I'm going to try to answer all your questions, just maybe not in the order you asked them - I think it'll make more sense this way.
First some additional info... the very cold weather in January highlighted that the handling of condensate could be better - the heat pump is on the northern side of the house (so no radiant heat from the sun) and it was rather humid, so it was damp and stayed damp. To cut a long story short our heat pump was running with a sheet of ice in the bottom of it for the better part of a week, so the performance was rather compromised but it still kept the house warm. Now resolved with more insulation and a trace heater. But back in January I tried a few different settings, one of which was to increase the flow rate and thus increase the mean emitter temperature without increasing the flow temperature. Our heat pump usually runs with a flow rate of 15-16 l/min, this seems to allow the heat pump to run continuously at mild OATs with good COP values while also working okay at lower OATs.
@robs Thank you for this. I attach a graph from the change you made. It is very interesting that you have reduced the flow to broaden the DT as this is what I have been advocating and some of the cognoscenti poo-poo this. What do you conclude from this? What do you think would happen if you reduced flow further? What is the ideal setting? Do you believe that this is the way to tame down over-sized HP’s?
As above, the change to 15-16 l/min was just going back to the usual settings. The 15-16 l/min is the minimum the water pump can do (unless the strainer is partially blocked then it can go down to 11 l/min!). I think for our heat pump and emitters this is close to ideal, much faster and the dT will narrow too much in mild weather, and much slower and the dT will be too wide in cold weather to maintain mean emitter temperatures at reasonably efficient flow temperatures.
I think flow rate can be part of the mitigations to over sized heat pumps, the control strategy used by the best performing Daikin on heat pump monitor was initially because he had a 9kW (software restricted 16kW) heat pump that was way over sized. The other is software limiting of the compressor through quiet or noise reduction modes. Our heat pump is over sized and it is usually set to the first (of three) quiet mode levels, as a result it generally won't use the 2kW compressor and the ramp-up on start up is gentler.
The other issue is the residual cycling at low OAT temperatures. It broadened out when flow temperature reduced. This persists later in January. This suggests that the flow temperature may still be too high as the AVERAGE flow temperature is a few degrees lower than peak. Each start-up seems to cause a higher power surge before dropping which could affect COP. Have you tried lowering the flow temperature further to eliminate this cycling?
The cycling isn't, they are defrosts. The 6kW compressor has a regular reverse cycle defrost, the 2kW compressor has a hot gas defrost - so it has two defrost mechanisms! If it runs the 2kW compressor I think it does a hot gas defrost but if the 2kW compressor isn't being used then it does a reverse cycle defrost using the 6kW compressor. The defrosts of the left side of your image are hot gas and so the water pump is stopped, while the defrosts of the right side are reverse cycle and the water pump remains on. The hot gas defrosts take longer and use more electricity but don't take heat from the house, in very cold weather not taking heat from the house is a bonus.
This certainly looks like an interesting machine. Does it still have the external long NTC leads for measuring the flow and return temperatures? What happens at mild temperatures - presumably it turns one of the compressors off? Do you know at which point that happens?
Well done. Looks like you made a good choice.
Yeah, it's an interesting machine, quite complex inside with two compressors shoe horned into it! I guess not, there are only two cables - power supply and multi-core power/data to the FTC.
It only uses the 2kW compressor if it needs it, most of the time it just uses the 6kW compressor as that seems to be able to do over 6kW even in freezing temperatures.
@robs Thank you. Interesting! I presume that the switch from 20 l/min to 15 l/min was made automatically by the heat pump, and not manually, to effect the switch from dual compressor to single compressor. So, in summary, for those who want to cover the full heating temperature range efficiently, get one of these. It has all the advantages of an under-sized heat pump covering the top temperature end efficiently, but when the few cold weeks arrive to clobber you, the 2kW compressor kicks in to cover it. I initially thought it would have been the other way round with the 2kW covering the mild end, but this makes perfect sense. The hot gas defrost also covers the worst defrost period below freezing when house heat loss is critical. My old Ecodan will be proud.
Private individual. No affiliation with commercial "Heat Geeks" of same coincidental name.
@robs Thank you. Interesting! I presume that the switch from 20 l/min to 15 l/min was made automatically by the heat pump, and not manually, to effect the switch from dual compressor to single compressor.
The switches to the flow rate were manual and just experiments, the system should work fine at 15-16 l/min if not for the condensate issue that's now solved. The two compressors will run together at the lower flow rate (e.g. 28 Nov), but the second and third quiet modes seem to restrict the compressor frequency (and hence output) to the point that the 2kW compressor isn't used.
So, in summary, for those who want to cover the full heating temperature range efficiently, get one of these. It has all the advantages of an under-sized heat pump covering the top temperature end efficiently, but when the few cold weeks arrive to clobber you, the 2kW compressor kicks in to cover it. I initially thought it would have been the other way round with the 2kW covering the mild end, but this makes perfect sense. The hot gas defrost also covers the worst defrost period below freezing when house heat loss is critical. My old Ecodan will be proud.
@robs Regarding the November operation as attached, the DT is increased to 8C to get 9kW at the flow of 16 l/min with both compressors. Mitsubishi obviously set the flow at constant 16 l/min because that is rate for 6kW at 5C DT. Minimum output at 2kW shows a 3:1 ratio so this all stacks up. This blows my theory of manually turning down the flow out of the water. The defrost cycles are clear with electrical energy used in the dips.
For the previous January graph, the dips at the reduced flow level to the right show that there is no electrical energy used in the dip. This suggests that this is normal cycling, not defrost cycles?
The only thing that puzzles me is that your reported room temperatures over the heating season vary in the range 16C-23C. This wouldn’t work for me. Is there any reason for this?
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