Pool Boiler Temperature Controller
This document is still a work in progress. This temperature controller is still under active development.
Overview
The boiler temperature controller was created to enhance control and monitoring over the unit used prior to 2022 (a Johnson Controls C450CCN-3).
During the 2021 season, there were multiple observed instances of the boiler operating at a sub-optimal temperature. To prevent condensation, the boiler should operate at about 120 degrees or higher. This is achieved by mixing inlet water (ideally at 80 degrees or warmer) with water recirculated from the boiler outlet. This circulation pump is shown in the Boiler Loop section on page 2 of the Process Flow Diagram.
The occurrence of low temperature was likely due to air trapped in the pump; a condition that is possible after air is introduced into the system when the strainer basket is cleaned or the filters are backwashed. The controller cannot prevent this condition, but it can detect it and alert the operator to minimize the impact of condensation.
The key features of the new controller are as follows:
- Provides feedback of the boiler temperature via LCD display and via Modbus (RS-485) to the automation system.
Like in the Pool Temperature Controller, the following principles are used in its design:
- The core function (temperature control) shall be performed by a proven, durable controller. In this case, a Euroterm 3504 was chosen due to its enhanced logic control capabilities.
- The automation system shall not be required for operation. Failure of other components should have minimal impact upon its operation.
Theory of Operation
Call for Heat
A call for heat is raised by the Pool Temperature Controller. It is detected when the first input (terminal #13 - see wiring note) is energized.
Low Operator
The low operator is activated when the following conditions are true:
- A call for heat is present.
- The RTD is producing valid feedback (not shorted or open-circuit).
- The boiler temperature is below the over-temperature shutdown threshold (timing rules apply).
The low operator may be released when any of the prior conditions are no longer met. In addition, it may be released when:
- The first stage is active and the boiler temperature is below the minimum temperature threshold. This condition is applied after a warm-up period has elapsed.
The low operator activates the first stage burner once the boiler interlock conditions and timing are satisfied.
Main Valve Active
The main valve (MV) active input is energized via boiler terminal #30. This is the same signal that provides "Boiler ON" feedback to the Pool Temperature Controller. When the main valve is active, the first stage is presumed to be operating. If the main valve is not active some period of time after the low operator is activated, a boiler fault may be assumed.
High Operator
The high operator is activated when the following conditions are true:
- All conditions for the low operator are met.
- The main valve (MV) is active.
- The boiler temperature is below the second stage threshold (timing rules apply).
Eurotherm 3508
As Shipped Configuration
| Order Code | Description | Comment | |
|---|---|---|---|
| Model Number | 3508 | 3508 Standard | |
| Function | F | Profibus | Profibus will not be used. |
| Supply Voltage | VL | 20-29 VAC/DC | Intended to operate on the boiler 24 VAC control circuit. |
| Loops | 1 | One Loop | |
| Application | XX | Standard | |
| Programs | X | No Programs | |
| Recipes | 1 | 1 Recipe | |
| Toolkit Wires | XXX | Standard 30 Wires | |
| Fascia | G | Eurotherm Green | |
| IO Slot 1 | LO | Isolated Single Logic Output | |
| IO Slot 2 | D4 | DC Control | Will not be used. |
| IO Slot 3 | TK | Triple Contact Input | |
| H Comms Slot | PD | Profibus with D-type Connector | Module is an AH026222U002. |
| J Comms Slot | XX | Not Fitted | An RS485 module will be installed. |
| Config Tools | XX | None | |
| Product Language | ENG | English | |
| Manuals Language | ENG | English | |
| Warranty | XXXXX | Standard | |
| Calibration Certificate | XXXXX | None | |
| Custom Labels | XXXXX | None | |
| Config | STD | Standard Config | The remaining codes are all blank; no factory configuration. |
Modifications
Additional modules were purchased and installed, modifying the as-shipped configuration.
- Installed an RS-485 module (Eurotherm AH025075U002) in the J Comms Slot for MODBUS communication.
- Changes to IO Slot 2:
- Removed the DC Control module.
- Installed a Change Over Relay (Eurotherm AH025408U002) for SPDT 2A control.
Inputs
- The Triple Contact Input module provides three switch inputs. TBD: What is the "common" pin connected to?
Network Interfaces
There are two digital communication (also referred to here as "network interfaces") on the Eurotherm 3508.
Port H
The Port H interface was factory-configured for Profibus with a Eurotherm Profibus Comms card (AH026222U002). A 9-pin D-sub connector (part number PROF9PIND) provides access to RS-485 compatible signaling on pin #3 (Tx/Rx +Ve) and pin #8 (Tx/Rx -Ve) along with GND on pin #5. The iTools software allows configuring the Profibus interface to speak MODBUS, thereby allowing it to become the primary software configuration interface (freeing up Port J for system-level communication).
Port J
The Port J interface is configured for RS-485 MODBUS communication at 19.2 kbps 8N1.