@fazel Just noticed your question on indoor temperature. I have the thermostat on the controller set to 23C in the hall area (we do like it warm, hence my initial high flow temp) but it was starting to get too warm towards the end of the night and the missus made me turn it off 🙂
So that def means the WC should go lower flow temp at 15C outside, you can do that.
@fazel Well thanks Random Internet Guy, good superhero name!
House-2 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
5kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60
Here's another factor to throw into the mix; for a given LWT and OAT, COP varies with output. This is for a 11.2kW Ecodan.
At 7 deg OAT and 35 deg LWT (a reasonable scenario for a mild winter's day) the COP would be better if the ASHP delivered 8kW half the time compared with 4kW all the time. Cycling losses may negate some of the gains but it's worth thinking about.
It also shows that running flat out may not be the most efficient mode of operation.
Raising the WC curve will indeed change the on/off ratio and will reduce the likelihood of cycling.
Yes but at the expense of running at a higher flow temperature (and thus lower efficiency) than is necessary to achieve the power output required from the emitters. This will in fact make things even worse than keeping it as low as possible to achieve the required room temperature.
This is where the problem is located, because the controller cannot keep the LWT low, because there is insufficient load , which is the reason why the larger heat pump will stop for a period of time. By raising the WC curve slightly allows a on - off type room thermostat to take over control and reduce the heat pump running time and cycling frequency.
The quantity of energy difference during milder weather conditions I feel will be minimal, my objective was more to reduce the cycling frequency which many owners find of concern.
Sorry but you are still ignoring the basic thermodynamics. If I want to maintain a given indoor temperature at a given OAT I need to deliver a certain amount of energy from the emitters to the house, let's say 5kW if I heat 100% of the time. But if I am only heating 50% of the time I will need to heat at 10kW to deliver the same amount of energy on average. To do that the delta t between my emitters and the room will have to be higher, which can only be the case if the flow temperature is higher. For radiators where the emitter to room deltaT is typically about 20C, that means the flow temp will need to be roughly 10C higher, which means the heat pump will be roughly 20% less efficient. It's all in the Carnot equation.
'how much of the electrical energy input is actually being converted into thermal energy output'? Any losses will be greater while the heat pump is actually running, so does a larger heat pump use more electrical energy running part of the time, as against a smaller heat pump running constantly, both producing the same thermal energy output, or is it the other way around?
Hopefully you get roughly 3 times (between 2 and 5 depending on OAT and ft) as much energy out as you put in, that's the whole point of a heat pump. It's not about 'losses', it's about the relative efficiency of the _gains_ due to the fundamental properties of a heat pump.
I am not ignoring the laws of thermodynamics, and I am also not making assumptions, but using actual data provided by manufacturers.
I have just completed a set of calculations based around the Midea data for their 4kW, 6kW, 8kW and 10kW heat pumps. Using the data for an OAT of 10C and LWT in the range from 30C to 35C.
Comparing the 4kW and 6kW pumps would indicate that there is little difference in the efficiency and energy usage, in fact when running constantly at minimum output the 6kW unit is slightly more efficient than the 4kW one.
When the 4kW unit is running constantly at minimum output, the 6kW would cycle whilst supplying the same quantity of thermal energy at a slightly higher LWT, but approximately the same electrical energy usage.
The 8kW unit would appear to be noticeably more efficient than the 6kW unit, so appears to be more efficient even when cycling compared to a constantly running 6kW unit.
Running a 10kW unit in cycling is slightly less efficient than an 8kW unit running constantly, but the difference is approximately 35W per hour.
So it would appear that undersizing a heat pump does not produce any significant improvement in overall efficiency or reduction in energy usage.
I obviously did not make myself clear.
I was referring to the probably losses within the compressor motor and associated electronics, since I very much doubt they will be 100% efficient in converting electrical energy input into useable thermal energy output to the heating system.
@kev-m Well if im reading the graph on the right properly, its suggests that at all plotted ambient temps, the 11 kW heat pump has its highest COP at an output of about 9 kW, suggesting that oversizing by about 15-20% might be optimal for efficiency??? Feel free to correct me.
If that holds true for other heat pumps, then I guess the only argument against a modest oversizing apart from cost would be possible reduced longevity??
House-2 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
5kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60
@Derek M @JamesPa Thanks both for an interesting and illuminating discussion. By way of a practical example Ive attached my heat pump graph for yesterday.
Im happy with things up to 14:22 but then we start to see cycling getting particularly frequent about 17:00. Should I reduce the lower value of my WC curve to lower the heat input to my house in the shoulder season? My delta T is also somewhat small at about 3C, is this due to my emitters being undersized? (I did reuse radiators as much as possible to keep costs down, but also have an extension planned for next year which will increase the emitter volume)
As suggested previously, a possible way to reduce cycling frequency could be as follows-
Try setting the warm end of the WC curve to LWT of 25C at OAT of 20C.
This should help ensure that the LWT is lowered as much as possible, but at some point the heat pump will not be able to reduce its output any further, so will start to cycle.
Since at this point cycling cannot be avoided, the objective is to extend the on and off periods, and hence reduce the cycling frequency.
If your system has one or more on - off type thermostats connected to the heat pump controller, this is where they come into play. Normally these thermostats would be set 1C or 2C above the desired IAT, so that the heat pump is normally enabled, but operates in WC mode.
You now have a choice dependent upon how warm or cold you would like the IAT to be.
If you would like the IAT to be slightly warmer, then set the thermostat at a temperature setting 1C above the desired value, then raise the WC curve setting by the offset or otherwise by say 2C. This should cause the required LWT to be increased slightly, which in turn will cause the IAT to increase until it reaches the setting of the thermostat, at which point the heat pump will be stopped. The heat pump will remain stopped until the IAT falls to the reset value of the thermostat, at which point the heat pump will restart. Because the heat loss and hence demand is low, the coll down time could be quite long, thereby reducing the cycling frequency.
An alternative method would be to leave the WC curve at its present setting, but lower the room thermostat setting until the heat pump is stopped. The heat pump will not restart until the thermostat resets, which will be lower than the normal desired IAT.
The idea is to basically operate the heat pump as you would a gas boiler, such that it switches on, dumps a dollop of heat into the heat emitters and raises the IAT, then remains stopped until the IAT falls and the process is repeated.
@derek-m Thanks, Im going to try some of these ideas now, however I noticed on Glyn Hudsons second video that his heatpump became unrelaible at abot 33C so I wont go lower than that as mine is exactly the same model. Ive tried reducing it from 37 to 35 today and I will see it that can keep us warm, if it does Ill reduce it again. 🙂
House-2 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
5kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60
If you want a heat pump not to cycle at low load, bellow minimum output, you need not let it come on automatically but control it to be 1h on 1h off or something similar, thus there's no difference in flow temp to give your theory a base.
Unfortunately you are wrong assuming the objective is to heat the house to a given temperature. If the heating is on/off, whether controlled by the hp or an external device, the flow temp must be set higher than if the heating is continuous otherwise the emitters simply won't emit enough power. So in my example where the pump is on 50% of the time they need to emit twice the power to deliver the same energy. The only way they can fo this is by adjusting the flow temperature upwards. The heat pump doesn't know that it needs to do this, in practice the user will do it by nudging up the wc curve.
There are some cases where this doesn't apply:
If the cycling is fast then the water won't cool down much during the off cycles so the emitters will continue to emit.
If you have fantails then the fan in the fancoil may speed up to extract more energy from the water even though it is at the same flow temperature
The first case is normally avoided (possibly for reasons that no longer apply!!) and the second is rare.
In regards to cycling efficiency, data from heatpumpmonitor doesn't show a tragic loss of efficiency, but in my view a tragic unnecessarily barrage of cycles of start/stop by letting heat pumps do it automatically
That's good to know (particularly the 'beserk cycling result') but unfortunately isn't the measurement relevant to the question posed by op. The measurement relevant to ops question is to compare the performance of two otherwise similar heat pumps one of which is well oversized and the others of which isn't. More specifically you need to compare them at a temperature where the oversized one is cycling but the other is not, and both have been adjusted to achieve the same room temp under identical conditions. What you will find is that the one that cycles will have its wc curve set higher at higher OATs so that the ft is higher to compensate for the cycling. It will thus be operating at a lower COP.
Of course this is impossible to do without a lab, so you have to fall back to theory or the manufacturer data to get an idea what will happen. Since op hasn't specified a manufacturer there is only theory to go on.
Once the quest is narrowed down its worth looking at the manufacturer data (a) because different manufacturers models do perform differently and (b) because some manufacturers, eg daikin, midea sell identical hardware with several sticker capacities. Thus several apparently different models have identical performance at the low end because all daikin gave done is restricted in firmware the max outpuy In these cases you may as well go for the highest capacity of the otherwise identical models unless the price difference or noise difference (if there us one) matters.
I hope you will eventually get your own toy to play with in real time, so you can come to a conclusion based on facts rather than virtual data.
As I set out above having a heat pump doesn't enable you to make the measurement necessary to contribute to ops question. You would need two heat pumps and two identical houses. As I do not plan to have a heat pump lab I will never be able to make this particular measurement. If you do it would be great if you reported the results from the comparison relevant to ops question.
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.
Comparing the 4kW and 6kW pumps would indicate that there is little difference in the efficiency and energy usage, in fact when running constantly at minimum output the 6kW unit is slightly more efficient than the 4kW one.
When the 4kW unit is running constantly at minimum output, the 6kW would cycle whilst supplying the same quantity of thermal energy at a slightly higher LWT, but approximately the same electrical energy usage.
The 8kW unit would appear to be noticeably more efficient than the 6kW unit, so appears to be more efficient even when cycling compared to a constantly running 6kW unit.
Running a 10kW unit in cycling is slightly less efficient than an 8kW unit running constantly, but the difference is approximately 35W per hour.
So it would appear that undersizing a heat pump does not produce any significant improvement in overall efficiency or reduction in energy usage.
That's interesting data provided that
a) in doing your calculations you adjusted the lwt in the case of a cycling pump so that the average power output from the emitters remains the same as that of the constant operation pump to which you are comparing it.. This is quite a substantial adjustment where the on/off ratio is lower than say 75% on. At 50% on its roughly a 10C increase in lwt for a typical radiator system.
b) the heat pumps in question aren't in fact identical apart from firmware limits on max output (Midea do this)
If those conditions are both met, and given that midea can't violate the laws of thermodynamics, then it implies that mideast low cap pumps are fundamentally less efficient than their high cap ones. That is of course a possibility at least up to a point and should be evident from their capacity/cop tables without doing any furt5calculation. However there is as yet no reason to suppose that the same applies to other manufacturers. Can you post more details of the midea calculations/figures
I was referring to the probably losses within the compressor motor and associated electronics, since I very much doubt they will be 100% efficient in converting electrical energy input into useable thermal energy output to the heating system.
Actual losses in the compressor or electronics are hopefully small compared to the gain from the heat pumping itself. Most of the losses will anyway end up as heat energy a fair proportion of which will find its way into the flow water (it has to go somewhere!). Fundamentally the point im trying to make here is that a heat pump doesn't really convert electrical energy to heat energy in any conventional sense at all. Rather it uses electrical energy to extract heat energy from the air passed over the fins and raise it to a higher temperature and, because of the wonders of thermodynamics, this turns out to need less a less electrical energy to drive the compressor than the energy extracted from the air, so its more than 100% efficient.
Think of a fridge, you don't turn electrical energy into 'cold', you use electrical energy to force heat energy to move from a cold body to a warm one.
Of course no heat pump reaches the theoretical maximum efficiency, so performance is always less than ideal, albeit still way over 100%.
So you can't really talk about 'losses'. You can talk about deviations from ideal, which we don't, or gain, which we do but call it cop.
@jamespa if you are chasing the efficiency of on versus off running temperatures at the lowest output, then you better not have the radiators sized for 45-50C in winter, but to 35C in winter when the highest load is used, thus making a real difference.
@jamespa if you are chasing the efficiency of on versus off running temperatures at the lowest output, then you better not have the radiators sized for 45-50C in winter, but to 35C in winter when the highest load is used, thus making a real difference.
If sizing for 35C leads to rads of a tolerable size then in principle i agree ( although you can do both) However I'm not chasing what you suggest, im responding to a question.
OP asked a simple question related to sizing. That sparked a debate about the merits/ demerits of over, right and under sizing. There are of course arguments both ways from a householders point of view who cares about both running cost and comfort.
Many installers don't care much about running cost, only about reducing call outs. That is most easily achieved by 'generous' (over)sizing.
Gross over sizing leading to cycling at low temperatures is definitely bad for efficiency. Modest oversizing where the onset of cycling is above, say 10C, less so. Adding 50% margin on a load estimate that is already 'generous' can easily take you into the realms of gross oversizing as others have reported.
It's a good idea to be aware of physics and thus most likely performance trends anyway, but particularly given the possible diffence between the motivation as an installer and the motivation as a householder when considering the answer to op's question.
Furthermore understanding the basics, determined by the thermodynamics, makes it easier to interpret the manufacturers data once the choice is narrowed down.
Since op has asked the question, it seems reasonable to assume he/she wishes to be an informed customer not an ignorant one.
This post was modified 2 years ago 6 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.
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