If the only material problem in your system is cycling, and that is accounting for a significant reduction in COP, then that is very interesting data indeed. If you could post any data you collect that may help others to optimise their systems. As things currently stand I am not aware of any quantification of the downsides of cycling.
some data from my system today that I think proves and quantifies this. It always cycles first thing when mild. Pretty sure, I know why, because three large rads in the bedrooms are off (by virtue of the evohome programmable TRV's). I believe downstairs isn't quite enough emitter area for stability. What I haven't yet figured out is the best way around it!
Mean LWT is 34-35C in both. Room temp 20-21C in both. ambient 6-8C in both.
first: with cycles, 0430-0620. COP 3.72
second: without cycles, 0630-1000, long clean run. COP 4.05
that is about an 8% hit. Plus some compressor startup wear/tear that would be nice to tune out.
Looking at the upper chart, it would appear the the OAT sensor is being affected by the operation of the heat pump, so the controller will probably be 'chasing its tail' so to speak. Can you remember the wind speed and/or direction during the two days?
If it is possible to do so, try moving the OAT sensor to a position away from the heat pump.
@rgledhill so having looked at your system descriptions earlier, it looks like you have 2 places in your system where your circulation is separated, making 3 circulation loops.
loop1 : external unit to your internal units Plate HEX "left hand side"
loop 2: internal unit Plate HEX "right hand side" to LLH "left hand side"
loop3: LLH "right hand side" to CH.
am I right?
Each of these will have its own flow rate which comes from the pump size / setting plus the hydronic circuit's head resistance: and a consequently different set of Flow and Return temps. when you say you are targetting DT of 5, and a flowT of 35, in which loop is this? Do you know the flow rates and temperatures in each of the loops?
at each of the two hydraulic separation points there is a potential for the flowT to be reduced and what is known as distortion loss (read this) to kick in, meaning each of those could be and likely is , costing you some COP.
Today I basically did what everyone's (correctly) proposing - I set all rooms to 25C on the TRVs, full valve opening on lock nuts and TRV setting, 35C flow temperature and set to maintain 5C differential between incoming and outgoing water. The system ran all day continuously at a steady, low flow, without cycling (as far as I can tell). Rather than faff with locknuts (I'm working from home, not retired!) I simply opened windows a crack when it got to 23C, to keep rooms safely below the 25C point where the EvoHome might intervene and partially shut valves.
The end result? An identical COP of about 3.2, same as yesterday, and I barely had to open any windows except for half an hour when it was sunny.
So I'm puzzled as to why it didn't get to around 4 as it did last weekend when it was heating the house from about 15C, though I guess the temperature difference in the rooms compared to the flow temperature would be higher so heat emission would be greater.
Today I've still been using the Grundfos pump in its auto mode based on the pressure on the system (i.e. the more open the radiator valves are, the lower the back-pressure, so the higher the pump speed needs to be to get decent flow rates). I have noticed that this is still a significantly slower speed than when on it's lowest fixed setting, which makes for a blissfully peaceful house but could also result in lower heat emission rates.
Tomorrow's test: back to normal target temperatures in the house, but with the Grundfos CHW pump set to speed I (lowest fixed speed but higher than it's been running previously). I shall report back...
Have you considered lowering the LWT, or using WC?
Looking at the upper chart, it would appear the the OAT sensor is being affected by the operation of the heat pump, so the controller will probably be 'chasing its tail' so to speak. Can you remember the wind speed and/or direction during the two days?
If it is possible to do so, try moving the OAT sensor to a position away from the heat pump.
known issue, it is on top of the HP casing on a short cable... which where samsung puts it. I wonder where other vendors put it. It does therefore get a bit of draught when the fan is running. Its not an ambient wind thing. I had considered investigating adding an extension cable to move it, but my WC line is very low gradient , almost flat , so there is minimal feedback effect anyway.
Looking at the upper chart, it would appear the the OAT sensor is being affected by the operation of the heat pump, so the controller will probably be 'chasing its tail' so to speak. Can you remember the wind speed and/or direction during the two days?
If it is possible to do so, try moving the OAT sensor to a position away from the heat pump.
known issue, it is on top of the HP casing on a short cable... which where samsung puts it. I wonder where other vendors put it. It does therefore get a bit of draught when the fan is running. Its not an ambient wind thing. I had considered investigating adding an extension cable to move it, but my WC line is very low gradient , almost flat , so there is minimal feedback effect anyway.
Temperature sensors are not like humans and are not affected by a draft, but they do respond to changes in temperature.
The change in OAT in the upper chart could possibly be a 'cold well' effect, which is not apparent in the lower chart because of different weather conditions.
@ianc Yes, that's correct, those are the three loops I have. The ASHP is targeting 35C leaving the output from the internal unit, 30C coming back into it. It doesn't have any external temperature sensors on the radiator loop, LLH etc.
@derek_m I'm afraid I don't know what LWT is. If it's Leaving Water Temperature, lowering it beyond 35C seems a bit low. Weather Compensation has just made things worse.
I stood and watched the system for about half an hour earlier, while it was using the above set-up. It's cycling about 6-8 times per hour, with the pump to the outside unit running almost continuously even though the actual compressor is only on for a couple of minutes at a time. I can see that it raises the circulating water temperature from 31-36C in just a couple of minutes. Therefore there's a very considerable amount of time spent with the pump running to the outside unit compared to amount of time the compressor is actually running.
I think the COP of around 4 was when I had the leaving temperature of 38C last week. I've also got the overshoot set to 4C which means it waits till the whole system gets to 31C, at which point it heats up to about 39C before turning off the compressor. Maybe the overshoot should be reduced? It was set to the maximum (4C) to try and reduce cycling.
I can see that it raises the circulating water temperature from 31-36C in just a couple of minutes. Therefore there's a very considerable amount of time spent with the pump running to the outside unit compared to amount of time the compressor is actually running.
With that llh in place it's quite possible that the water circulates primarily round the heat pump loop which would cause the very short cycling you are seeing. How are the two pumps either side of the llh controlled? I'm 95% certain that the llh needs to go BTW.
If you really do mean 2 mins it suggests that roughly 45kg = 45l of water is in play 8kWx60x2s/(4200 J/kgK ×5). This assumes that there is no output from the emitters, which is obviously not the case. Its less water if there is output from the emitters)
@derek_m I'm afraid I don't know what LWT is. If it's Leaving Water Temperature, lowering it beyond 35C seems a bit low. Weather Compensation has just made things worse.
It is leaving water temperature and you will get best performance, assuming the setup is otherwise well adjusted (which I fear it isn't) by operating at lowest temp consistent with just heating to house up to the desired temp.
This post was modified 1 year ago 2 times by JamesPa
4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.
Yes, I think that's causing the very short cycling when the heating pump is off. I think it's actually pushing the water around the heating system, through the CHW pump, as I can feel the pipe getting warm on the rads side of the CHW pump and the rad system pressure goes up from about 1.7bar to 2bar.
The leaving water temperature of 35C proved to be pretty much spot on today to just about keep the whole house warm.
If it's not an idiot's question... what's the point of having the LLH and the Grundfos CHW pump? Why can't you just directly connect the out and return pipes from the ASHP internal unit to the radiator circuit?
Yes, I think that's causing the very short cycling when the heating pump is off. I think it's actually pushing the water around the heating system, through the CHW pump, as I can feel the pipe getting warm on the rads side of the CHW pump and the rad system pressure goes up from about 1.7bar to 2bar.
The leaving water temperature of 35C proved to be pretty much spot on today to just about keep the whole house warm.
If it's not an idiot's question... what's the point of having the LLH and the Grundfos CHW pump? Why can't you just directly connect the out and return pipes from the ASHP internal unit to the radiator circuit?
Are you telling me that there are times when the heat pump side water pump is on and the central heating side water pump is off. If so, no wonder you get poor cop. Surely I have misunderstood you.
If it's not an idiot's question... what's the point of having the LLH and the Grundfos CHW pump? Why can't you just directly connect the out and return pipes from the ASHP internal unit to the radiator circuit?
Er didn't I say several times that the llh has to go? Without a system diagram one can't be absolutely sure but I'd say 95%+ the llh is a problem not a solution.
This post was modified 1 year ago 2 times by JamesPa
4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.
When I set the operation mode to be based on Leaving Temperature (difference vs return temperature) then yes, the internal ASHP pump is on, while the Grundfos CHW pump is off. When I set the operation mode to be External Thermostat (which means running in tandem with the CHW Grundfos pump) then no, as at that point it's turned off the compressor.
Surely I just have this system? Why do I need the LLH and external CHW pump?
When I set the operation mode to be based on Leaving Temperature (difference vs return temperature) then yes, the internal ASHP pump is on, while the Grundfos CHW pump is off.
Struggling to find a reliable heat pump installer? A poor installation can lead to inefficiencies and high running costs. We now connect homeowners with top-rated installers who deliver quality work and excellent service.
✅ Verified, trusted & experienced installers
✅ Nationwide coverage expanding
✅ Special offers available
