I thought I'd plot compressor frequency on a typical AA cycle to try and understand what's going on under the hood (rather annoying that frequency isn't chartable in Melcloud, so I've had to take manual readings every 5 mins). From this set of data it appears to be soft starting - starting at a low frequency and ramping up. There is no apparent effort to back off (although I guess the 52 number may be the minimum it want's to run at after soft starting from a lower number). Once again the target temp is an informed assumption on my part.
Note: frequency numbers added at the bottom of the chart. I'm still running thermo diff adjust values of +3 and -9.
Thank you, good thinking. It would be useful to compare your findings with other systems if anyone has the time to spare. I presume that this is compressor motor frequency?
When operating in AA mode, and the compressor has been pulled back to minimum operating speed, the FTC controller has basically 'lost' control. As the LWT increases above the calculated value, the normally action would be to reduce the compressor speed, but this is no longer possible. The system therefore reverts to a type of on - off control, that is designed to minimise cycling. The LWT is allowed to increase until it reaches the upper temperature limit, at which point the compressor is stopped. The compressor does not restart again until the LWT falls to the lower temperature limit, which will hopefully be a lengthy period of time.
Whilst the room temperature will vary due to the above on - off operation, the variation should be within acceptable limits.
When I look at the energy consumption when the unit is cycling I often see a dramatic short but high increase in power draw before it goes into a rest period. I think: why is it doing that? Inefficient high power! The underlying gradient of total energy consumed remains very linear, just with a little bump in it. So I think the FTC is prioritising maintaining a consistent delivery of heat required as expressed in the the required flow temp by adding a bit extra to balance out through the stop. We, on the other hand, probably prefer it to always work as efficiently as possible and just have a little rest now and again.
But if you cap the overshoot hard and don't allow extra heat production then aren't you effectively forcing it to run below the required flow temp on average over a period of cycles. Which should be fine (?) as long as the heat curve is not too tight to the actual requirements.
Does this just mean not allowing it to overshoot within a bit of the cycle results in it having to work harder during the rest of it?
The adjustability must be there for a reason, right?
Mitsubishi EcoDan 8.5 kW ASHP - radiators on a single loop 210l Mitsubishi solar tank Solar thermal 3.94kW of PV
I thought I'd plot compressor frequency on a typical AA cycle to try and understand what's going on under the hood (rather annoying that frequency isn't chartable in Melcloud, so I've had to take manual readings every 5 mins). From this set of data it appears to be soft starting - starting at a low frequency and ramping up. There is no apparent effort to back off (although I guess the 52 number may be the minimum it want's to run at after soft starting from a lower number). Once again the target temp is an informed assumption on my part.
Note: frequency numbers added at the bottom of the chart. I'm still running thermo diff adjust values of +3 and -9.
Out of interest, what is the setting of the 'room temp control' time interval on your system?
Thank you, good thinking. It would be useful to compare your findings with other systems if anyone has the time to spare. I presume that this is compressor motor frequency?
Thank you, good thinking. It would be useful to compare your findings with other systems if anyone has the time to spare. I presume that this is compressor motor frequency?
I believe so.
Code 016 in the service manual.
So as you stated, the compressor is making a soft start at 36Hz, and then gradually ramping up to 52Hz, where it appears to remain constant, until the LWT reaches the upper limit and the compressor is stopped. Unfortunately we cannot see what is happening to the room temperature during this period.
One thing that I would be interested to see is the same test being performed with the time interval set at 60 minutes, and also the room temperature included if possible. Obviously I appreciate that you may not have time to do this.
Unfortunately we cannot see what is happening to the room temperature during this period.
Room temp at the time of sampling (12:00) was perfectly at set temp of 18c. I must say that AA mode is good at hitting it's target and holding it to within 0.5 degrees. Only on mild days does it go 1 degree over and the thermostat turns the system off.
What's your thoughts/expectations behind the 60 min interval test?
Unfortunately we cannot see what is happening to the room temperature during this period.
Room temp at the time of sampling (12:00) was perfectly at set temp of 18c. I must say that AA mode is good at hitting it's target and holding it to within 0.5 degrees. Only on mild days does it go 1 degree over and the thermostat turns the system off.
What's your thoughts/expectations behind the 60 min interval test?
Is the blue line the setpoint and the black one the measured value? What is the resolution of the display, is it +/- 0.5C or +/- 0.1C?
Edit.
I think that the way that AA works is somewhat similar to an industrial type control system, though without the normal range of adjustable tuning parameters. The only apparent adjustable parameter being the 'time interval'.
During your test the compressor restarted, and began ramping up its speed because the LWT was below the calculated value. The controller will also have been sensing the room temperature, which may have fallen below the desired setpoint.
Unfortunately I don't know how the control algorithm has been programmed, so must make assumptions as to how I think that the controller will respond.
What I think may happen is that by increasing the time interval, it may slow down the rate at which the compressor is ramped up, thereby lengthening the cycle period.
I noticed something on the chart I posted yesterday (with the frequency values added - below) that got me thinking. I noticed that the compressor frequency appeared to be increasing when Delta T was starting to widen (leading to a self fulfilling loop of ever increasing frequency and LWT until the cycle tops out and is stopped). Here I appear to have a 'healthy' Delta T of around 4-5.
So I decided to up the pump speed to narrow the Delta T to see what happened. The result is quite surprising (to me anyway).
Conditions at the time were near identical (outside temps 1 degree higher) and I've laboriously sampled the compressor frequency value every 5 mins manually during the cycle period and added to the chart as before. Firstly to note the cycle lasted some 1hr 28mins (versus 56mins on the previous chart). Secondly the frequency was held as low as 24-26 for some time before ramping up. Again, once it gets to target, to seems to want to increase the frequency and blast through the target as quickly as possible (to reach target + thermo diff adjust upper limit, then stop). Delta T has narrowed to 2.5-3 as a result of the pump speed increase.
Also of note (whilst it's marginal) the higher pump speed does 'feel' like a slightly more even distribution of heat throughout all the radiators.
“Operation settings” adjustments offer some hopeful results.
I’ve been trying different settings and got our best COP results so far.
Control interval
We adjusted our room temp control interval to 10minutes because we found the room occasionally got uncomfortable with bare arms.. We have rads and are well insulated so followed the logic that the room responds quickly and that this is a monitoring function, not necessarily a “turn-it-up” function.
Heat pump thermostat diff. Adjust
lower limit -7 because -9 shut down the HP for too long in our opinion. Interpretation of function-This setting appears to lengthen or shorten the period before the HP will re start if the room temp is reached or passed.
Upper limit +3 which is the smallest add-on to the compensation curve set temp. (Our logic- if the compensation curve is accurate we think AutoAdapt only needs a few degrees variation to keep to the target room temperature.)
anyway here are the results
HP operation without cycling but with 2 room temp overruns followed by 2 x 2 hour shut downs. Throughout the test day the room never went above 1c higher than set temp and never dropped below 1.5c below target temp. COP 3.9 and 10kwh consumption.
Room maintained at 19.5-21c with a night time set back of 16c. Switched itself on at 5am.
HP Energy use graph, 24hours
Blue=Flow, Black=Return. no line means power is off.
24 hour room temperature gradient
Blue=room set temp, Black=actual room temp. The blue drop is the 16c setback. Green=outside ambient
Our HP basically operated 3 hours on, 2 hours off, 3 hours on, 2 hours off, 3 hours on, 11 hours set-back - off.
What we don’t know is
Did the algorithm purposefully raise the room temp in order to carry the room comfort level through the 2 hour shutdown?
@sunandair that's spooky, I also settled on the 10 -7 +3 combo for the exact same logic/reasons.
Also see my previous post with the charts with compressor frequency numbers added. It is apparent there is a purposeful ramp up at the end of each cycle.
However, wanting to try one more thing based on what I've observed over the past few weeks staring at charts and reading through the wealth of information on this forum..
I decided to re-visit the weather comp curve. This time at the cold end of the curve I've set 38C at -5 (now knowing that 38C will see a comfortable temp maintained in the house) the big difference was what I changed at the warm end of the curve. Now knowing I'll get increased cycling at a certain ambient temp / LWT, I've narrowed the warm end of the curve to suit - I've set 33C at 7C. As expected cycling is increasing once temps hit 6-7C ambient, but I've decided that 3-4 cycles per hour at these ambient temps and above using a slightly higher LWT than is needed is an acceptable compromise, versus trying to keep a lower LWT and the more excessive cycling that results (i.e. I'm matching the warm end of the curve to my 'cycling threshold point'). Also combined with the fact that once at those milder ambient temps I'm hitting target temp +1 more frequently so the wireless controller turns the heating off (thus the 3-4 cycles per hour periods don't last that long anyway).
So far it's working well. I hit a COP above 3 for the first time yesterday (not that I believe the energy numbers and resultant COP are entirely accurate). Comfort levels are spot on, and energy use appears to be reduced. Have I finally hit upon the sweet spot for my system - it's only taken me 6 weeks of near incessant monitoring testing and tweaking.. lol
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