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Regulation of Boiler Cascade

Use this setup if you want to achieve desired temperature of hydronic circuit using 2 or more boilers connected in cascade.

In the example scenario, we will show configuration for 3 boilers, that can be controlled by digital outputs.

Preparations

You should already have following devices defined:

Thermostat with temperature of returning water, measured on the pipe

Thermostat

Digital outputs for each boiler (Boiler A, Boiler B, Boiler C)

Boiler ABoiler BBoiler C

Create virtual Analog Output that will be used as a variable for calculating overall heat output.

Overall Output

Basic configuration

Create Smart Rule Temperature PID Controller that will calculate Overall Output based on difference in Thermostat Set point and Actual temperature

Input device

→ Smart Rule →

Output device

Thermostat

→ PID Controller→

P = 2
I = 0.2
D = 0

Overall Output

Based on overall output ranges, 3 (or more) boilers will be turned on
Configuration for mode where priority of boilers is: Boiler A → Boiler B → Boiler C

Input device

  • Overall Output

→ Smart Rule →

  • Equation

Output device

  • Boiler
<1%

--

--

>1%

Equation:
If Overall Output > 0.01

Minimum duration: 5min

TRUE: Turn on Boiler A
FALSE: Turn off Boiler A

>45%

Equation:
If Overall Output > 0.45

Minimum duration: 5min

>90%

Equation:
If Overall Output > 0.90

Minimum duration: 5min

Cycling of boilers

In order to achieve even usage of boilers, extra configuration needs to be applied.

Create Multi-value switch with N+1 states, where N is number of boilers. Last state can be defined as Heating Inactive system with zero heating.

Name: Boiler Priority


States:

  • Priority A (Boiler A → Boiler B → Boiler C)
  • Priority B (Boiler B → Boiler C → Boiler A)
  • Priority C (Boiler C → Boiler A → Boiler B)
  • Heating Inactive


Create Smart Rule Sequencer that will rotate priorities (A → B → C → A → ... )

Input device

→ Smart Rule →

Output device

Virtual push button to start the sequencer

Sequencer
Step 1: Priority A
(duration 18 hours)
Step 2: Priority B
(duration 18 hours)
Step 3: Priority C
(duration 18 hours)



(tick) 

(/) Rotate sequence

Boiler Priority


Define separate set of smartrules limited for each Boiler Priority. See Smart Rules for Priority A in table below:

Input device

  • Overall Output

→ Smart Rule →

  • Equation

Output device

  • Boiler

Limiting Condition

<1%

--

--


>1%

Equation:
If Overall Output > 0.01



TRUE: Turn on Boiler A
FALSE: Turn off Boiler A

Type: Multi-Value Switch
Smart Rule will only work if...

  • Boiler Priority is set to Priority A
>45%

Equation:
If Overall Output > 0.45



TRUE: Turn on Boiler B
FALSE: Turn off Boiler B

Type: Multi-Value Switch
Smart Rule will only work if...

  • Boiler Priority is set to Priority A
>90%

Equation:
If Overall Output > 0.90

TRUE: Turn on Boiler C
FALSE: Turn off Boiler C

Type: Multi-Value Switch
Smart Rule will only work if...

  • Boiler Priority is set to Priority A

Similar set of Smart Ruless should be applied with Limiting Condition for Priority B and Priority C:

Input device

  • Overall Output

→ Smart Rule →

  • Equation

Output device

  • Boiler

Limiting Condition

<1%

--

--


>1%

Equation:
If Overall Output > 0.01



TRUE: Turn on Boiler B
FALSE: Turn off Boiler B

Type: Multi-Value Switch
Smart Rule will only work if...

  • Boiler Priority is set to Priority B
>45%

Equation:
If Overall Output > 0.45



TRUE: Turn on Boiler C
FALSE: Turn off Boiler C

Type: Multi-Value Switch
Smart Rule will only work if...

  • Boiler Priority is set to Priority B
>90%

Equation:
If Overall Output > 0.90

TRUE: Turn on Boiler A
FALSE: Turn off Boiler A

Type: Multi-Value Switch
Smart Rule will only work if...

  • Boiler Priority is set to Priority B

Input device

  • Overall Output

→ Smart Rule →

  • Equation

Output device

  • Boiler

Limiting Condition

<1%

--

--


>1%

Equation:
If Overall Output > 0.01



TRUE: Turn on Boiler C
FALSE: Turn off Boiler C

Type: Multi-Value Switch
Smart Rule will only work if...

  • Boiler Priority is set to Priority C
>45%

Equation:
If Overall Output > 0.45



TRUE: Turn on Boiler A
FALSE: Turn off Boiler A

Type: Multi-Value Switch
Smart Rule will only work if...

  • Boiler Priority is set to Priority C
>90%

Equation:
If Overall Output > 0.90

TRUE: Turn on Boiler B
FALSE: Turn off Boiler B

Type: Multi-Value Switch
Smart Rule will only work if...

  • Boiler Priority is set to Priority C