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?