue. The explanation for cycling that is often given is that the ASHP can only modulate down to around 30% of its peak power output. If the heat given out by the radiators is less than this minimum figure, then the ASHP cycles off. The ASHP modulates the output down in this scenario as low as possible by reducing the difference in temperature between the leaving and returning water, known as Delta T. A PWM water pump can also be made to reduce the flow round the radiators further reducing the heat output. You will have fixed speed water pumps, so reducing the flow rate dynamically is not an option.

I have a similar G L installed 12kW Samsung system to yours, only I have now had my buffer tank removed. I appear to experience less cycling as a result, although it still occurs occasionally. The cycle time is invariably around 8-9 minutes, which I assume is not healthy. Contrary to the explanation above, I have found that cycling, when it does occur, is associated with heat outputs well above the 30% figure. The two screenshots below show regular cycling in the evening, when heat output was over 7kW and then only a single cycle the following morning followed by continuous low output of 3.3kW. The LWT and outside temperatures are very similar, so I have no explanation for the difference in behaviour.

By the way, ignore the fact that the flow temperature is labelled Hot Water Flow Temperature. It is in fact Heat Pump Heating Flow Temperature. (Another G L issue).

@mike-h

Hi Mike,

I think that I may be able to answer your query. It is all to do with DT's, but not the alcohol related variety. 😋 

Looking at the actual data rather than just the charts, shows that in the one where the system is cycling, the IAT is 17.24C and the OAT is 8.79C, giving a DT of 8.45C. The second chart where operation is more stable has an IAT of 17.82C and an OAT of 7.67C, so the DT is 10.15C.

The heat loss and hence heating demand is dependent upon the IAT to OAT DT, so in the second chart the heating demand is approximately 20% higher than that in the first chart, even though the difference between the two DT's is only 1.7C.

Another factor is the time of day, the cycling occurs during the evening period, after the outer fabric of your home has been accumulating solar energy, which in turn means the actual heat loss is lower than that calculated from the temperature readings. The second chart is when the outer fabric of your home has been cooled overnight, so may actually be presenting a higher heat loss.

As far as thermal energy output is concerned, in the first chart you state this was 7kW, but because it is cycling the average will probably be 3 point something, which is not too far from the 3.3kW under steady operation.

One way to probably reduce cycling would be to raise the desired IAT setting, which would slightly increase the heating demand. 

@lfranklin

My personal experience with a Samsung 12kW Gen 6 unit with external thermostat connected is the control method of the external thermostat is often not well designed to work with heat pumps.

The external thermostats mostly seem to use a pulsing method to call for heat to "save" energy.  With my Wiser Heat Hub set to Gas boiler it does this 6 times per hour, change a setting to heat pump and it changes the algorithm to 3 per hour.  Better, but still causes cycling in warmer shoulder months.

I recently took the external thermostat out of my system and I am using the Samsung controller as an experiment.  The cycling is gone and efficency has improved significantly.

I suspect that the Hive unit does similar, but I'm not familiar with it's setup.  Might be worth checking your Hive settings to make sure it's in Heat Pump mode if it has one. 

However, I can't find any refernce to Hive being compatible with heat pumps online.

I also note the flow rate in your pictures seems quite high compared to the values specified in the manual for 12kW system - possibly your flow pump is set too high.

 I have independently found that the thermostat is to blame for cycling our 12 Kw Samsung HP 4 or 5 times an hour, and so turned the permitted cycle number down to 1.  Naturally the installers should have done this, or rather, they should have sorted out a better thermostat system, but that appears to be quite rare in the first place.