What is Energy Integral

[IGP]

Good thread on it here too, which posted in another thread yesterday. 
 

https://energy-stats.uk/vaillant-arotherm-firmware-351-06-07-problems-energy-integral/amp/

[Dreadnaught]

Thanks @IGP. Yes, I had read that post too. All very interesting.

 

Another thing I have noticed is regarding ΔT.

 

My understanding is that heat pumps generally target a ΔT of about 7º (or sometimes I have seen 5º referenced), which is unlike gas boilers that target something around 20º.

 

However in @Dan F's chart above, the ΔT is just 2º after about 20 minutes of heating (24.5º supply, 26.5º return), which surprised me. Its just an observation and I am sure there is a good explanation for it.

 

I wonder if this is just a consequence of the modulation flexibility of the compressor in the Arotherm Plus. The compressor can only modulate down to a minimum 25% of full speed, not lower.

 

(I do note that the ΔT is user definable (setting "Compressor hyster. Heat." in  the installer-level heat-pump controller settings.)

[JohnMo]

28 minutes ago, Dreadnaught said:

My understanding is that heat pumps generally target a ΔT of about 7º (or sometimes I have seen 5º referenced), which is unlike gas boilers that target something around 20º.

 

However in @Dan F's chart above, the ΔT is just 2º after about 20 minutes of heating (24.5º supply, 26.5º return), which surprised me. Its just an observation and I am sure there is a good explanation for it.

Bit of a generalisation on gas boilers, my Atag boiler and most low temp boilers do not work at a dT of 20, when I was flowing 30 degs the dT was closer to 5.

 

dT is variable dependant on heating system and flow rate. During a heating cycle our ASHP will start at about 7 dT, and slowly close this down to about 3.5. reason is simple the return temp rises over time, and target flow temperature is fixed.

Edited September 14 by JohnMo

[Dreadnaught]

12 minutes ago, JohnMo said:

dT is variable dependant on heating system and flow rate. During a heating cycle our ASHP will start at about 7 dT, and slowly close this down to about 3.5. reason is simple the return temp rises over time, and target flow temperature is fixed.

 I wonder how then is compressor modulation controlled by the system? A direct response to ΔT (within the operating limits of 25% to 100%)?

[JohnMo]

14 minutes ago, Dreadnaught said:

 I wonder how then is compressor modulation controlled by the system? A direct response to ΔT (within the operating limits of 25% to 100%)?

Generally when the compressor starts it tries to get the target dT fixed with as little heat input possible. Once target dT is there it manages dT by either modulating output down (if it can) or to add additional heat to flow temp as return gets hotter. Once at target flow temp it allows dT to reduce. Once the heat pump controller is no longer happy the compressor is switched off.

 

Mine running - 25 mins running.

Yellow/tan shaded area is power output big load initially and then quickly goes to the main modulation it can. Red flow temp slowly increases and dT between green (return) closes

 

 

[SteamyTea]

Integration, in mathematics, is continuous addition.

So I suspect that it works in a similar way as a heating controller.

It keeps adding, or subtracting, until the desired outcome is reached.

The outcome will be one of at least 3 bounds, the other two will be minimum and maximum output if the unit.

The unit controller will probably have an initial set point (in the middle of the performance curve) and then is manually set to add, or subtract 'degree minutes'.

Degree minutes are the product of the temperature and the time it is delivered i.e. multiply flow temperature by how long the system is delivering (it may be the difference between flow and return as that is the energy delivered).

 

The same can be achieved by simply switching the system on and off for varying amounts of time, but that is inefficient with a heat pump (but works well with a resistance heater).

[Dreadnaught]

3 hours ago, JohnMo said:

Mine running - 25 mins running.

 

Thanks @JohnMo

 

I see the ΔT shown in your chart is about 3.5º.

 

- Is your's set at the factory-default of 7º ΔT?

- Would you be able to overlay on that chart a line for compressor modulation (%)?

 

My theory is that, with the well-insulated houses that are common to those on this site, such heat pumps will hit their lowest-level limit of modulation for much of the time (which is 25% max compressor speed) because heat demand from the house is just so low. That includes yours during the period shown in your chart and would explain the ΔT.

[JohnMo]

This is what my manual says

 

8.1 EVAPORATOR WATER FLOW

The nominal water flow rate refers to a 5°C temperature difference between the evaporator inlet and outlet. The maximum permitted flow rate features a 3°C temperature difference while the minimum one has an 8°C temperature difference at the nominal conditions as shown in the technical sheet.

 

My circulation pump speed is varied by the controller, so I have no real control over dT.

 

Generally heat pump will modulate but are just as likely to modulate a bit and switch off especially at low flow temperature. Some are better than others, your modulation is also dictated by your flow temp, low flow temp generally equals less modulation, from what I have seen. Mine currently only modulates down to 64% of nominal capacity at 12 degs

 

From an energy used perspective, a heat pump pulling 800W 20 mins in the hour is better than one 400W 60 mins an hour.

[Dreadnaught]

 

4 minutes ago, JohnMo said:

8.1 EVAPORATOR WATER FLOW

 

 Oh that's interesting, @JohnMo. Which manual is that? Its not in my manual.

[Dreadnaught]

4 minutes ago, Dreadnaught said:

Which manual is that?

 

Ah, @JohnMo, Google tells me you have a Viesman heat pump, possibly a "Vitocal 100-A" or similar? Is that so?

 

Mine is a Vaillant Arotherm Plus 5kW.

 

For the Arotherm, the circulation pump speed is effectively fixed (860 l/hr for the 5kW model) and there is a setting for the ΔT (albeit a setting that can only be accessed by an "expert").

Edited September 14 by Dreadnaught

[mk1_man]

@JohnMo  :  What make / model is your heat pump?  Your traces are very similar to mine.  I get the initial spike followed by a period of modulated output then after a period of less than 1hour heat pump turns off, waits about an hour then does same again. Mine is an Arotherm 7kW :

 

[Dreadnaught]

6 minutes ago, mk1_man said:

Mine is an Arotherm 7kW

 

Ah, @mk1_man, yours shows a ΔT of about 3.2º. Interesting again. Are you able to look at the compressor modulation % during that period?

[PhilT]

2 hours ago, Dreadnaught said:

My theory is that, with the well-insulated houses that are common to those on this site, such heat pumps will hit their lowest-level limit of modulation for much of the time (which is 25% max compressor speed) because heat demand from the house is just so low.

Which is not good. There is a significant variation in efficiency/COP throughout the modulation curve, for example this research chart shows peak efficiency at around 40% of the modulation range. JohnMo's heat pump is a 6'er I think, so in mild temps the modulation range could be as much as 3 - 8kW. His chart shows a constant 5kW output which would be around 40% of the modulation range - bang on the optimum according to this chart. Mine does exactly the same in Auto mode, but in straight WC mode it cycles on and off at minimum modulation level during mild outdoor temps, and the performance hit is very clear to see.

[Dreadnaught]

@PhilT, thanks. That's interesting. What's the reference source for that graph?

 

Is it as google found:

 

"PV Optimized Control of Modulating Heat Pumps regarding PV Self-Consumption" (January 2019) by Christina Betzold and Arno Dentel?

 

Link. I had a quick scan read.

 

When I encountered: "The bi-quadratic-polynomial equation for describing the heat pump characteristics" I thought I might leave it to @SteamyTea and went to pour myself a drink instead.

Edited September 14 by Dreadnaught

[SteamyTea]

2 hours ago, Dreadnaught said:

leave it to @SteamyTea 

Thanks, just leave out the odd indices, then it becomes a lot shorter

[JohnMo]

6 hours ago, mk1_man said:

What make / model is your heat pump

 

6 hours ago, Dreadnaught said:

Viesman heat pump, possibly a "Vitocal 100-A" or similar? Is that so?

It a Maxa i32V5, which is same unit Viessmann bought in and rebadged as a 100-A.

[Dan F]

On 14/09/2024 at 08:23, Dreadnaught said:

I wonder do you happen to know whether "Hc2ActualFlowTempDesired" is targeting the temperature of the flow to or return from the home?

 

flow

[Dan F]

On 14/09/2024 at 08:49, Dreadnaught said:

My understanding is that heat pumps generally target a ΔT of about 7º (or sometimes I have seen 5º referenced), which is unlike gas boilers that target something around 20º.

 

However in @Dan F's chart above, the ΔT is just 2º after about 20 minutes of heating (24.5º supply, 26.5º return), which surprised me. Its just an observation and I am sure there is a good explanation for it.

 

I wonder if this is just a consequence of the modulation flexibility of the compressor in the Arotherm Plus. The compressor can only modulate down to a minimum 25% of full speed, not lower.

The aroTHERM works a bit differently. It uses a constant flow rate and varies delta-t between around 2 and 10K.

[Dreadnaught]

13 minutes ago, Dan F said:

and varies delta-t between around 2 and 10K.

 

Thanks @Dan F. Very interesting.

 

With the Arotherm Plus, do you happen to know how the user-selectable setting of "Compressor hyster. Heat.", which seems to define a ΔT, is relevant?

 

Its default is 7℃ and the range of possible settings is 0 to 15 ℃. (The setting is not accessible via the VRC 720 but only via the heat-pump controller - which in my case is part of the UniTower).

 

In operation, my guess has been that the controller targets a ΔT according to the value set for this setting and uses the "target flow temperature" only for the "Energy Integral" calculation, to define when the heat pump turns off (or on), but I might be over-simplifying.

 

With a constant flow rate (which we know is an idiosyncrasy of the Arotherm Plus), one might think that targeting both "ΔT" and "Target flow temperature" at the same time would more often than not be contradictory when the only parameter subject to control is the compressor speed, hence my enquiry.

[Dan F]

12 minutes ago, Dreadnaught said:

 

Thanks @Dan F. Very interesting.

 

With the Arotherm Plus, do you happen to know how the user-selectable setting of "Compressor hyster. Heat.", which seems to define a ΔT, is relevant?

 

Its default is 7℃ and the range of possible settings is 0 to 15 ℃. (The setting is not accessible via the VRC 720 but only via the heat-pump controller - which in my case is part of the UniTower).

 

In operation, my guess has been that the controller targets a ΔT according to the value set for this setting and uses the "target flow temperature" only for the "Energy Integral" calculation, to define when the heat pump turns off (or on), but I might be over-simplifying.

 

With a constant flow rate (which we know is an idiosyncrasy of the Arotherm Plus), one might think that targeting both "ΔT" and "Target flow temperature" at the same time would more often than not be contradictory when the only parameter subject to control is the compressor speed, hence my enquiry.

 

It targets a flow temperature and adjusts the compressor speed to achieve this, it doesn't target a ΔT.  The "Compressor hyster. Heat." is not a target ΔT, but rather the maximum difference between target and actual flow temperature allowed before the compressor shuts down.

 

In a large volume system when heat demand is low (autumn/spring) the primary (or potentially only) mechanism that decides when to turn compressor on/off is the energy integral calculation. This is because the flow temperature never overshoots enough to hit the 7K hysterisis limit.  But, with a low-volume system, or one with too many zone controls, the 7k hysteresis will likely be the primary mechanism that controls when compressor turns off. 

 

 

Edited September 16 by Dan F

[Dreadnaught]

@Dan F, thank you.

 

That makes full sense now.

 

It's effectively a double limit for when the compressor is turned off: (i) the energy integral; (ii) and a ΔT between the "target flow temperature" and the "actual flow temperature" (not the return temperature) . Local and system conditions will define which limit is hit first, as you say.

 

An example: if the "target flow temperature" is set at 25º, the compressor will operate until the flow temperature reaches (25+7) = 32º (the return temperature is irrelevant), OR if the "Energy Integral" has reached "0" (from its user-selectable starting value, with default -60 ºmin).

 

Is that correct?

 

Regarding when the compressor restarts after being shut down, do you happen to know whether it is only the "Energy Integral" that does so, or whether "Compressor hyster. Heat." also does so at the other extreme (in my example above 25-7 = 18º)?

Edited September 16 by Dreadnaught

[Dan F]

9 minutes ago, Dreadnaught said:

@Dan F, thank you.

 

That makes full sense now.

 

It's effectively a double limit for when the compressor is turned off: (i) the energy integral; (ii) and a ΔT between the "target flow temperature" and the "actual flow temperature" (not the return temperature) . Local and system conditions will define which limit is hit first, as you say.

 

An example: if the "target flow temperature" is set at 25º, the compressor will operate until the flow temperature reaches (25+7) = 32º (the return temperature is irrelevant), OR if the "Energy Integral" has reached "0" (from its user-selectable starting value, with default -60 ºmin).

 

Is that correct?

 

Regarding when the compressor restarts after being shut down, do you happen to know whether it is only the "Energy Integral" that does so, or whether "Compressor hyster. Heat." also does so at the other extreme (in my example above 25-7 = 18º)?

 

Yes, thats correct.   Yes, hysterisis of 7K will mean compressor comes on at 18C.  You ideally want your system designed and installed to never really use the hysterisis though IMO.

 

If your system volume is good, but not very big, it can be a good to increase the hystersis just so that when the compressor comes on (before it dials back to minumum) it doesn't hit the 7K and turn off prematurely.  This does however depend on if you have a buffer, are/aren't using mixed circuit and what your emitters are though. In some cases it may not be a good idea to have water 7-15k above target even for a short period of time. In my case I have a buffer + mixed circuits so it only means the buffer gets hotter, the circuits don't.  But if you had open-loop with radiators this might not be a good idea.

 

 

 

Edited September 16 by Dan F

[Dreadnaught]

This all now presents another question: what are the best settings for a well-insulated house with a simple heating system* for …

• "Compressor hyster. Heat."
• "Compr.start heat. from"
• "Room temp. mod."
• "Set-back mode"
• "Set-back temperature"

Are there any other settings that are relevant for heating that I have missed?

 

* Mine, unlike yours, is, hydraulically, a very simple system: flow is to the UFH manifold without a secondary pump and without buffer or low-loss header. Single zone, no controllable zones; no wiring centre; all manifold valves are fully open. Low system resistance (big fat pipes throughout). Basic system diagram code "8". As simple as can be. I calculated that my total system volume is about 100 litres. 

 

Edited September 16 by Dreadnaught
I note that my system is not like yours 🙂 And to add system volume.

[Dreadnaught]

Reflecting on what you've told me, for my system …

• "Compressor hyster. Heat." could easily be set at 15º (its highest possible value). Nothing bad will happen to the UFH pipes or my 55-tonne concrete-raft foundation-slab if the water circulating is 15º hotter than the "target flow temperature" and it will mean that, in the circumstances where "Energy Integral" is not shutting the compressor down, that the compressor will run for longer – good for CoP.
• "Compr.start heat. from": similarly I can set this at -120 ºmin (the lowest possible value, a reduction from the default at -60 ºmin) for similar reasons, so that the "Energy Integral" takes longer to shut the compressor down, again helping CoP.

Edited September 16 by Dreadnaught

[Dan F]

5 minutes ago, Dreadnaught said:

Reflecting on what you've told me, for my system …

• "Compressor hyster. Heat." could easily be set at 15º. Nothing bad will happen to the UFH pipes or my 55-tonne concrete-raft foundation-slab if the water circulating is 15º hotter than the "target flow temperature" and it will mean that, in the circumstances where "Energy Integral" is not shutting the compressor down, that the compressor will run for longer – a good thing for CoP.
• "Compr.start heat. from": similarly I can set this at -120º min (the lowest possible value, a reduction from the default at -60 ºmin) for similar reasons, so that the "Energy  Integral" takes longe to shut the compressor down, again helping CoP.

 

Exactly.  Main reason to keep hysteresis at 7K or to keep the integral start at -60 ºmin is comfort (or any limits you need to consider for the flooring type if open loop).  But in a system like ours, this isn't a concern.

 

Using -120º will have a direct impact on the amount of cycling when demand < min power and in turn a small impact COP.  Adjusting the hysteresis probably want make any difference in practice, but no harm in increasing.

 

This brings me to another question though, do you have buggy firmware that doesn't allow this to be configured, or newer one?