@burtis I’m still waiting for my Octopus Mini (18 months or so and counting) and have to rely on the Enphase and Tesla apps but can’t help feeling the mini (which is, after all, what I will be metered by) might give me a better indication. As best I can ascertain, my Daikin performs in a similar ballpark wattage range to your Midea. Regards, Toodles.
Toodles, he heats his home with cold draughts and cooks his food with magnets.
The effect of cycling is very different with different heating systems too. If modulation was not an important factor heat pumps would still use fixed speed compressors. All modern ASHPs use modulating inverter compressors, no matter how good their control strategy, so it seems a little foolish to dismiss modulation range from consideration.
I have been working on our minimum operating output using these charts for about a year now. There is no notes on how to interpret this chart especially where the chart states “Capacity”. (example if I have a bucket which has a capacity of 2 gallons there is nothing stopping me from just putting a pint in it but the capacity is still 2 gallons)
I might be wrong but this thread seems to be interpreting/reading capacity as minimum output whereas I’ve been reading this word as the top output at a given compressor setting with some modulation below that capacity. So I’m interested to hear how to interpret this chart.
Also, while Capacity is one side of the question, the other issue is maintaining a sustained, modulated output. This second part of sustained operation does not appear to be answered in these charts. Especially at +10c ambients.
While we have found flow temperatures which offer reduced cycling at these warmer ambients, it has invariably been at higher outputs than the chart implies. Ie typically 300 to 500watts higher output than the chart stated capacity.
Has anyone got a definitive way to interpret this chart?
incidentally @robs this chart is for the ecodan R32 model 8.5. It was rumoured that this model also had a split compressor.
This post was modified 5 months ago 2 times by SUNandAIR
I have been working on our minimum operating output using these charts for about a year now. There is no notes on how to interpret this chart especially where the chart states “Capacity”. (example if I have a bucket which has a capacity of 2 gallons there is nothing stopping me from just putting a pint in it but the capacity is still 2 gallons)
I might be wrong but this thread seems to be interpreting/reading capacity as minimum output whereas I’ve been reading this word as the top output at a given compressor setting with some modulation below that capacity. So I’m interested to hear how to interpret this chart.
I'm pretty sure it just means output, using the word capacity in this way isn't unusual in Japanese documents (and if something has a capacity then it will have a minimum and maximum capacity). So max is maximum output (from max compressor speed) at those conditions and min is minimum output (from min compressor speed) at those conditions. The capacity-ambient temp graphs on page 109 show this quite well, your ASHP at 35C flow temp and 10C ambient has a range of about 3kW to 10.6kW (min compressor speed to max compressor speed).
Also, while Capacity is one side of the question, the other issue is maintaining a sustained, modulated output. This second part of sustained operation does not appear to be answered in these charts. Especially at +10c ambients.
While we have found flow temperatures which offer reduced cycling at these warmer ambients, it has invariably been at higher outputs than the chart implies. Ie typically 300 to 500watts higher output than the chart stated capacity.
If the heat pump can achieve it, and the document suggests it can, then that would suggest that the rest of the heating system may be the cause. I believe the need for a higher flow temperature at high ambient temperatures implies that the emitters can't shed the heat they are being fed fast enough (due to the emitter to room deltaT being so low). And as a result the return temperature rises to a level that causes the heat pump to shut off, hence you get cycling. Bigger emitters (radiators or UFH) make it easier to shed heat even with low emitter-room deltaT values, so maybe with larger emitters a stable low output is possible.
incidentally @robs this chart is for the ecodan R32 model 8.5. It was rumoured that this model also had a split compressor.
The document I linked to has the details of the compressors in the heat pumps, the PUZ-WM85 has a single SVB220FEGMC-L1 model compressor (see page 9 and the refrigerant system diagram on page 77).
I have been working on our minimum operating output using these charts for about a year now. There is no notes on how to interpret this chart especially where the chart states “Capacity”. (example if I have a bucket which has a capacity of 2 gallons there is nothing stopping me from just putting a pint in it but the capacity is still 2 gallons)
I might be wrong but this thread seems to be interpreting/reading capacity as minimum output whereas I’ve been reading this word as the top output at a given compressor setting with some modulation below that capacity. So I’m interested to hear how to interpret this chart.
point1. I'm pretty sure it just means output, using the word capacity in this way isn't unusual in Japanese documents (and if something has a capacity then it will have a minimum and maximum capacity). So max is maximum output (from max compressor speed) at those conditions and min is minimum output (from min compressor speed) at those conditions. The capacity-ambient temp graphs on page 109 show this quite well, your ASHP at 35C flow temp and 10C ambient has a range of about 3kW to 10.6kW (min compressor speed to max compressor speed).
Also, while Capacity is one side of the question, the other issue is maintaining a sustained, modulated output. This second part of sustained operation does not appear to be answered in these charts. Especially at +10c ambients.
While we have found flow temperatures which offer reduced cycling at these warmer ambients, it has invariably been at higher outputs than the chart implies. Ie typically 300 to 500watts higher output than the chart stated capacity.
point2. If the heat pump can achieve it, and the document suggests it can, then that would suggest that the rest of the heating system may be the cause. I believe the need for a higher flow temperature at high ambient temperatures implies that the emitters can't shed the heat they are being fed fast enough (due to the emitter to room deltaT being so low). And as a result the return temperature rises to a level that causes the heat pump to shut off, hence you get cycling. Bigger emitters (radiators or UFH) make it easier to shed heat even with low emitter-room deltaT values, so maybe with larger emitters a stable low output is possible.
incidentally @robs this chart is for the ecodan R32 model 8.5. It was rumoured that this model also had a split compressor.
Point3. The document I linked to has the details of the compressors in the heat pumps, the PUZ-WM85 has a single SVB220FEGMC-L1 model compressor (see page 9 and the refrigerant system diagram on page 77).
I thought I'd post the pages in the data book you referred to:
Point1.
Capacity to ambient temp graph ; page105 the 4 lines on this graph show Max output, Partload1 output, Partload2 output and Min output. at a specific water outlet temperature. the discussion relates to the lowest output and how it changes depending on the outdoor ambient temperature.
Regarding our installation it is stable operating in 10 to 12c ambient at an outlet temp of 35c. It operates with 25 minutes on-cycles and roughly 5 minutes off-time. It modulates when on between DT3 to DT5.
The issues become apparent when operating at 30c outlet temperature, when our weather comp curve was set that low. It was clear that excessive cycling became an issue.
Point2.
Of course you are right; The sizing of our emitters (total capacity) appear to be restricting the heat loss into the living space at these very low demand levels. This must be a challenge for many Rads-only installations.
I wonder how frequently installations get designed with maximum heat demand criteria when it is -3 outside and never designed for minimum operating capacity of the heat pump at the moderate weather days?
We have a total rad capacity of 21kw (at DT50+room temp21) but by the time that is calculated down to a flow temp 30c in a room temp21 the heat loss is miniscule. Our TRVs are now fully open. So we are left with a min WC setting of 35c which gives us a CoP of somewhere between 3.9 and 4.2, which is ok but we could squeeze a bit more efficiency out.
Point 3.
It was interesting to see the diagram on the new twin compressor on the latest R190 model PUZ-WZ80VAA. Complete with its twin plate heat exchangers.
This post was modified 5 months ago 3 times by SUNandAIR
Regarding our installation it is stable operating in 10 to 12c ambient at an outlet temp of 35c. It operates with 25 minutes on-cycles and roughly 5 minutes off-time. It modulates when on between DT3 to DT5.
The issues become apparent when operating at 30c outlet temperature, when our weather comp curve was set that low. It was clear that excessive cycling became an issue.
Of course you are right; The sizing of our emitters (total capacity) appear to be restricting the heat loss into the living space at these very low demand levels. This must be a challenge for many Rads-only installations.
Yes, radiator based systems are likely to be limited in stable low flow temperatures. But larger radiators should allow lower stable flow temperatures.
I wonder how frequently installations get designed with maximum heat demand criteria when it is -3 outside and never designed for minimum operating capacity of the heat pump at the moderate weather days?
We have a total rad capacity of 21kw (at DT50+room temp21) but by the time that is calculated down to a flow temp 30c in a room temp21 the heat loss is miniscule. Our TRVs are now fully open. So we are left with a min WC setting of 35c which gives us a CoP of somewhere between 3.9 and 4.2, which is ok but we could squeeze a bit more efficiency out.
What was the total output capacity of your radiators at a flow temperature of 30C? Is it less than the minimum output of your heat pump at 30C flow temperature?
It was interesting to see the diagram on the new twin compressor on the latest R190 model PUZ-WZ80VAA. Complete with its twin plate heat exchangers.
Indeed, but I think the dotted line around the plate heat exchangers indicates that these are a single item. There is a similar dotted line around the fans in the schematic but the unit only contains one fan.
Heating needs to go on when it's 12 or less outside. My gas boiler can keep the house warm with a 30 ish flow when it's that temp outside. Might work with an even lower flow temp, but due to the boiler being stupidly oversized, it would just cycle like crazy.
This is the link for the Daikin EU data portal. If you choose a pump size and tank (mine is Altherma EDLA 6kw & 180litre tank) you can then play with temperatures, outputs, flow temps etc., In other words a capacity chart that is interactive. It's quite useful. Hope it helps.
We have a total rad capacity of 21kw (at DT50+room temp21) but by the time that is calculated down to a flow temp 30c in a room temp21 the heat loss is miniscule. Our TRVs are now fully open. So we are left with a min WC setting of 35c which gives us a CoP of somewhere between 3.9 and 4.2, which is ok but we could squeeze a bit more efficiency out.
What was the total output capacity of your radiators at a flow temperature of 30C? Is it less than the minimum output of your heat pump at 30C flow temperature?
it took me a while to learn how and to work out output correction factors for 30c and 35c flow temperatures. Here’s my best attempt as written back then…
At a 21kw combined rad load
30c LWT = 1.785kw total output
35c LWT = 3.47kw total output
i also calculated outputs when all the TRVs were closed as can be seen.
We had various discussions on correct ways of calculating radiator outputs on this forum over the past 18 months. So I hope I can now do it justice and try and describe the method I used for working out the DT and correction factor.
see below
The first issue I recall, was to notice that the manufacturers correction charts do not go down low enough or fine enough to take account of 5c circuit DT (temp drop) and 30c LWT. See correction chart. I also hope my calc method looks about right
So after a bit of trial and error I was left thinking the 30c outlet temperature at under 2 kw output it was the reason for the cycling we encountered.
Variability of heating demand throughout a year. | Protons for Breakfast
Friends, as I drift off to sleep at night, I often reflect on the subtle wonder of heat pumps, and last night I imagined this article in my head. However writing the article has proved more difficult than I had envisaged in my sleepy reverie. There are lots of technical details, so in case you…
So to summarise:- Of the 365 days of the year our heat pumps will try to operate for 160 days at less than 20% capacity. (If they can cope with the modulation, that is.)
great info.
would be interesting to see the split over autumn, winter and spring.
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