Hi Mick.
I can’t help but wonder if the 5K isn’t a bit of “that’s how we’ve always done it”?
The ASHP controller will decide what energy is needed (either from weather-compensated LWT control and/or roomstat).
The water/glycol circulation flow may be fixed (if non-PWM) or variable (if PWM).
You are 100% correct that this may mean high flow/low DT or low flow/high DT (from Q=m.Cp.DT).
The main consideration is that a lower DT (say 3degC compared with 5degC) will result in increased emitter output (because the average radiator temp is higher, so dT to room is higher), but at the cost of increased circulation pump energy. @Andre_K poses a good question, which I suspect that you have side-stepped slightly. Circulation rate (hence DT) doesn’t set the duty - it’s merely the transfer mechanism to get the necessary heat from the ASHP to the house.
It’s going to depend on the specific ASHP potentially, I’m not sure you can make any generic assumptions regarding this. Some models and/or some refigerants may work better at 5K. What we do know is that the arotherm+ i) doesn’t aim for specific delta-t ii) it operates efficiently in this mode.
Good points, Daniel.
My comments above assumed 1) an element of LWT control, and 2) the use of a PHE for refrigerant heat transfer (with attendant close temperature approach, which varies little with water DT).
You’re both right. The end result of the calculation (heat output) is the same regardless how you manipulate the DT, flow rate and SHC.
BUT and this is the big but as Daniel alludes to. The Arotherm works by fixing the flow rate and modulates by adjusting the DT. Narrow DT when it needs low output and opens the DT up when it needs more output. There isn’t really anything to tweak unless you start dropping flow rate.
Personally, I would not get obsessed with DT5 at the radiators. The outdoor unit will still do DT2 when low output.
I got obsessed with DT5 at the rads and ended up closing down my rads so much that I dropped my system volume and ruined my COP performance.
Aim to open all the rads as much as possible so you have as much system volume as possible, but at the same time, the rads need to be closed down just enough to ensure resistance so that all the rads in the circuit get some heat. In my case, fully opening up every rad meant my index rads (the ones furthest away) didn’t get much heat.
Once you’ve got heat to every rad, you can tweak the lockshield slightly to adjust room temp.
So it’s a balance!! (no pun intended).
My radiators are fortunately quite nicely adjusted for now (that took time!). As you suggested, they’re alle fully open. I don’t have any lockshield valves but I adjusted the flow rates in the basement where all radiator pipes connect to a UFH manifold with individual flow control. An uncommon setup, I know, but that’s how I bought the house and it makes adjusting and seeing individual radiator flows easy. Now I’m just trying to see whether there is still room for improvement.
I would word that a bit differently. In my understanding, you cannot modulate DT in itself - the delta is a response of the heating system and depends on the power emitted by the radiators at the given flow temperature. The only thing under control of the heat pump is the flow temperature and with increasing flow temperature the DT will automatically increase as more heat is emitted by the radiators.
Yes, heat output is the same, I think the (more academic) question is whether the condenser temperature is also the same for both scenarios? Heat transfer between condenser and water depends on the delta between the two and the flow rate so that should also shift accordingly for the low/high return temperature scenarios. This sounds like a nice exercise for some coupled heat transfer numerical calculations …
FYI I’m now on 09.02 and integral is working as designed (and reported by @Andre_K). Hope to now see reduced cycling on default settings, as well as have the ability to extend cycles even further by reducing “compr start heat” to -100.
Fair play to Vaillant for resolving this within just 2 days!
I’m not sure Integral dropped to -89, I think cos i’m in Setback… but it’s a good starting point.
11:10 Integral -89
11:32 Integral 0
(yellow line on graph)
Flow Temp versus Target Flow Temp across that 22 mins: 39.
(difference is purple, ie 2.3 at 11:21)
ie, this is the time & DT where flow temp is higher than target flow temp (the yellow shading!)
So the Integral changes twice as quick as it should?
I do see the Integral changing every 30 seconds when I drill into the data points. Which isn’t right? It should be every minute according to the manual.
The energy balance is the integral from the difference between the flow temperature actual value and targe tvalue, which is added up every minute. If the set heat deficit is reached,the heat pump starts.
If the supplied heat volume corresponds to the heat deficit, the heat pump is switched off.
I have also played with “Compr. start heating from” and tuned this down to -100, but my unit ignores this and starts at -60 regardless.
Hard to see on your graph, but yes, those are the two issues. I added a calculated integral on mine which helps visualize it. While the lines offsets won’t match, you can at least compare the gradient which is what you are interested in. (the division step needs to be based on your sample rate. If it’s 15s this should be 0.25, 30s then 0.5 etc.)
The integral can and does change more often than once per minute (polling it every 10s over ebus you will see this), it’s the magnitude of the change that 's wrong on the old firmware. If you have a 2°C difference between flow and return temperature, the integral should change by 2°min over the course of 1 minute. The old firmware apparently doubles this, showing a change of 4°min.
In your graph, between 11:25 and 11:30, you have a desired flow temp around 26°C and an actual flow temp of 27.5. Over 5 minutes, the integral should accumulate by 1.5° * 5 min = 7.5°min. Eyeballing your integral, it rises from around -24 to -9, which is a delta of 15°min and hence twice as much as it should be.
On a related note: Your ebus flow&return temperatures seem to make sense and not have a terrible offset like mine. I’ll use that as evidence to hopefully convince Vaillant of a repair.
This probably means that Vaillant is slightly under-reporting output and COP in my case, as the 0.3C offset in return temp means lower delta-t vs. heat-meter.
I wonder if there is a way to improve that return temp by adjusting it’s position, using some heat-transfer paste, or insulating around it?
There’s a roughly 1.2 °C more or less constant offset between them, easily to be seen by the remaining delta when the compressor is off. Yello/Blue are Vaillant ebus measurements, Red/Green my own PT1000.
@dfeist@Zarch Thanks for those graphs!! There’s quite a spread there. I wouldn’t mind about an absolute offset but the delta between them when the compressor is off is very telling. I have the suspicion that this might also contribute to my cycling problem since the system always thinks the flow temperature is too high. Let’s see what Vaillant has to say.
If it’s alright with you I will add our three graphs to the Vaillant inbuilt metering vs MID thread for reference as it was discussed there as well.
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