Fröling T4 boiler circulation pump turns continuously on and off in standby mode.

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    Wir betreiben einen Fröling T4 Hackschnitzelkessel mit einer drehzahlgeregelten Wilo Stratos Umwälzpumpe (0–10 V Steuersignal). Der Kessel ist außerdem mit einem Rücklaufmischer ausgestattet. Wir verfügen über einen 3000-Liter-Pufferspeicher. Wir verwenden Hydrauliksystem 4 mit vier Pufferspeichertemperatursensoren.


    Der Kessel läuft beim Heizen einwandfrei, aber im Leerlauf/bei ausgeschalteter Feuerung spielt die Pumpensteuerung verrückt. Kurz nach der Feuerung schaltet sich das Pumpensteuersignal kontinuierlich ein und aus, oft nur für etwa eine Sekunde. Die beigefügten Oszilloskop-Fotos zeigen das Pumpendrehzahlsteuersignal über einen längeren Zeitraum und ein vergrößertes Beispiel. Beachten Sie, dass die Pumpe ständig mit Strom versorgt wird, aber über das Drehzahlsteuersignal ein- und ausgeschaltet wird.


    In Beispiel 2 decken die kleinen Abbildungen den Kesselbetrieb vom Heizbeginn über „Stoker aus“, „Wartezeit Abschaltung“ bis hin zum „Standby“ ab. Die kleinen Abbildungen 51 bis 54 decken das Pumpensteuersignal etwa eine Stunde nach der Feuerung ab. Man sieht, dass das Drehzahlregelsignal der Pumpe sehr schnell ein- und ausgeschaltet wird – viel schneller, als die Wilo Stratos nach Aktivierung des Drehzahlregelsignals starten soll. Nach einigen Stunden beruhigt sich das Regelsignal, die Kesseltemperatur sinkt auf die Puffertemperatur und darunter.


    Diese Art der Pumpenregelung widerspricht den Spezifikationen von Wilo. Ich vermute, dass dieses ständige Ein- und Ausschalten die Pumpe ausfallen lässt. Wir haben innerhalb von sechs Betriebsjahren zwei Pumpen verloren, während andere Pumpen seit 45 Jahren problemlos laufen.


    Seit dem Ausfall der zweiten Pumpe habe ich das Pumpenregelsignal vorübergehend angepasst. Auf den Fotos ist zu sehen, dass das Regelsignal immer bei 3 V oder darüber liegt. Das bedeutet, dass die Pumpe tatsächlich dauerhaft mit Mindestdrehzahl oder mit höherer Drehzahl läuft, wenn der Kessel eine höhere Drehzahl anfordert. Ohne diese Mindestdrehzahl von 3 V würde das Regelsignal noch stärker schwanken. Ziel war es, das ständige Ein- und Ausschalten der Pumpe zu vermeiden. Später habe ich diese Schaltung leicht modifiziert, sodass die Pumpe abschaltet, wenn das Kesselsteuersignal für etwa 20 Minuten oder länger ausfällt. Dies ist jedoch nur eine vorübergehende Lösung zum Schutz der Pumpe.


    Soweit ich weiß, schaltet der Kessel die Pumpe im Standby-Modus ein, sobald die Kesseltemperatur die Pufferspeichertemperatur um etwa 2 Grad überschreitet. Das Problem scheint zu sein, dass überhaupt keine Hysterese vorhanden ist. Unser Techniker vor Ort weiß nicht, warum sich die Pumpendrehzahlregelung so verhält. Das Werk hat bisher keine Hilfe geleistet (ich habe vor zwei Jahren zum ersten Mal um Hilfe gebeten). Mir wurde gesagt, dass der Hydraulikmodus mit zwei Pufferspeichersensoren gut funktioniert. Ich verstehe nur nicht, warum sich der Standby-Modus vom Hydrauliksystem 4-Modus unterscheiden sollte. Soweit wir wissen, scheint es keinen Parameter zu geben, der das Verhalten beeinflusst, weder einen Hystereseparameter noch sonst etwas, das helfen könnte (weder im Standard-Benutzermodus noch im -x-Benutzermodus).


    Google hat mir mitgeteilt, dass mindestens zwei weitere Fröling-Kunden (US-Markt) das gleiche Problem haben und beide den Hydraulikmodus 4 nutzen. Sie haben das Problem mit einem Pumpenzeitrelais gelöst, verwenden aber eine Kesselumwälzpumpe mit fester Drehzahl. In Kombination mit dem 3-Wege-Mischer sollte es eine bessere Lösung geben.


    Ich frage mich, ob jemand aus dem Hauptmarktgebiet von Fröling dieses ähnliche Problem bei der Nutzung des Hydraulikmodus 4 beobachtet und eine Lösung dafür gefunden hat.



    In English if you prefer:

    We have a Fröling T4 wood chip boiler with a Wilo Stratos variable speed circulation pump (0-10 V control signal). The boiler is also equipped with a return feed mixer. We have a 3000 litre buffer tank. We use hydraulic system 4 with 4 buffer tank temperature sensors.


    The boiler runs perfectly when heating but in idle/fire off state the pump control is crazy. Soon after fire off the pump control signal turns on and off continuously, often on for about once second only. The attached oscilloscope photos show the pump speed control signal over a longer period and a zoomed in example. Note that the pump has power all the time but is switched on and off via the speed control signal.


    On example2 the small figures cover the boiler operation from start of heating to "stoker off", "shutdown wait" and finally "standby". Small figures 51 to 54 cover the pump control signal about an hour after fire off. One can see that pump speed control signal turns on and off very rapidly, much faster than the Wilo Stratos is supposed to start after speed control signal activation. After a few hours the control signal calms down, the boiler temperature drops to the buffer top temperature level and below.


    This kind of pump control is against the specifications Wilo gives for the pump. I believe this continuous on and off control makes the pump fail. We have lost two pumps within 6 years of operation while we have other pumps that have been running continuously for 45 years without any problem.


    Since we lost the second pump, I have modified the pump control signal temporarily. On the photos one can see that the control signal always stays at 3V or above. This means the pump actually runs continuously at minimum speed or at a higher speed if the boiler asks for a higher speed. If this 3V minimum speed was not added, the control signal would be fluctuating even more on and off. The intention was to avoid switching the pump continuously on and off. Later on I have modified this circuit a bit so that the pump turns off if the boiler control signal is off for some 20 minutes or more. Still just a temporary fix trying to protect the pump.


    My understanding is that the boiler starts to turn the pump on in standby mode when the boiler temperature exceeds the buffer tank top temperature by some 2 degrees. The problem seems to be that there is no hysteresis at all. Our local dealer technician does not know why the pump speed control behaves like this. The factory has not provided any help at least so far (I have requested help 2 years ago for the first time). I've been told the two buffer tank sensor hydraulic mode works well. I just don't understand why the standby mode operation should be any different from that in hydraulic system 4 mode. There does not appear to be any parameter that affects the behaviour, no hysteresis parameter or anything else that might help as far as we understand (not in standard user mode or the -x user mode).


    Google tells me at least a couple of other Fröling customers (US-market) suffer from the same issue, both using hydraulic mode 4. They have handled the issue with a pump timer relay but they use a fixed speed boiler circulation pump and together with the 3-way mixer a better solution should exist.


    I wonder if anyone from the main Fröling market area has observed this similar issue when using hydraulic mode 4 and if anyone has found a solution to fix the issue?

    Hello and welcome to German Holzheizer Forum!


    I have no T4 so I am not aware of the specific hydraulic module and settings, but there should be a 2K hysteresis between upper buffer tank temperature and boiler temperature.


    Did you check the temperature signals of the boiler and the upper buffer tank temperature sensors? Maybe there is irregular noise/disturbances on one of the lines which leads to the 2K hysteresis?


    Best regards

    Thank You for your reply.


    The T4 seems very similar to other Fröling boilers as far as I've seen wiring diagrams. Return sensor is connected to the core module (wood chip module) as well as the boiler pump, buffer tank sensors are connected to the hydraulic module. Buffer tank temperature sensors are less than 10 metre long. I have measured the AC-voltage at the boiler control boards with a basic multi-meter and found it to be negligible. I cannot remember the exact figures, I have to measure this again with my better multi-meter and report the exact figures.


    Also temperature readings at the H3200 controller screen appear very stable but the screen does not update measurements very fast. I would assume a properly built control software implementing decent time hysteresis too to reduce the control errors from input noise. Then again, an acceptable implementation may assume proper input signal quality. Better to double check this signal noise anyway.


    Yes I agree with you, proper hardware and regulator implementation should cancel all noise. As you said, better double check, best with the oscilloscope directly at the temperature inputs on the board. But as the on-times of the control signal for the pump is in the range of few seconds, noise is maybe not the problem, but who knows how the regulator internally works. First thing is to check all the signals, then go deeper into controller settings (but, same as you, I am not aware of a parameter which could change the hysteresis between boiler and buffer tank. Maybe one should go into development level of the controller and take a look there).


    I have problems from time to time with the temperature sensor for drinking water. It shows sometimes much higher or lower temperature than it actually should measure. But I can see these wrong readings in the display although it's refresh rate is actually slow, as you already mentioned.


    Best regards

    I have now checked temperature sensor readings with a 4 digit multi-meter. Voltage figures were in the 2.635 V range and the mV digit was absolutely stable. 0 V in AC-mode, even peak hold min/max zero. But this was a stationary situation, no pump switching or such. Someone at the Hearth.com forum reported his top buffer tank sensor showing a one degree drop when the pump starts, then one degree up when the pump stops. Obviously need to try to monitor sensors during these transient cases. Our sensors have a common ground at the boiler connectors, the pump or the mixer should not affect sensor readings but one never knows. Reading from the console though sensors remain stable at the 0.5 degree resolution but the slower update rate may hide some spikes.


    I already tried to capture sensors with my memory oscilloscope but realized the cheap PC-scope has an 8 bit ADC and no option for analogue DC-offset to increase gain and the scope is not capable of reading sensors at sub-degree accuracy, more like a couple of degrees resolution which is a no go.


    Somewhere at this forum I found discussion about access modes other than user and -7, I wonder if these give better access to hysteresis parameters and such.


    Meanwhile I also got a response from the factory (via our dealer, the factory does not want to talk directly with a customer) where they suggested the boiler running quite hot and not able to transfer heat the buffer properly. This is nonsense, at the start of standby mode, the lower buffer temperature is less than 70 degrees while the boiler temperature is in the 81 to 82 degrees range. However the boiler does not open the mixer and run the pump properly but instead keeps the return temperature very close to the boiler temperature.


    I should have mentioned that we have a check-valve after the pump in the return feed line. The check valve was included with the boiler installation parts and intended to allow a cascade setup. The check-valve prevents incorrect water flow when a second boiler is heating and the one in question is off. I'm afraid the check-valve prevents natural water circulation from temperature difference between the boiler and lower part of the buffer. The pump might not need to come on to cool the boiler if there was no check valve. The mixer however would need to be opened by the boiler. No big deal to run the pump too at the same time for a short while (but longer than a second or so as currently).


    I'll do some more measurements and report later. Hoping also feedback from other hydraulic mode 4 users as well as any other ideas.

    Measuring the temperature signals with a multimeter will not lead to success because multimeter will also not really show peaks on the lines and in addition update rate of the multimeter readings could be too slow. Can you measure the signals with the Picoscope with AC coupled input mode? Then you could set the Y axis to highest resolution and only measure AC signal components.


    Signals may have a common ground which have nothing to do with the pump current at first glance, but inductive or capacitive coupled disturbances may flow through paths which are not obvious, most times they will flow through earth loops back to the source of disturbances.


    When you are not able to measure the signals, you could try to add parallel capacitance to the temperature signals inputs in range of few hundret pico farads to ten nano farads (multilayer ceramic capacitors, best C0G/NP0 dielectricum in this case), and see if this makes the problem better. But I don't know how much pure capacitive load is allowed at these inputs, as these are not purely inputs but some sort of voltage or current outputs to drive the sensors. Too high capacitive load could lead to oscillation, so you should observe the signal with the oscilloscope when adding the capacitors. Another method to supress noise at the inputs would be to use common mode chokes.


    I did take a look at all the parameters with access mode "installation" and "internally", but "installation" gives less parameters than -7 and "internally" only gives in addition some regulator settings for the fan and the mixer. None of the parameters I saw could change the 2K hysteresis. In addition there is the development access, but unfortunately I don't have the code for it.


    Fröling service is quick and friendly, but for deeper technical questions useless. They don't have a real plan of what is going on inside the controller. I think for good support they would have to forward your case to the development department, which is unfortunately unusual in many companies (it has something to do with the pride of the service department and/or with the fact, that development department has "always" something better to do than look at the individual problems of very few customers, which is a shame because the company should have deep interest to improve their systems....)


    Best regards


    PS: One additional idea - when the problem occurs, you could try to connect fixed value resistors on the temperature inputs of the boiler and the upper tank, which could simulate f.e. a boiler temperature of 80 °C and a upper tank temperature of 85 °C. If then the pump control signal will be 0 V (or 3 V in your actual case) and is not alternating anymore, then you know it has something to do with the temperature sensors and/or the wiring. If the alternating pump control signal will then still be visible, the error maybe occur directly in the controller hardware or firmware. At this point you could make pressure on Fröling to solve the problem because it happens inside the controller and nobody but Fröling can influence the functions.

    Thank You Holzheizer for all your ideas.


    I believe my Fluke multi-meter is quite fast in peak measurement mode but probably difficult to use when trying to capture transients from switching pumps and mixers etc. specifically when the actual temperature also slowly changes.


    Very good idea to use AC-coupled mode with the oscilloscope. I just hope my cheap Chinese USB-scope does have a true AC-coupled input option instead of implementing AC-coupled input via digital signal processing at the laptop.


    I'm hesitating to touch sensor wiring, also the boiler gives a fault code and switches off if a sensor is disconnected. Adding a parallel resistor to either of the temperature sensors would be possible on-the-fly and without disconnecting existing wiring. In this case I guess reducing the boiler temperature reading by a couple of degrees with a parallel resistor.


    Very nice that you have looked at different parameters being available with different access modes. A pity if there does not appear to be any adjustable hysteresis for pump control.


    I agree earth loops and inductive or capacitive coupling may be an issue even if this feels unlikely when our boiler house does not really have other devices except those for boiler operation and all wiring there is coming directly from the boiler circuit boards. Should have a common ground but one never knows.


    I'll think about all the proposals but will probably first try the scope with AC-coupling to capture any temperature sensor transients during the period where the pump control signal misbehaves.


    Our local dealer has also suggested capturing a log with their add-on logging device. This should be informative as it would give exact readings of all measurements exactly as the boiler controller sees them. I just don't know if log analysis needs to be done at the factory or if the local dealer can decode the data.

    I don't even think that the problem is caused by disturbances, but the method to narrow down the source of the problems should be "from outside to the inside". So the regulator has it's input signals (temperature of boiler and upper tank) and this will generate some sort of output signal for the pump control signal. So my suggestion would be to first check the input signals for the regulation loop before looking deeper into the controller settings.


    But even when there would be a parameter in development level, which could change the 2K Hysteresis, why should it be changed at your controller since you never accessed the development level?


    When you observe this oscillating pump control signal, how are the temperature values at this time?


    Maybe try to disconnect the pump control signal cable, which carries the 0-10 V signal, and then observe the signal if it is oscillating further (only with scope as load on the analog output). Just to check if the output is working properly and the load (cable and pump control input impedance) is not causing any problems with the output. I mean the output is just working fine in "normal heating mode", but I think that control parameters for the output signal could be different in heating and standby mode (in standby mode, output signal generation is maybe more "agressive" and less smooth than in heating mode - but I don't know it exactly).


    Logging is another good idea, decoding the data shouldn't be a problem for you and/or your local dealer. As far as I know it is some sort of CSV data which can easily be read with Excel or something simllar.


    Best regards


    Edit: Last solution would be to buy a PWM controlled pump next time ;) but before this, you should measure in PWM mode if that does the job properly


    Edit2: Another idea would be to buy a cheap PWM to 0-10 V adapter module and use the PWM output of the controller with the actual pump (implied that PWM doesn't show this faulty behavior)

    My comments below inserted to your text:


    Yes, I agree that there should be some filtering in hard- or firmware which cancells most of the noise.


    If temperatures are close to the 2K threshold, small changes in amplitude will cause the controller to turn on the pump. And if there is really literally no hysteresis, then we have some sort of oscillation on the pump control signal like you showed it.


    But if there is really no hysteresis, same problem should occur with PWM signal. Maybe you could check this. Then you know that the problem is not the analog control signal but the non-existing hysteresis.


    You could easily check if you temporarily connect potentiometers on the boiler and upper tank temperature inputs and simulate the behavior.


    Did you try a firmware upgrade? Maybe your local dealer could do this for you.


    Best regards

    Agreed, any noise in the sensor signals defeat the hysteresis if the controller does not have any time hysteresis (not even for a couple of seconds). Difficult to be sure if the hysteresis is missing or if some noise of the order equal to about one degree on both sensors kill the hysteresis.


    I should be able to test with some self introduced hysteresis without detaching sensor wires. A parallel resistor on the boiler temperature sensor connectors would make the controller see "low enough" boiler temperature relative to the buffer tank top temperature. I've just been a bit busy lately and need to find the time to test this shortly after boiler shutdown, at standby state.


    Our boiler SW is from November 2020, actually thought it was updated later. This is when the older SW stopped supporting Fröling Connect application.


    Now I've been handed contact info for a factory expert too. I will soon explain them the issue hoping they could provide a simple solution. Will report of course if it is something that could benefit others.

    A progress update here: we are going to have the logging device installed to the boiler. It should show if sensor readings are noisy or misbehaving and a lot of other useful info. I'll give an update when the log is available and has been analysed.

    Quote from Holzheizer12345

    Any update?


    Thank you for your interest! Some yes, I installed the logging device yesterday. I'm supposed to keep it for a week or two although I think one heating cycle should be enough. Will be interesting to see if the log file is something readable like CSV or if I need to wait decoding by the factory expert.


    Meanwhile I've also started to think if there could be some cavitation that kills the pump. The point being that the pump mostly runs at less than 40% drive signal (30% equals to the minimum pump speed) but after the heating state, during empty stoker and during shutdown wait the pump tends to run at maximum speed or very close to maximum while most of the time the mixer is closed or marginally open. The bypass (not sure about the correct name, the pipe that feeds the pump from the boiler when the mixer closes the buffer tank line) line pipes are a bit smaller than the main pipes but the main pipes are supposed to handle two boilers. I have no knowledge about this topic, not sure if it could be any issue. I anyway reduced the pump max speed to 80% (from 90%), perhaps could go a lot further. Just stupid to try to solve any issue this way, if it even is an issue. I wonder why how the boiler decides on the pump speed e.g. at shutdown wait state. Would be interesting to know how this detail works with hydraulic system 1. This pump short cycling anyway is a clear fault and I hope we can fix at least one issue at a time.

    Hello again everyone. We've now had the boiler log running for a week. I've sent the log to the factory for analysis/comments, obviously takes some time fort them to have a look.


    I had a quick look myself, here a graph showing the pump speed (pinkish line) and the boiler to buffer top temperature difference (multiplied by 20 to make it better readable, blue line). The graph starts from the point the boiler enters empty stoker phase. Sampling period is 5 seconds, at 154 the boiler enters shutdown wait state, at 358 it enters standby state.


    The pump stops running for almost 15 minutes at the beginning of the standby state (or actually just the pump speed control signal goes to zero, I still have my modification for the pump speed signal keeping it running at minimum speed from the latest active pump drive signal, meaning all the time within this shown period). The interesting thing occurs after this 15 minutes pump drive pause. Here the boiler to buffer top temperature difference cycles between 2 and 1.5 degrees (40 to 30 in the figure). Every time the temperature difference increases to 2 degrees, the pump is driven at max speed (80% setting currently). Immediately when the temperature difference drops to 1.5 degrees, the pump drive drops to zero. Most often the 2 degree difference occurs for a single sample period only. In practice the changes are most often shorter but not captured at the 5 second sampling period.


    Obviously there is some minor unavoidable noise in the temperature measurements in the 2 degree difference region. A 2 degree difference then can be observed for five seconds only. No problem there if the pump control then had some hysteresis (at least on the temperature readings, preferably time hysteresis too) but there is none!


    My opinion is that the 100% match with the pump drive and the shown temperature difference pretty much proves that this boiler to buffer top temperature difference is the pump speed decision factor. This would be OK if decent hysteresis was applied. Actually even if there was a hysteresis of a couple of degrees, I'd still be puzzled about the actual pump speed applied. Running the pump at the minimum speed would be perfectly sufficient but the speed is less of an issue, the lack of hysteresis appears to be fatal (likely reason for pump failures?).


    Thank you for the detailed update!


    So the pump starts, when boiler temperature is 2 K above the tank temperature- that's normal. And the pump is controlled with max. speed signal - that's normal too.


    The pump stops, if boiler tank temperature is just 1,5 K above the tank temperature? That would not be normal. Hysteresis should be 2 K.


    You will send this log to Fröling, right? The reaction of Fröling would be interesting. If you like, you could give us an update when Fröling answered.


    Best regards


    Edit: Did you check in the meanwhile, if the same problem occurs on the PWM output?

    I have sent the log file to Fröling almost two weeks ago but I got an out-of-office reply. Perhaps the expert is on holiday or sick leave or it takes some time for them to figure out what's wrong.


    I did monitor the true pump signal with my oscilloscope while the log was running. I should have included these measurements here too. Attached a sample covering a longer time at the start of standby state. I also took a zoomed screen capture which shows that the pump drive goes on and off faster than the 5 second sampling rate that the log is using. 5 second sample rate however does capture quite many of the on/off pulses.


    Unfortunately I cannot synchronize the oscilloscope view exactly with the log file signal. Could be useful to find out a synchronized zoomed sample.

    I think I forgot to mention one odd occurrence with the pump drive. At some point deep into standby state the pump turns on for three log samples (15 seconds) at full speed even if the maximum speed is set to 80%. I don't think this is anything critical over the rest of the pump short cycling but just another bug in the code. Makes me wonder how many other bugs there are. The oscilloscope log confirms this odd occurrence.


    I also followed some of the buffer temperature sensor values from the log. There is no visible noise, values go up or down smoothly at 0.5 degree resolution. At the exact point moving from x to x-0.5 one may see a few samples back and forth which is quite unavoidable (no hysteresis supposed to be at this point). The AD-converter however seems to be of a traditional type and working at its resolution limit. At some temperature I noticed the temperature reading skip on 0.5 degree step, otherwise changing equally smoothly as elsewhere. Must be a linearity error at some higher bit of the ADC changing state. If the control hysteresis was true 2 degrees, this would still leave sufficient hysteresis for pump control.


    Some updates on this topic. Our contact at Fröling was out of office when I sent the log data. Since I had not heard of them for a month, I sent another email to check if the expert is in the office and if they have had a look at the log data. This time I got a short reply along with their graph from the log, covering one heating cycle.


    The conclusion at Fröling is that "the buffer charge level is high, the buffer load criterion should be reduced". No comment at all about the missing hysteresis. The pump starts when the boiler temperature reaches 2 degrees more than the buffer top temperature (this is OK). How come the pump is switched off immediately at 1.5 degree temperature difference "because the buffer charge level is too high"?


    Fröling did not include the buffer charge percentage in their graph, nor the boiler bottom sensor figures. Seems to me that this factory expert looks at the graph and finds out the pump stops short cycling once the boiler temperature drops and concludes the issue being "too high buffer top temperature".


    Perhaps they assume at Fröling that the buffer bottom temperature is too high. If this was the case, the pump would need to keep running but it would remain constantly on, not short cycling. The aim would be to pump cool water from the buffer bottom to cool the boiler down to buffer top temperature (I assume the missing hysteresis should be from boiler 2 degrees above the buffer top to boiler equal to buffer top). In our scenario the buffer bottom temperature is at least 10 degrees lower than the buffer top, meaning it would be easy to cool down the boiler from say 82 degrees to 80 degrees. This just does not happen currently because the mixer has no time to open up, the pump stops immediately when the boiler to buffer top temperature reduces to 1.5 degrees (this isn't even 0.5 degrees drop in practice but the reading fluctuating between two values at 0.5 degrees measurement resolution).


    I also would have expected more accurate guidance on "reducing the buffer load level criteria". The point being that in hydraulic system 4 case, the main parameter to control the buffer charge level is the buffer charge percentage to shut down the boiler. This particular parameter how ever has not been implemented currently. The parameter can be adjusted but the boiler always uses a fixed 96% buffer charge level to switch off. Actually quite a decent figure but if the advice was to reduce buffer loading level, this parameter is of no use. The buffer temperature can be reduced by reducing the boiler temperature set point. This we do differently for the winter compared to summer. Reducing boiler temperature set point makes the buffer bottom part cooler but the buffer top does not change much (which is fine as such). More importantly, this does not change the pump drive behaviour at all. Because of the lacking hysteresis, cooler water at the buffer bottom does no make a difference. The pump only switches off properly once the boiler cools down for other reasons than the circulation pump cooling it. The short cycling pump helps marginally but eventually residual boiler heat dissipates via some flue gas flow and leak to the boiler room.


    We should remember that the graph shows the boiler turning the circulation pump off but because of my modified speed signal, the pump actually keeps running some 15 minutes at minimum speed after the boiler turns the pump off. This is not sufficient to cool down the boiler because the boiler keeps the mixer closed. This just protects the pump from too frequent switching on and off. The mixer leaks a bit even when closed and the short cycling as seen from the graph would look even worse without the pump drive modification.


    Now I am quite piss** off when I have explained the missing hysteresis to the factory expert but they completely ignore that fact. Feels like they are not seriously trying to help and solve their problem but I'm still hoping for some useful feedback (helps them getting rid of me from complaining). I asked for some reference log data from another customer using hydraulic system 4 with a speed controlled pump and a mixer. I'm starting to assume everyone else is using their default mode, hydraulic system 1 (except the couple of customers from the US but they don't have speed controlled pump, neither the mixer and an external timer relay works fine in that scenario to solve the pump short cycling). I am already quite ready to move to hydraulic system 1 too but first I'd like the company to confirm that their implementation of hydraulic system 4 is not working.


    Any observations from the Fröling graph that I may have missed would be welcome.


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