Hello again. So I’ve just hand my heat pump serviced. All good no errors or faults, water in system good etc. Also the guy doing the servicing understands how they work. Now i was telling him how my heat pump cycles, comes on, compressor ramps up to max, and stays on till desired temperature is reached, then goes off and stays off for a few hours depending. Now he was a bit puzzled by this, he was talking about slow and low. So he took a look at my heat curve which is 0.8 and only thing he adjusted was Minimum Target temperature to 20 deg, and Max Target temperature to 50 degrees. But still wjen it comes on it ramps up to max speed. Ive noticed that what will happen is it will overshoot its target flow temperature, so lets say 40 degrees, and even go past maximum target temperature. Stays this way until Sensocomfort display registers the desired temperature. Is something not right? I’m thinking now once it reaches target flow temperature it should reduce compressor speed in order to maintain the target until required to turn off.
I also questioned Vaillant why it turned on even if the current temperature was matching the desired temperature. Their response was the SensoConfort will measur in 0.1 increments but can only display in 0.5.
Hi André, not sure how you read the numbers, 5.7 and 6.6 are outside air temperatures. First 4 numbers are for first hr, second 4 numbers second hr.
Average outside temp of 6C should give COP around 5 with compressor running slowly. Heat curve would require above 35C, but actual temperature during that period is 33 or 34C. Calculated COP 3.1-3.2
Aah sorry, my bad, I misinterpreted your lines. Column vs. row major in a 4x4 matrix…
This should be what you mean (I assumed setpoint of 20°C indoor temperature)
Electric
Heat
T_Out
T_Flow
COP
0.84
2.70
6.60
36.22
3.22
1.10
3.60
6.30
36.50
3.27
1.20
3.70
5.70
37.07
3.08
1.20
3.60
5.40
37.34
3.00
You’re right to expect a COP around 5 and the 3.1 looks low in comparison even with an assumed maximum measurement error of your probes in the range of 20%. I think at this point Vaillant should get involved - at least I don’t know what else to check.
the overshoot is normal when the pump is cycling (i.e. not enough power drawn from the emitters when it’s quuite warm outside or the pump is oversized).
How do you know the compressor is running at full speed? There is an initial higher power segment where it ramps up flow temp but afterwards it should go to minimum again:
See how electrical power goes to a higher value in the beginning to ramp up the flow temp but then drops to the minimum (around 1kW for me). Temperature still overshoots.
Regarding the overshoot: As soon as actual flow temperature is above the desired value, the heat pump begins incrementing its “energy integral” measure by 1°min for each minute the flow temperature was 1°C above its target. When the energy integral rises above 0°min, the compressor is switched off. When the actual flow temp falls below the target, the energy integral decreases by the same logic and the compressor is switched back on when a certain threshold is reached (default: -60°min). This is how that looks in practice:
Have you ever looked into the Cylinder Charging Offset setting? It seems the value is not really respected - even when I put 5 K or even 1 K the flow temperature to charge is at least 10K above the DHW cylinder temperature:
Thanks. So to summarize there is no way to influence the flow temperature used for making hot water, it is just based on current tank temperature and I can in no way force the system to use a longer DHW cycle with lower offset from the current tank temperature to have better COP?
I’ll take a closer look at the graphs but i know my compressir is running at full tilt looking at the kW gauge on the heatpump controller in my tank cupboard and my smart meter. My Arotherm Plus is the 7kW model. In short however can my system run low and slow or is it a case of ramp up for as long as needed then stop? The only time in heating mode I’ve seen my system run at lower capacity is when the weather compensation probe is about 14 degrees plus.
I am using the Eco mode already thanks to your great comparison of the modes, it’s just that I find a 10 K offset to charge the cylinder a bit excessive. I think there would be quite some potential to improve COP further by having a lower offset between flow and cylinder temperature. The option to do that is in the menu, it just seems it’s ignored. The installer even mentioned this option on initial setup.
I am experimenting with having the HP delivering the day’s worth of energy in to the building during the warmest part of the day, during which the pump can be running constantly/longer cycles. This may/should give better efficiency because: 1, the pump is cycling less, 2 the air temperature is higher than the daily average & 3 the pump/standby power is reduced.
This only works if the fabric of the building can absorb and buffer the energy over the day period, e.g. slab UFH.
After running for a solid block yesterday, I switched off the heating using the myVaillant app. at 20.00 last night.
This morning, looking at emoncms, it started up again at 01:53:20secs, as the outdoor temperature recorded 0°C.
Some antifreeze activity is expected, but the system has remained on since then, delivering a more or less constant 2kW apart from defrost cycles. Essentially we have pumped 15 kWh of heat we don’t need in to the building.
The myVaillaint app. is still showing the system as off.
I have not configured time schedules, the system is either manually on or off.
Is there a setting I am missing to allow the pump to do its antifreeze business without it switching on/running in standard heating mode?
I have not explored the ‘Away’ settings, but would seem odd to be using that when we want the building at temperature, but don’t need the system running for a period to maintain that temperature.
It’s on auto but for DHW mode that translates to 37 l/min (2200 l/h) which should be the maximum. Return temperature is around 8K above tank temperature (I get that via ebus, at whichever position that is measured…), dT is around 3.5 K. One hot water cycle takes around 30 minutes currently and I wouldn’t mind it taking 2x that long while heating up more slowly, i.e. using a lower flow temperature offset from the tank.
Thanks! Did all that already except for the coil which would not be economic to replace along with the storage tank. I was just hoping to have another option to improve DHW COP. It is a bit weird that the charging offset option is even in the controller if it is unused.
Is the hydraulic station (I have the VWZ MEH 97/6) also used for gas boilers? The setting on the sensoComfort simply overrides the internal setting on the MEH for the cylinder charge offset (according to the manual). So I get it might be a leftover from gas boilers on the sensoComfort because that’s also used for those setups but I thought the MEH was for heat pumps only.
