---
title: "Boiler Cascade"
description: "Run 2 or more boilers in cascade: a PID Smart Rule computes the total heat output, a sequencer switches the boilers by priority and rotates them."
url: "https://taphome.com/en/docs/functional-areas/heating-cooling/boiler-cascade/"
language: en
lastmod: 2025-11-21
generator: hugo
ai:
  topics: ["boiler-cascade","pid-controller","heating-control","boiler-rotation","smart-rules","sequencer"]
  questions: ["How do I configure a boiler cascade with multiple boilers in TapHome?","What PID values should I use for cascade boiler temperature control?","What percentage thresholds trigger each boiler in a cascade setup?","How does automatic boiler rotation work to ensure even boiler wear?","How to create a multi-valued switch for boiler priority states?","How to use a Smart Rule sequencer for automatic priority rotation?","What minimum duration should I set for boiler switching in a cascade?"]
---

# Boiler Cascade

Configuration and setup of cascade boiler systems to achieve desired hydraulic circuit temperature using PID controller.

Last updated: 2025-11-21

Three boilers A, B and C connected in cascade to one supply pipe and one return pipe.


Use this setting to achieve the desired hydraulic circuit temperature using two or more boilers connected in cascade.

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

## Preparation
You should already have the following devices defined:

- **Thermostat** with return water temperature measurement on the pipe

## Digital outputs for each boiler
- **Boiler A**
- **Boiler B**
- **Boiler C**

## Virtual analog output
This output will be used as a variable to calculate the **total heat output**.

### Basic configuration
Create a **Smart Rule** PID temperature controller that will calculate the total output based on the difference between the thermostat setpoint and the current temperature.

| Input device | → Smart Rule → | Output device |
|----------------|---------------|----------------|
| Thermostat | → PID Controller → P = 2, I = 0.2, D = 0 | Total output |

Based on the range of the total output, 3 (or more) boilers will be turned on.  
Configuration for the mode where boiler priority is: **Boiler A → Boiler B → Boiler C**

| Input device  | → Smart Rule → Equation                                     | Output device |
|----------------|------------------------------------------------------------|----------------|
| <1%            | --                                                         | --             |
| >1%            | If the **total output** > 0.01 <br> Minimum duration: **5 min** <br> **TRUE**: Turn on **Boiler A** <br> **FALSE**: Turn off **Boiler A** | Boiler A |
| >45%           | If the **total output** > 0.45 <br> Minimum duration: **5 min** | Boiler B |
| >90%           | If the **total output** > 0.90 <br> Minimum duration: **5 min** | Boiler C |

## Boiler rotation

To ensure even use of the boilers, additional configuration is required.

## Creating a multi-valued switch
Define **N+1 states**, where **N** is the number of boilers.  
The last state can be defined as **Heating inactive**, which means the system is not 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**

## Creating a Smart Rule sequencer
This sequencer will **rotate priorities** as follows:  
**A → B → C → A → ...**

| Input device | → Smart Rule →                      | Output device |
|----------------|----------------------------------|----------------|
| Virtual button to start the sequencer | Sequencer | Boiler priority |
|                                            | **Step 1:** Priority A (**duration: 18 hours**) | |
|                                            | **Step 2:** Priority B (**duration: 18 hours**) | |
|                                            | **Step 3:** Priority C (**duration: 18 hours**) | |
|                                            | ✅ **Rotate sequence** | |

Define a separate set of **Smart Rule rules** restricted to each boiler priority.  
See **Smart Rules for Priority A** in the table below:

| Input device | → Smart Rule → Equation                                | Output device | Restricting condition |
|----------------|------------------------------------------------------|----------------|----------------------|
| <1%            | --                                                   | --             | --                   |
| >1%            | If the **total output** > 0.01 <br> **TRUE**: Turn on **Boiler A** <br> **FALSE**: Turn off **Boiler A** | Boiler A | Type: Multi-valued switch <br> Rule only applies when **Boiler Priority** is set to **Priority A** |
| >45%           | If the **total output** > 0.45 <br> **TRUE**: Turn on **Boiler B** <br> **FALSE**: Turn off **Boiler B** | Boiler B | Type: Multi-valued switch <br> Rule only applies when **Boiler Priority** is set to **Priority A** |
| >90%           | If the **total output** > 0.90 <br> **TRUE**: Turn on **Boiler C** <br> **FALSE**: Turn off **Boiler C** | Boiler C | Type: Multi-valued switch <br> Rule only applies when **Boiler Priority** is set to **Priority A** |

A similar set of **Smart Rule rules** should be applied with a **restricting condition** for **Priority B** and **Priority C**.

**See also** — [PID Cascade Regulation](https://taphome.com/en/docs/functional-areas/heating-cooling/pid-cascade/), [Equithermic regulation](https://taphome.com/en/docs/functional-areas/heating-cooling/equithermic-regulation/), [Circulation Pump](https://taphome.com/en/docs/functional-areas/heating-cooling/circulation-pump/).