Pool Boiler Temperature Controller
Background
Pre-2022 Design
In the prior design (pre-2022), a xxx provided two-stage burner control and over-temperature shutdown.
The new controller adds important features:
- Shutdown of both stages if the boiler temperature does not reach a minimum temperature threshold after a warm-up period. This may occur if the recirculation pump fails, or is providing insufficient return water into the loop (due to air in the pump, improper valve position). Operating the boiler at low temperature for an extended period may result in condensation, which may reduce the life of the boiler.
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 boiler temperature is above the minimum temperature threshold (timing rules apply).
The low operator activates the first stage burner once the boiler interlock conditions and timing are satisfied.
Main Valve Active
The main valve 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 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 | Purchased used; not relevant. |
| 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 | Profibus will not be used. |
| 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?