There may be some timing issues, but when averaged comes out to 5.7.
But Samsung appear to claim (boast) COPs of 4 point something (with LWT at 35 degrees) and we must assume that is a downhill with a following wind COP which means you are getting a COP that is ~25% better than the manufacturer's boasts. Is that credible?
The first sun sight I did with a sextant in the English Channel put me in Liverpool. All the raw numbers looked credible, and the sums were done correctly. But the final result was not credible, ergo something somewhere was wrong. On that occasion, it was probably human error (it was my first attempt at astro nav) but it might have been the sensor (sextant) was out, or something else. That's the point of sanity/reality checks: if the result doesn't seem credible, you need to check and re-check, and then check again.
Well the rad room can be controlled, increase flow temperature you increase the ability to emit heat - to a point, the room temp will likely catch up
Yes, but that is not how we run things, Generally we want a stable room temp, so in effect the room temp is fixed, to the temperature we want. And, as you say, it is a dynamic system, and in any event, the thing you are controlling here is LWT, the rad/room delta t only changes in response. In practice, the only thing we/the unit can vary is the LWT.
Which leaves the question: what is changing the LWT/RWT delta t?
I think the answer may in fact be in the chart I posted earlier. The LWT controls the LWT/RWT delta t. When the LWT is high (and other parameters are stable), relatively more heat is lost to the room, resulting in a bigger LWT/RWT delta t. At the other extreme end of the scale, if the LWT is the same as the room temp, no heat will be lost to the room, and the LWT/RWT delta t will be zero (or very close to zero).
In summary, the only parameter that controls kW output in the real world is LWT. As the desired output gets lower and lower, achieved by lowering LWT, we reach a point where the heat demand to achieve the lower LWT falls below the compressor's minimum output. At that point, the heat pump's only remaining option is to cycle.
It's like trying to drive a car at a snail's pace. With the engine running in gear all the time, you can't do it, the only way you can gets the car to average a snail's pace is to take it in and out of gear, ie cycle it. The heat pump does the same thing to lower it's output, but because it doesn't have a clutch/gearbox, it has to stop/start the compressor.
Bottom line: cycling is unavoidable at low LWTs.
Midea 14kW (for now...) ASHP heating both building and DHW
I agree, that it is quite difficult to avoid cycling when the LWT is low. I will continue to persevere though. With [eventually] 8.5 cubic meters of concrete with UFH pipes that is likely equivalent to a 1700 liters of water I think I will find a way .... hopefully as the plan is maximse COP.
I wonder what peoples' views are on using a volumiser to reduce cycling. The idea as I understand it is that by increasing the circulating volume (which is all that a volumiser does, it being a passive tank of say 50L added in line to the circuit), both the on and off phases last longer, on longer as there is more fluid to heat, off longer as there is more residual heat to be lost. Seems like a sensible low tech idea to me. It it is placed within the heated part of the house, any heat loss from it isn't really lost, as it contributes heat to the house. Any thoughts/experiences?
I believe that the primary benefit of installing a volumiser tank is to provide additional thermal energy, back to the heat pump, during a defrost cycle, but as CR has pointed out, adding additional capacity to the system should also help reduce cycling during milder weather conditions.
If you want to stop or reduce the cycling, you should consider reducing the LWT.
I have been trying to get to the bottom of the occasional cycling of my heat pump. Once my heat pump gets to its lowest modulation level, I actually have to raise the LWT in order to increase the rate of heat being dissipated by the "radiators" (the dT goes up a little bit when I do this as I have a constant flow rate) this then stops the cycling, of course then the room gets warmer than I want it. I have come to the conclusion that you can't stop the cycling at this point and it is better to switch off the heat pump. I think that if your heat pump is sized correctly [i.e, the entire system is properly engineered] you would not have this issue as you could keep tuning your system without hitting limits.
Mine is a bit oversized for the radiators [but correctly sized for the heat loss @ -2], since I did not change them as the whole house is going to UFH, over time.
Amazingly my 16kW Samsung can modulate down to around 700 watts input [without PWM ... this on my todo list].
This is about
1/5 the rated output [at specific conditions]... A2W Condition #1 : (Heating) Water In/Out 30℃/35℃, Outdoor Air 7℃[DB]/6℃[WB]
1/10th of the max rated input of 6kW
I could probably achieve 1/10th rated output for the same input (700W) given an outdoor temperature low enough to reduce the cop to about 2.5.
There will always be occasions when heat pumps will cycle, just as gas boilers and oil boilers also cycle. For optimum efficiency it is necessary to reduce the cycling to a minimum.
To be able to reduce cycling to a minimum, it is necessary to understand the root cause of the cycling, which may also vary under differing operating conditions.
During cold and damp weather an ASHP will have to perform defrost cycles, which are unavoidable, but could possibly be reduced by operating the heat pump at the lowest acceptable power output.
Cycling occurs during milder weather conditions due to completely different reasons, with the frequency of cycling being dramatically affected by how the heat pump has been configured, and is being operated.
The thermal energy produced by a heat pump is derived from two main sources, one being the electrical energy driving the compressor, with the second being the thermal energy absorbed from the outside air. To maximise overall efficiency, it is necessary to reduce the former as much as possible, and increase the latter.
So how is it possible to achieve this?
Thermal energy obeys the laws of Physics and Thermodynamics, so to cause it to transfer from one place to another there will need to be a difference in temperature. Thermal energy will flow from a higher temperature to a lower one, since it will try to equalise the temperature of both.
To initiate the transfer of thermal energy within a heat pump, it is necessary for the compressor to increase the pressure, and temperature, of the refrigerant gas. As this hot gas enters the Plate Heat Exchanger (PHE) called the Condenser, it will eventually become hotter than the water at the outlet side of the PHE. Thermal energy will therefore be transferred from the refrigerant gas, through the PHE, into the water, hence increasing the temperature of the water. As the temperature of the refrigerant gas reduces, it will fall below the dewpoint, and the gas will start to condense into a liquid. If there is no flow around the system, then eventually the refrigerant gas would condense to a liquid and the temperatures on both sides of the PHE would equalise.
To continue the transfer of thermal energy it is necessary for flow on both sides of the PHE.
On the refrigerant gas side, the condensed liquid flows through an expansion valve, on its way to the Evaporator. As the name suggests, the liquid is caused to evaporate by a combination of reduced pressure and absorbing thermal energy from the outside air, which is being pulled through the Evaporator coils by one or two fans.
The quantity of thermal energy being produced by the heat pump will be dependent upon how hard the compressor needs to work to raise the pressure and temperature of the refrigerant gas, and how much thermal energy can be extracted from the outside air, which will also be dependent upon how quickly the refrigerant gas is flowing through the Evaporator. When the refrigerant gas is flowing at a slower rate it has more time to absorb thermal energy.
To maintain a reasonably constant indoor temperature, a heat pump needs to provide sufficient thermal energy to supply the present heat loss, which it does so by heating the water supplied to the heat emitters. If the outside air temperature increases, the heat loss will be reduced, and if the LWT and water flow rate remain the same, the indoor temperature will start to increase. To prevent the indoor temperature from increasing, it would therefore be necessary to reduce the water temperature and/or reduce the water flow rate. The most common method is to lower the LWT, by reducing the speed of the compressor using Weather Compensation (WC) control, which also causes the refrigerant flow rate through the Evaporator to be reduced and hence allow for more thermal energy to be absorbed. A win - win situation which improves efficiency.
The compressor has a minimum operating speed, which when reached would prevent the LWT from being reduced further, thereby limiting the minimum quantity of thermal energy that can be produced by the heat pump. To reduce the quantity of thermal energy being transferred to the heat emitters will now require a reduction in the water flow rate. Reducing the water flow rate should indeed lower the quantity of thermal energy being transferred to the heat emitters, but may now lead to cycling due to the heat pump exceeding the required LWT.
Once the outside air temperature increases above a certain value, and the heat loss falls below the minimum output of the heat pump, then cycling will be unavoidable.
How best to control, or at least reduce the frequency of cycling?
Having thought about what is actually happening and what may be the best solution, I think that in most instances the better option would be to slightly increase the required LWT when operating in milder weather conditions, which should have at least two benefits. One benefit would be that it should reduce the cycling frequency, with the second benefit being that this method should help store some thermal energy in the mass of the building, which would hopefully reduce the energy requirement later in the day when the outside temperature is lower.
Whilst I normally advise against the use of on - off type thermostats, this is one of the few occasions when one may be of use. If the desired indoor temperature is 21C, then having a thermostat set to switch the heat pump on at 21C, and off at 22C, this should cause the heat pump to run for a period of time to raise the indoor temperature to 22C, then switch the heat pump off until the indoor temperature falls to 21C. This should help reduce the cycling frequency and increase efficiency.
A further method to help reduce cycling and improve efficiency would be to perform DHW heating during the mildest period of the day.
@william1066 my heat pump (same as yours) I have observed to have the same stable min output (3.4 kw). However, I'm measuring input power as 1.0 - 1.1kw at that same time. LWT 35. BUT this power is measure with a CT clamp into an ESP - although I calibrated it with a resistive load, I'm not trusting its numbers with the complex load of the heat pump, so I will be changing it to a proper inline meter and I hope it will go down to close to your numbers.
These low LWT's on milder days are a really interesting one - I tried running with lower LWT's (30) : the house stayed warm, but cycling got worse. Radiator output was so low there wasn't enough emitter power available, so RWT would steadily approach LWT at which point compressor switches off and we get a cycle. as it stands now we run a slightly warmer house now (21C instead of 20C) for stable output.
side point: I wonder if this has been analysed much - we definitely run our house warmer and at a higher comfort level than when we were on gas.
For the other discussions, I have a 50L volumiser on the flow side of the heating circuit. that decision was made following advice from graham hendra posted previously as to where to put it, and the consensus from the (heatgeeks and john cantors etc) that having this volume helps reduce cycling. the samsung 16kw also has a min required circulating volume of 50L and I wasn't sure of getting there with just the downstairs rads.
If you want to stop or reduce the cycling, you should consider reducing the LWT.
I think this depends on the "flavour" of the cycling. I've see that my HP sometimes cycles when the call for heat is constantly on, if LWT is too LOW, because the emitters can't get the heat out, the RWT approaches the LWT and , I assume due to some programming inside the ASHP which means that this is a non-desirable condition, when they become very close, the compressor cycles to off. the circulation pump keeps going, and a few mins later after LWT and RWT have dropped (To the same value, implicitly, as the compressor is off), the compressor cycles back on. I see the options here as either accept it, or raise the RWT (and have a warmer house) and allow the call-for-heat to be the cycle cause if and when that gets triggered (at which point that is the cycle cause that I think you refer to?), or get better / more low-temp emitter setup.
I have been trying to get to the bottom of the occasional cycling of my heat pump. Once my heat pump gets to its lowest modulation level, I actually have to raise the LWT in order to increase the rate of heat being dissipated by the "radiators" (the dT goes up a little bit when I do this as I have a constant flow rate) this then stops the cycling, of
how are you raising LWT? have you automated it, or just running to the controller and pressing buttons?
are those COPs credible? I ask, because if they are not, then the underlying data is suspect.
The Samsung UI is showing (34/7.4 = COP of 4.60) My data suggests a COP of 5.3. This is about a 15% difference. This could be due to Samsung assuming glycol is in use. Since I am not using glycol, I am calculating based on specific heat capacity of water. I will adjust my code to assume a specific heat capacity for a glycol system and see if I get closer to the Samsung data.
The Samsung UI is showing (34/7.4 = COP of 4.60) My data suggests a COP of 5.3. This is about a 15% difference. This could be due to Samsung assuming glycol is in use.
It might account for about a 10% difference (specific heat 4.2 vs 3.8), but it all depends on how Samsung (and Midea and all the others) do their calculations (trouble is, we just don't know). But I still think the figures are suspiciously high, in the best Stalinist tractor production is up tradition, though perhaps not yet reaching the all pigs fed and ready to fly standard.
I also note you appear to be using extremely low amounts of energy compared to me. The charts are seriously confusing, they seem to have a crystal ball inside, as they appear to see into the future, but actually I think it is just bad layout and captioning. I think what they show is daily use over the week 24-30th April, and at this point in time most of that week hasn't happened, so the values are zeros, all we can see is the use for 24th April, which was 7.4kW. But as you posted the images at 0558 (GMT) on 24th April, either there is a crystal ball behind the charts, or the 7.4 figure is usage from midnight to whenever you grabbed the images. The point I am slowly getting to here is that if your usage is low (and/or you use short time frames), then any fixed error could have a disproportionately large effect (error of 1 on 10 vs 1 on 100). Generally, the shortest interval I use to calculate a COP is 24 hours.
I tried to find Samsung's equivalent to Midea's Engineering data book which has detained outputs (and COPs) at different ambients and LWTs but didn't have any luck - maybe you have the data? It would provide another benchmark to compare your figures to.
What this discussion underlines for me is just how uncertain all this monitoring is. Might reading the tea leaves do just as well?
Midea 14kW (for now...) ASHP heating both building and DHW
From trying to follow the discussions in this topic, I would suggest that a volumiser with varied volume is what's required!
OK... so that doesn't exist. But the same effect could be achieved by using a controlled mixer-valve on the Return
An intriguing idea. But might it add additional complexity without any real benefit? Put another way, is there any harm associated with having a volumiser in the circuit when the heat pump isn't cycling?
Midea 14kW (for now...) ASHP heating both building and DHW
Thinking about installing a heat pump but unsure where to start? Already have one but it’s not performing as expected? Or are you locked in a frustrating dispute with an installer or manufacturer? We’re here to help.
