I have an AroTherm Plus 7kW heat pump and am trying to understand its behaviour during warmer weather.
Does anyone know what might be causing the significant drop in COP during each cycle in this video? This effect seems most noticeable when it’s warm outside (like today in the South of England). Its worth noting my system volume is quite low (my installer told me its about 53litres)
I’d not be too concerned as the daily COP for heating is still very good. It’s likely part of the operation where is modulating right down at its lowest limit, can’t go that low and brings the output up again, that’s my uneducated guess, but hope it’s helpful.
I’d be more interested in the DHW performance and trying to get longer runs, less often. That would significantly improve the daily overall COP when heating demand is low.
The dip is normal and appears - to varying extent - when the heat pump transitions from the initial high-power ramp-up period to its lower power steady-state operation.
Very short cycles suffer a COP reduction due to this, but it cant really be helped. The only thing you can do is set the Comp. Start. Heat. setting to -100, this will result in longer cycles that are more spread out. The downside is that you might get more indoor temperature fluctuations, depending on what the thermal capacity of your house is.
Thanks for your replies. @HydroSam - I have a Heat geek mini store - so my DHW more or less operates as a combi - so I sadly have little control over its runs. I can force them by raising the temperature target with my Home Assistant myVAILLANT integration, currently I do that during cheap tariff times - increasing the target temp to 65.
@Andre_K When I compared this to other 7kW arotherms on the heat pump monitor site I don’t see such a severe drop on other systems.
Also strangely looking at my OEM today - the 9.28 cycle shows the same drop in COP after the initial ramp up.
I cant see a huge difference in the parameters between them - they both start at similar flow temperatures, the external temperature is only slightly higher in the second run.
Also another separate question. Here is someone elses system. Between cycles their OEM is not calculating a COP - which is presumably not being counted towards their SCOP overall figure.
The dip is quite excessive, I agree. But you see that the cycle with the dip features some more dynamics on the electrical input (short rise and then fall). The second example just happily chigs along at the minimum power. I think that’s related and a hiccup occurring early in a cycle.
Do you have a heat meter in your system or where is the data coming from? What you’re seeing indicates an offset in the temperature probes, as flow measures a higher temperature than return when the compressor is off. It’s a well-known issue with Vaillant’s sensors, which you’d be using if you get the data off ebus into the heat pump app
If you enable the plot of Flow Rate, you’ll see that your system continues to push water around the system between cycles, whereas other systems may have stopped their pumps. There’s probably a setting for that on the Vaillant.
The System Volume is inferred from other data, so I wouldn’t read too much into that.
Ok got it, is there anything I can do to accurately model the system volume at any given time. I know I have 2 zones, perhaps the second zone is closed during this dive in COP.
Also what do you think about the 53litre system volume (confirmed by my installer) do you think thats limiting my system performance?
With that low a system volume you should definitely not have any TRVs or similar in use - keep all circuits open at all times.
Vaillant always pushes water around between cycles. However, there shouldn’t be an appreciable dT between flow and return when the compressor is off, which however seems to be the case here.
You are quite right. My thinking was perhaps at the start of the cycle, the second zone is closed, then it opens which could add extra system volume (and cooler water) which could reduce the flow temp.
The first zone is all normal radiators.
The second zone is a Daikin Altherma heat pump convector - it has a minimum turn on temperature of 35 degrees - although daikin technical support said it can activate ± 2 to 3 degrees. It also has quite a long run with 28mm flow and return pipes so I imagine it has a decent amount of water volume.
Looking at the good run vs the bad run.
The good run the flow temp was at about 32.6 where I would expect the sharp dip to begin. Wheres on the bad run its 30.7, certainly not enough to activate the Daikin Altherma heat pump convector. Could this be a factor?
Everything is fully open. I have no TVRs installed, just lockshields which are all fully open.
Also something I didn’t mention (please see my post from a minute ago) - I have a second zone with a heat pump convector radiator which can only turn on under relatively high flow temps.
Not always. When in expanded mode and it’s the indoor temperature that clicks the heating off, there is no pump overrun.
Pump Overrun Observations
Inactive mode: The circulation pump continues to overrun non-stop between cycles that are caused by Energy Integral (albeit at a reduced flow rate)
Active mode: The circulation pump continues to overrun non-stop between cycles that are caused by Energy Integral (albeit at a reduced flow rate)
Expanded mode: The circulation pump continues to overrun non-stop between cycles that are caused by Energy Integral (albeit at a reduced flow rate) but DOES NOT run between cycles when indoor target temperature is met where this causes the heating to stop. The circulation pump does not fire up again until start of next cycle (ie, room temp is 0.1875C lower than target)
Isn’t it just the fan that turns on? Does water not flow through the radiator even when the fan is off? Or is there some valve that stops the flow on this zone?
Another observation. I’m pretty clueless on what this means.
Circuit 1 (the normal radiators) has a
mixing valve status of -15%
target temp 23 degrees
actual flow temp 25 degrees
Circuit 2 has a
mixing valve status of 0%
target flow temp 25 degrees
actual flow temp 25 degrees
max flow temp is the same for both circuits
heat curve is the same for both circuits
My question is, why would the target temps for the zones be different? and what is the mixing for? Google seems to suggest this is to mix cold water to temper the flow temperature - which would make sense with circuit 1 as the temp is above target. Surely this is a complete waste of energy mixing cold water with warm?
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