OBJECTIVE
This technical information details the new PID cascade algorithm that can be selected within the OpenTherm page of the RegConfig Configurator.
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The following image shows, within the OpenTherm page of the RegConfig, the position of the Heating System Cascade control Algorithm Control section (red) and the Secondary Heating Buffer tank Compensation Temperature Compensation section (blue), closely related to the PID cascade algorithm:PHOTO TBD
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The cascade algorithm can be of three types:
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Enabled OpenTherm generators are those generators that have a number (which can range from 1, highest priority, to 8, lowest priority) in the Generators section (highlighted in green) on the OpenTherm page and not OFF in the box corresponding to the “Heat/Cool” column. The number indicates the priority of the call, so in the image below G1 and G3 generators have priority 1 (they are called first), while G2 has priority 2 and G4 priority 3:
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Attenzione: non utilizzare G0 in quanto è il generatore OpenTherm utilizzato con il prodotto DOT, non con il sistema REG, quindi nella logica di cascata non viene mai considerato. Warning
Do not use G0 as it is the OpenTherm generator used with the DOT product, not with the REG system, therefore it is never considerate in the cascade logic.
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However, it is important to underline that “PID Cascade” and “Buffer Tank Compensation Temperature” are two different and separate functions, which can operate together or separately (e.g., the “Buffer Tank Compensation Temperature” can be also used for other types of cascade algorithms, and also for Parallel functioning), which incidentally “share” the same sensor, as the requirement for both algorithms is to monitor the “final” temperature provided by the group of generators.
Note
The “Buffer Tank Compensation Temperature” is not the subject of this technical part, which instead focuses on the “PID Cascade”.
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so, if we consider a delta T of 4 K and a delay T of 5 minutes before switching on a second generator, then the value of Pid I to be entered is 87 (1740 / 4 * 5). The lower the Pid I parameter, the more time will pass before an additional generator is added.
Note
The values 160 and 1740 were derived from formulas implemented for this specific algorithm!
N. Start Generators: indicates the number of generators that are switched on starting from the condition “all OpenTherm generators are off”. For example, let us assume that the Start generators number is 2: you are in the condition “all OpenTherm generators are off” and the cascade algorithm results in only one generator switched on, but at least two generators are switched on at the same time. After switch-on (and therefore at least after the time indicated by “Stop between changes”) the algorithm may assess if the number of generators is appropriate to the demand of the system (and therefore leave it unchanged) or decrease or increase it; the number of generators at the start is useful if there are systems with many generators. Let us suppose we have a system with 8 generators: if the system starts with only one generator and then every “Stop between changes” (typically 3 minutes) adds another one, it will pass at least 21 minutes before the system goes at full speed; instead, if we set the initial number at 4, the system starts immediately with 4 generators, and then we decide if increase this number (and, in any case, in 12 minutes it would go to the maximum) or decrease it.
N. Minimum Generators: indicates the minimum number of active generators. If, for example, it is set to 3 and 3 generators are currently active, even if the cascade algorithm were to determine the need to switch off a further generator, none would be switched off; in another way, they would have never left on less than 3 generators on (Note: this means that the REG System enables 3 generators which can then decide to switch off the flame/compressor, because they determine that their water outlet temperature is satisfied).
N. Maximum Generators: indicates the maximum number of generators that can be switched on at the same time. If 5 generators are enabled, but the maximum number is 3, this means that the algorithm can switch on maximum 3 generators, even though 4 would be needed. This choice has the direct consequence that the set point may never be reached or that may be reached but over a long period of time.
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The lower part of the OpenTherm page shows for each generator the result of the heating/cooling and DHW requests (red box) and the feedback from the generators to the REG System (yellow):PHOTO TBD
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Highlighted in red you have:
The demand for H = Heating, W = DHW, and C = Cooling;
The required temperature in heating or cooling [°C];
The power required in heating or cooling [%].
Warning
If the generator is used for DHW production only both the required temperature and power are zero, the values that are passed in case of DHW production are those that are displayed in Generator Demand Status (top right of the OpenTherm page)
The OpenTherm page can manage both boiler via OpenTherm protocol (without dedicated firmware) and heat pumps or boilers via Modbus protocol with dedicated firmware. In yellow are highlighted the values transmitted by the boiler or the heat pump and are closely linked to how the generators’ manufacturer has implemented the OpenTherm protocol on the boiler or the Modbus protocol on the heat pump/boiler (always refer to the documentation of the generator or, in the case of dedicated firmware, to the documentation supplied with the firmware). By hovering the mouse over the various rectangles/squares, a tag with extended description appears:
Stato GeneratoreGenerators Status: indica in che stato si trova il generatore (G = guasto, R = riscaldamento, SA = Acqua sanitaria, F = fiamma, C = condizionamento, ecc.). Se non implementato nessun quadrativo verrà popolato con le spunte.
Temperatura mandata, Temperatura ritorno, Temperatura ACS [°C]: sono le letture dei sensori del generatore se presenti e/o se mappate nel firmware dedicato.
Pot.: potenza erogata dal generatore in %, in kW elettrici, in kW termici (fare riferimento al manuale del produttore della caldaia o al manuale del firmware dedicato).
Errore OT: errori Opentherm più frequenti (Ra = richiesta assistenza, Rr = reset remoto abilitato, Bp = Bassa pressione acqua, ecc.). Nel caso di firmware dedicato fare riferimento alla documentazione fornita per la decodifica.
Errore OEM: indicazione del numero dell’errore, fare riferimento al manuale del generatore per la diagnostica o alla documentazione del firmware dedicato.
Reset: reset dell’errore, funziona solo se implementato o se la caldaia lo gestisce, fare riferimento al manuale del generatore per la diagnostica o alla documentazione del firmware dedicato.
ESEMPIO
Si ipotizza di avere selezionato 1 come numero di generatori Start e Minimo, mentre il numero Massimo è 9:
PHOTO TBD
L’impianto in oggetto ha 4 generatori OpenTherm:
G1 e G3 hanno priorità 1, G1 è abilitato solo in Riscaldamento, mentre G3 è sempre disponibile
G2 ha priorità 2 ed è disponibile solo in Raffrescamento
G4 ha priorità 3 ed è sempre disponibile
Partendo dalla condizione in cui tutti i generatori OpenTherm sono spenti la sequenza di accensione è la seguente:
In Estate G3 → G2 → G4
In Inverno G1 → G3 → G4
A parità di priorità l’accensione segue l’ordine numerico dei generatori per la prima accensione, mentre per le successive accensioni i generatori vengono fatti ruotare.
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indicates in which status the generator is (F = fault, H = heating, D = domestic hot water, F = flame, C = cooling etc.) If not implemented, no square will be ticked.
Water outlet temperature (T outlet), Water inlet Temperature (T inlet), DHW Temperature [°C]: are the readings of the generator sensors if present and/or if mapped in the dedicated firmware.
Pow.: power delivered by the generator in %, in electric kW, in thermal kW (refer to boiler manufacturer’s manual or to the dedicated firmware manual).
OT error: most frequent OpenTherm error (Ts = service request, Rr = remote reset enabled, Lp = low water pressure, etc.). In the case of dedicated firmware, refer to the documentation provided for decoding.
OEM Error: indication of the number of the error, refer to generator manual for diagnostic or to the dedicated firmware documentation.
Reset: reset of the error, it only works if implemented or if the boiler manages it; refer to the generator manual for diagnostic or to the dedicated firmware documentation.
EXAMPLE
Let us assume we have selected 1 as the number of Start and Minimum generators, while the maximum number is 9:
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The hypothetical system has 4 OpenTherm generators:
G1 and G3 has priority 1, G1 is enabled only in Heating, while G3 is always available.
G2 has priority 2 and is available only in Cooling
G4 has priority 3 and it is always available
Starting from the condition “all the OpenTherm generators are switched off”, the switch-on sequence is as follows:
In Summer G3 → G2 → G4
In Winter G1 → G3 → G4
With equal priority, the power on follows the numerical order of the generators for the first power on, while for the subsequent power on the generators are rotated.
In Summer no generator has the same priority, so the order will always be the same (G3 → G2 → G4).
In Inverno Winter G1 e G3 hanno la stessa priorità, durante la prima accensione la sequenza è G1 e poi G3, alla seconda accensione verrà acceso prima G3 e poi G1. G4 seguirà sempre in quanto ha una priorità più bassa.
Per quanto riguarda lo spegnimento vale la regola del “First On First Off”, quindi, prendendo il caso invernale, se per primo è stato acceso G1 poi G3 e infine G4, il primo a spegnersi sarà G4 in quanto ha priorità più bassa, poi tra G1 e G3 (che hanno la stessa priorità) il primo a spegnarsi è il primo che si era accesso, quindi G1.
Per quanto riguarda i parametri del controllo PI si è deciso di inserire 600 s, il che significa che per almeno 10 minuti dopo l’accensione o lo spegnimento di un generatore il sistema non modificherà il numero di generatori accesi. Mentre il delta T per cui il sistema deve subito inserire un generatore è 2 K, quindi si utilizza il valore 80 per Pid P, il tempo di ritardo per l’accensione del secondo generatore è di circa 20 minuti inserendo un Pid I di 40and G3 have the same priority, during the first ignition the sequence is G1 and then G3, at the second ignition G3 will be switched on first and then G1. G4 will always follow as it has a lower priority.
Concerning the switch off, the “First On First Off” rule applies, therefore, taking the winter case, if G1 was switched first, then G3 and finally G4, the first to switch off will be G4 as it has a lower priority, then between G1 and G3 (that have the same priority) the first to switch off is the first one that was switched on, therefore G1.
Concerning the PI control parameters, it was decided to include 600 s, which means that for at least 10 minutes after a generator is switched on or off, the system will not change the number of generators switched on. While the delta T for which the system must immediately enter a generator is 2 K, so the value 80 is used for Pid P, by entering a Pid I of 40 the delay time for switching on the second generator is approximately 20 minutes.