2024 build bungalow
Southern england 179 m2 High level of insulation Underfloor heating All 12 circuits are fully open all the time 1 thermostat in family room 7KW heat pump 50 litre buffer tank (4 port)
3.6KW solar panels Energy used by heating 2527 KWh - 7527 KWh (SCOP 3.5 approx)
I hope this may help with the modulation discussion. Comparing the Hitachi 2.5 HP to the Mitsubishi R290 6kW (just one compressor):
Min output at 7C OAT, Hitachi 1.8 kW, Mitsubishi 1.8kW
Max output at -7C OAT, Hitachi 6.2kW, Mitsubishi 6.2kW
The Hitachi is able to output 8.6kW at 7C (vs 7.2kW for the Mitsubishi) but that isn't very useful if the house only needed 6.2kW at -7C.
So GH is right if you look at overall modulation ranges, but the max output occurs at higher OATs when that output is not needed for heating. The usable modulation ranges are smaller, for example the Hitachi 2.5 HP and 3 HP are both a little better than 1/3 (-7C to 7C) and the 6kW Mitsubishi is the same. Only the 8kW Mitsubishi, with its two compressors, is significantly better.
@heatgeek@robs I was going to say exactly what @robs has said. The interesting metric is the ratio between min output at high OAT/low FT and max output at low OAT/higher FT (how much higher depends on installation of course), which is much less than the overall modulation range.
The Farnborough Daikin does seem to have a modulation ratio of 7.5:1.6 according to this metric.
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.
@jamespa Quick one for your information. Specs for Hitachi 3HP heat pump which one member of this forum has. Note min/max power. Ratio 2.1/11 = 19%.
Thank you for the info. If I am reading it right, it only quotes the output power. From the data can you work out the min input power?
@trebor12345 it should be possible to work out the (nominal) input from the data provided since it has COP and output data.
The chart would suggest the input is known but perhaps not listed. Im curious as to why you want to know about minimum input? Since output is what would be used to work out heat pump size and emitter size for nominal operation and low output operation vs minimum HP capacity.
From an overview sense it’s worth noting the r32 ecodans are not as low modulating as the new r290 models.
This post was modified 2 months ago 2 times by SUNandAIR
@jamespa Quick one for your information. Specs for Hitachi 3HP heat pump which one member of this forum has. Note min/max power. Ratio 2.1/11 = 19%.
Im curious as to why you want to know about minimum input? Since output is what would be used to work out heat pump size and emitter size for nominal operation and low output operation vs minimum HP capacity.
Only so I see on my energy monitor when the heatpump is running at it's minimum power.
This post was modified 2 months ago by trebor12345
2024 build bungalow
Southern england 179 m2 High level of insulation Underfloor heating All 12 circuits are fully open all the time 1 thermostat in family room 7KW heat pump 50 litre buffer tank (4 port)
3.6KW solar panels Energy used by heating 2527 KWh - 7527 KWh (SCOP 3.5 approx)
@jamespa@robs @trebor 12345 Attached is a more detailed power table. CAP and IPT are nominal integrated capacity and input power with defrost correction factor included. The power of the 2.5HP is 6.7kW @ -2C. With minimum of 1.8kW, the nominal range is therefore ~25%.
Hitachi seem to recognise this drop-off of power at low temperatures and say the following for their automatic method for use of the in-built electric heater.
Key to this discussion is compressor performance. John Cantor addresses this at the end of his very useful YouTube video on ashp cycling. He shows one performance curve, which does have a ‘soft’ lower end, down to ~15%. He also states that few manufacturers give their curves.
Each manufacturer will do their control differently. The resultant trade-off between being on the reducing COP part of the curve and overall system performance will depend on variables such design volume.
2kW + Growatt & 4kW +Sunnyboy PV on south-facing roof Solar thermal. 9.5kWh Givenergy battery with AC3. MVHR. Vaillant 7kW ASHP (very pleased with it) open system operating on WC
The power of the 2.5HP is 6.7kW @ -2C. With minimum of 1.8kW, the nominal range is therefore ~25%.
1.8kW is at 7C, the more interesting figure I think is the min at say 12-15C. That doesn't appear to be specified for the Hitachi, but for the heat pumps that do specify it is typically quite a lot higher than the minimum at 7C.
It's a real shame that relatively few manufacturers actually give us the information we need!
Key to this discussion is compressor performance. John Cantor addresses this at the end of his very useful YouTube video on ashp cycling. He shows one performance curve, which does have a ‘soft’ lower end, down to ~15%. He also states that few manufacturers give their curves.
Each manufacturer will do their control differently. The resultant trade-off between being on the reducing COP part of the curve and overall system performance will depend on variables such design volume.
Thanks for that. The ecoforest graph of cop Vs compressor speed explains a lot. Perhaps it even explains how Daikin appear to do a single compressor heat pump with nearly a 5:1 modulation ratio between max output at low OAT and min output at high OAT, but a later model of theirs and several other manufacturers appear to be around 3:1. The ecoforest graph clearly shows it's a design choice and as you say different manufacturers will go about this different ways. It's a pity that, also as you say, relatively few manufacturers provide comprehensive data.
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.
2024 build bungalow
Southern england 179 m2 High level of insulation Underfloor heating All 12 circuits are fully open all the time 1 thermostat in family room 7KW heat pump 50 litre buffer tank (4 port)
3.6KW solar panels Energy used by heating 2527 KWh - 7527 KWh (SCOP 3.5 approx)
@jamespa These COP curves might explain something else that was puzzling me. Although the Daikin is running continuously at 1.6kW, its COP is not as good as one might expect it to be. It actually appears worse than others running with cycling. Maybe that explains why later models have less range.
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
