Due to my eyesight problems, I’m rubbish with reading details in graphs but I think what you have written supplies a clue…

Have you looked at what temperature your system is running? The leaving water temperature (LWT) may be set rather too high and this would account for the frequent bursts of activity followed by idling. If you are able to ascertain your LWT settings and reduce them a few degrees or more, you may find this will reduce the effect you are experiencing and allow more of the ‘low and slow’ running you desire. The LWT might for arguments sake be at 45 degrees C and only need to be 40 degrees C though the actual figure you experience / desire may be lower or higher but, try reducing it perhaps 3 degrees initially and see if things improve. The lower you are able to run the system, the more efficient it should be. If you are running with ‘Weather Compensation’ (WC) switched on, it is probably set correctly as you say you are comfortable and that the emitters are running with all valves open. So, it is just the way the heat pump is set to provide the heat you require that requires adjustment I think. Regards, Toodles.

We have a NIBE F2040-8, but have had it for over 7 years and at that time performance monitoring wasn't required so I have no idea how much energy ours uses at any given time so cannot comment on that but would suggest that in cold weather conditions 40+ kWh might be expected. I only have whole house consumption for an all electric house and our highest daily consumption in very cold weather has been 38 kWh. This will of course depend on size of house and your larger heat pump would indicate you have a larger house than ours so would expect higher consumption.

I don't understand the detailed technical side of things, but I'll make some comments based on experience.

We also find ours does not run continuously even in cold weather and the compressor does switch off even when the UFH side is calling for heating. I think this is because the return temperature has equalised with the flow temperature. However, NIBE systems have a thing called 'degree minutes' which delays the compressor from restarting too soon. I have yet to find anyone, including a NIBE engineer, who can explain 'degree minutes' but was told it was to prevent the compressor starting too often! The NIBE engineer implied that this was normal behaviour. 

Following from Toodles' comments, do you use weather compensation and if so what is your curve setting? If the flow temp is higher than you need it could cause cycling. We have ours on 6 (for those with other systems this setting is 35C flow at 0C outside temp and 25C flow at 15C temp) and that keeps our house at 22-23C. We have UFH but radiators will require higher flow temps.

What are your water heating settings? Are they on the automatic setting that will reheat when the water in the tank drops to a certain level? That would account for some of the spikes, especially if you use the 'luxe' setting which takes the temperature to 53C but uses the immersion heater in the tank. We use the automatic reheat but on economy which reheats to 48C when the temp drops to 42C or below. This means that sometimes the system is heating the water rather than house heating but just for short periods and it doesn't affect the temperatures in the house. The charge pump also ramps up speed when heating the water. Some people prefer to schedule the water heating, particularly on tariffs with cheaper overnight rates.

My sympathies re installer. Ours ceased to operate altogether and finding another suitable heating engineer has not been easy.

I hope these comments make sense. Others who are more knowledgeable than I am will hopefully advise on low loss headers, buffers and pump size.

PS NIBE owners are few and far between on this forum. 

Posted by: @lizzie

↑

I have yet to find anyone, including a NIBE engineer, who can explain 'degree minutes' but was told it was to prevent the compressor starting too often! The NIBE engineer implied that this was normal behaviour. 

Vaillant have the same concept.  Basically its the sum (actual FT - target FT) * Time.  So if the actual FT is 30 and the target 35 and this remains the case for 10 mins then the degree minutes is -5*10 = -50.  Its a way to decide when to switch the compressor back on when its cycling, balancing frequency of cycling with comfort and also ensuring that, over the cycle, the amount of energy from the heat pump 'balances' the target.

All heat pumps (and indeed boilers) cycle if the load is less than the minimum output that it can reduce (modulate) down to.  For a heat pump that is well matched in capacity to the house, this might typically occur when the temperature gets about something like 10-13C.  If the heat pump is well 'oversized' then the onset of cycling will be at a lower temperature.  Eventually this lower temperature will be at or below the onset of cycling due to defrost, which typically occurs at 4-5C in which case the appearance will be that it always cycles.

There are good thermodynamic arguments why cycling is likely to be less efficient than constant running, but little if any sound experimental confirmation.  With old heat pumps, before inverters were introduced, the 'hard start' was bad for the longevity of the compressor, which is, I suspect, the principal reason why 'short cycling' became thought of as 'bad'.  I am not aware of any evidence showing that this is still the case with modern inverter driven heat pumps that start their compressor slowly rather than switching it on abruptly.

BTW listen to your fridge or freezer.  It (probably) cycles too, for exactly the same reason!

Hello @jamieheatpumpnew

I'm also NIBE owner for 2 years, recently joined.  12kW and with an SM020 controller.

Do you have access to MyUplink data? The SM020 has WiFI so it could connect to your home WiFi if you have it. The data available on it is comprehensive. 

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. We have UFH throughout and about 19C indoor temperature 

I'd like to get a good understanding of the NIBE degree-minutes and then explain it @lizzie, (I suspect it's got a different name in control systems design), so glad of input from any one else.  Just seen @jamespa 's explanation above-  thanks!.

Posted by: @s_gatorator

↑

I suspect it's got a different name in control systems design

Yes, its the 'integral' term in a proportional - integral - differential' controller.  Vaillant actually call it the 'energy integral', the units are degree-minutes.

thank you everyone for your insightful replies

@toodles yes we do have weather compensation I think, with a heat curve
@lizzie ours has been on 7, struggled a bit in the recent cold but mostly has been ok at that level and maybe in normal conditions we could experiment with 6. There's a heatgeek Nibe installer near us, I'm tempted to book them for a service / consultation. Water is restricted to a couple of Octopus Cosy times of day.

@jamespa you might well be right, as a new owner this is part of the infinite mystery of ASHPs and the heating systems, maybe they're running exactly as they're meant to, maybe they're not

@s_gatorator  we don't have myuplink yet, for some reason we'd have to have physical cable to connect, interesting that yours is running mostly continuously. Out of curiosity, in your chart which is the line that shows continuous running?

Cycling might happen if it is controlled by a room thermostat. The heat pump runs hard (and noisily) to get to a high +40 LWT until the room stat stops calling for heat. Maybe you could try pure weather compensation with much lower flow temps?

Cycling can also occur if the heat pump is oversized for the heat loss. For example, my heat loss at around -2.5 is 2 kW and my heat pump's minimum modulation is around 2 kW so for me the cycling only stops a few degrees below 0. Some cycling at higher temperatures is always to be expected even if the heat pump is sized correctly.

Posted by: @jamieheatpumpnew

↑

@s_gatorator  we don't have myuplink yet, for some reason we'd have to have physical cable to connect, interesting that yours is running mostly continuously. Out of curiosity, in your chart which is the line that shows continuous running?

Hi @jamieheatpumpnew - I've just checked - we have an SMOS40, with a Wifi facility. Your SMO20 does not. If there is a wired facility somewhere on your installationand it's difficult to get a cable to it, a WiFi extender operating in Ethernet Bridge mode may be a solution. 

The line that shows continuous running is the blue one, "Current Compressor frequency".  Both the compressor (inside the box) and the fan are running. if the value is above zero. 

Here's a less cluttered view of the same timeframe (compressor frequency now in orange):

@dorkypark we're running on weather compensation with the radiators on and the UFH thermostats set well above the temperature. It's a bigger heat pump so you might be right that it's oversized for the heat loss. How would I go about calculating our actual heat loss rather than what was estimated?

@s_gatorator thanks for the second graph, would guess from our electricity usage that ours spikes more frequently. Planning to get the ethernet bridge at some point and wire it up. Our is NIBE F2040-16 which is 16kw, right? We're also end of terrace actually. 

@s_gatorator Maybe I am not understanding, but it looks like the compressor is at 0 frequency about 10 times in the graph, so it isn't running continuously. Have I got that wrong?

Hi @lizzie 

Yes, it's stopping for defrost in the first part of the day. 

1.  

   Posted by: @s_gatorator

   ↑

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.