Heat Pump Cycling

[Luke1]

I've got a Nibe heat pump and I've been monitoring the cycling of it over the recent cold period.

 

It seems to be on for 20 mins and off for 30 mins. It works by degree minutes and is set to start at -60 degree minutes and cut off again at 0. It usually gains a few minutes immediatley after as the flow temp exceeds the requested flow temp for a few minutes.

 

It ramps up to 80Hz in the colder outdoor temperatures of around 0C and you can visibly see it frosting up, albeit it doesn't go into a defrost cycle.

When the outside air temperature is around 7C it drops down to 40Hz

 

Delta flow temp is in the region of 7C 

 

Would it be of benefit for me to alter some settings, perhaps the delta flow temp to get a longer run time at a lower compressor frequency? 

Is there any consensus on how often a heat pump should cycle for best COP and minimum wear?

[J1mbo]

Sounds like the HP is oversized or the circulation speed in the house is too low, meaning the HP rapidly exceeds the set point (by circulation through the buffer) and so uses up the 60 degree-minutes pretty quickly. That said 20 minutes compressor run-time is hardly short cycling.

 

Is the house warm enough? 

Edited November 30, 2021 by J1mbo

[Luke1]

The system has 3 pumps.

A charge pump running between heat pump and buffer - this is controlled by the ASHP.

Then a pump to the UFH and a pump to the radiators. (These are controlled by a dumb thermostat which I have set to a high temp to keep continual flow as you mentioned in another thread) The

 

The house seems to sit at a steady 18C which is the temperature I have programmed it to. You can see as the house temperature is reached, it starts to dial back the calculated flow rate. 

 

I guess my question is, would the heat pump be more efficient if it ran at say 20Hz continually, or 40Hz for 50% on 50% operation? 

[J1mbo]

IMO 20 minutes to recharge the buffer is probably OK. The system is probably near peak efficiency at 50% load.

[SteamyTea]

13 hours ago, J1mbo said:

The system is probably near peak efficiency at 50% load.

Generally, in physical systems, if you can transfer 50% of the energy you are at peak efficiency.

 

[Luke1]

Well it seems like the Nibe controller knows what its doing then!! 

I'll continue to monitor closely and see if I can improve the weather compensation settings over time.