I raise this issue as it is causing me and maybe others some confusion. In mild weather we all know we are supposed to avoid heat pump cycling for reasons of efficiency and wear and tear. On radiator emitters this ideal is usually in conflict with a low flow temperature.
However on my Samsung HTQ there are clearly two types of cycling…
Type 1 cycling modulation. where the room temperature control algorithm turns the heat pump ON for say an hour and OFF for an hour in mild weather - this is not ideal so I reduce the flow temperature as that is more efficient.
Type 2 cycling modulation. On my system the compressor is turned off every thirty minutes or so but only for 3 minutes whereupon it resumes. You see a brief blip in the electrical demand but the heat output stays more or less steady. It can be prevented by increasing radiator emitter capacity, which I have now done, but it still does it when LWT is at the ideal 32deg C where the RWT hovers around 28 to 27deg C. The house is kept warm at this setting so I want to keep it for mild weather. This pause is clearly triggered by the return temperature in mild weather.
My question… Is this compressor pause OK at a rate of twice an hour? It seems to be part of an intentional modulation strategy. If I raise the LWT by 5 deg C it usually stops it but that is less efficient and causes full on/off heat pump cycling.
Yes it’s fine, it sounds like you have tuned the weather compensation or flow temperature setting as much as possible. You are hitting the minimum modulation of the compressor and so at that point the system needs to cycle to provide lower average heat output.
Your type 1 cycling, I call it “cycling on room temp when the weather compensation curve is set too high” is where most people loose efficiency today. This Electrification of heat example is an exhibit A of this much less than optimal cycling, a lower weather compensation setting (lower flow temperatures) resulting in longer on periods would deliver much better results than the COP of 2.8 seen here, this is discussed in more detail here Analysis of Electrification of Heat trial data — OpenEnergyMonitor 0.0.1 documentation
The issue with the Samsung controls is that is uses the LWT for the hysteresis of the type 2 modulation you described. When LWT reach 2 degrees above the target it turns off the compressor. However, the bottom of the hysteresis is the target LWT, which it almost immediately reaches when the compressor turns off, so it turns back on again straight away. But because RWT hasn’t had a chance to drop, it quickly reaches LWT target +2 again.
On my controller, I have a few FSVs that help. 1 that allows you to adjust the bottom of the hysteresis (FSV#1061). Which means that it doesn’t switch on straight away. You can see that in action in the graph below with that FSV set to 3.5, making the total hysteresis 5.5 degrees.
Another that sets the minimum compressor on time (FSV#6022), which means that even if it reaches the top end of the hysteresis it will stay running, this MIGHT on its own, mean that the LWT doesn’t drop immediately below the LWT target, and so could reduce cycling. The first setting seems to eliminate the need for the 2nd though.
From what I remember, this one is only accessibly through the Simple Settings, and not through the menu. Others have said that neither of these settings are available on older MIM controllers, like those sold with the HTQ.
My gen 7 8kw is the same and always has been. While I accept what is said above I dont understand why it needs to do a wasteful defrost every time at these ambient and room temperatures, looks like a lot of things and conditions have not been fully thought through by Samsung….well at least to me!
It was doing 4746w out and using 904w, defrost used -400w on this screen shot.
I don’t think that’s a defrost per se. I don’t remember what the exact phrase is, something like “flash-cooling”. I think @SarahH used this phase in her ‘when is a defrost not a defrost’ thread, but I might be wrong on this.
It’s a simple consequence of shutting the compressor off, the refrigerant very rapidly cools and you see this micro heat loss as it draws heat from the circulating fluid.
Hopefully she will come along and set us both straight
Edit: I may have also missed your point as to why it cycles and switches off the compressor in the first place.
This is because the target LWT falls rapidly (maybe because the sun starts shining on the outdoor temperature sensor and it thinks the outdoor temp has increased quickly) which means the LWT is now over the +2 degree hysteresis.
I agree, this seems like bad control when the compressor frequency is not near its minimum, when it could lower lower the compressor frequency instead of shutting off. But there might be more to it that Samsung know and we don’t.
Quite right, @jakeymd1, at least on a Samsung, because when the compressor stops, the main EEV opens wide and the compressor low flow recycle valve opens, to depressurise the refrig circuit. So any residual liquid at the HP end (basically the condenser) flash cools to a temperature lower than the circulating fluid, temporarily reversing heat flow in the condenser.
Yes reading your well thought out reply makes it sound familiar….sure its been explained like that to me before by Sarah, but whatever, some HP seem to do it some dont from looking at the monitoring.. I’ve recently had my thermostat changed to 1degree hysterics by a Samsung engineer. I explained about the “flash cooling” or whatever it is and showed him it on my monitoring and he was truly fascinated by the data we can see. It seems to me that Samsung is not really very good at optomising its products and generally feels like its lost its way somewhat, I find that app very much like that. I also asked the engineer about a weekly schedule for my hot water and he couldnt make it work because it doesnt exist so officially confirmed. Generally speaking I’m pleased with Gen 7 as its quiet, seems well made etc but the control system and interface must make it less appealing the more time goes on. My son wants to get rid of his gas boiler and I dont think a Gen 7 is in the race especially with an indoor unit like it is….and he’s used to dealing with industral plant of varying ages…..
We may be getting a bit off topic here - just to clarify my Samsung HTQ does not type 1 cycle when using the internal Samsung thermostat it will only type 2 cycle by pausing the compressor to modulate the output when the temperature demand has been met. It integrates the temperature delta over time so that it avoids having to turn off unless type 2 modulation fails which will of course happen during mild weather. On this basis I am happy it does its job by combining room temperature delta with water law to modulate output to keep the room temperature on target without cycling and this is the ideal situation we all seek for heat pump operation.
