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 ensures that the pool boiler internally operates at an optimal temperature. It does so by providing Low Operator and High Operator interlocks.
This controller does not manage the pool temperature. That functionality is handled by the Pool Temperature Controller.
In the system, a circulation pump (shown in the Boiler Loop section on page 2 of the Process Flow Diagram) force-returns some amount of hot water from the boiler outlet to the inlet. This helps to maintain an elevated operating temperature (i.e.. 120 °F or greater), reducing the risk of condensation forming on the heat exchanger that can cause damage over time. Unfortunately, various factors may prevent the circulation pump from maintaining the proper temperature such as improper valve settings, trapped air, or pump failure. Air may be regularly introduced into the system while cleaning the strainer basket or backwashing the filters may cause the pump to operate improperly. The boiler temperature controller in place prior to 2022, a Johnson Controls C450CCN-3, did not detect low-temperature operation.
The controller cannot resolve fault conditions. Instead, it notifies the operator of a fault condition, which usually requires manual intervention.
The design has the following key principles:
- The controller functionality shall help improve boiler function and longevity without adding unnecessary complexity.
- The core function (temperature control) shall be performed by a proven, durable controller. In this case, a Euroterm 3508 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.
While the automation system is not required, it will have a communication link to the controller for monitoring purposes.
User's Guide
LCD Display
An LCD display on the front of the controller indicates the measured temperature of the boiler manifold (top-most value). A graphical bar shows the value relative to the typical minimum and maximum operating bounds. Temperature thresholds for a Low Temperature Fault, Stage 2 operation, and a High Temperature Fault are displayed on the subsequent lines. All temperatures are displayed in degrees Fahrenheit.
The OP1 and OP2 indicators (beacons) indicate that Stage 1 and/or Stage 2 operation is active, respectively. An ALM indicator is associated with a fault condition; user intervention may be required to restore boiler operation. Blinking H and/or J indicators indicate communication port activity.
Stage Monitoring
At this time, no feedback is provided to the temperature controller to verify which stages are active; external conditions/faults may prevent firing stages. See Pool Boiler Operation for further details on how the controller contributes to the boiler interlocks.
Operating States
The simulated screen captures below show various operating conditions that the user may encounter. The measured temperature and thresholds shown may vary.
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Normal Operation
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No Stage 2
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Boiler Fault
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Low Temperature Fault
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High Temperature Fault
Normal Operation
During normal operation, no user intervention is required. Both Stage 1 and Stage 2 heating is enabled.
No Stage 2
The boiler temperature is greater than the Stage 2 threshold, and Stage 2 is disabled as a result. This is not a fault condition, though it may indicate sub-optimal operation. Only Stage 1 heating is enabled.
The Stage 2 threshold is the temperature value to the right of the Stg2 identifier on the display, and should typically be set to TBD. To adjust this threshold, see the Threshold Adjustment section below.
If this state is active, adjustment of the recirculation valve position should be considered. Since operating only in Stage 1 produces less heat, the ability to heat the pool is reduced. This is primarily an issue where the amount of heat loss in the pool exceeds the capability of Stage 1 heating (i.e. the pool cools faster than it can be heated).
Boiler Fault
The boiler has not fired Stage 1 is response to a Call for Heat. This is an informational fault only; it does not disable either stage. This fault is only applicable if the Call for Heat signal is wired to the controller.
Low Temperature Fault
The boiler temperature is less than the low temperature threshold and both stages are disabled as a result. The low-temperature threshold should typically be set to TBD. This is a latching fault and must be cleared via the front panel before the boiler will operate.
Before attempting to clear the fault, the source of the fault should be cleared. See the Pool Boiler Low Temperature Fault section for further information (recirculation valve adjustment may be needed).
High Temperature Fault
The boiler temperature is greater than the high-temperature threshold and both stages are disabled as a result. The high temperature threshold should typically be set to TBD. This is a latching fault and must be cleared via the front panel before the boiler will operate.
Threshold Adjustment
Threshold adjustment is typically not necessary or recommended. Instead, it is recommended that the system be evaluated for deficiencies.
If a parameter must be adjusted, press the scroll button (left of the down arrow) to move the up/down cursor to the desired parameter. Press the up/down keys to adjust the temperature. Internal bounds limit how high or low a parameter may be adjusted. Use extreme caution in adjusting each parameter, as each could have significant impacts upon boiler operation.
Other Buttons
The "A/MAN" and "RUN/HOLD" buttons are not presently active. Pressing either button has no effect upon 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 (the relay contact is closed) when the following conditions are true:
- The RTD is producing valid feedback (not shorted or open-circuit).
- The boiler temperature is below the high temperature fault 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 (the relay contact is closed) 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).
Controller Hardware
As-Shipped Configuration
When shipped from the factory, a long configuration code is printed on the side of the controller to identify the configuration. The code on the installed controller is decoded below:
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 | May not be used. |
IO Slot 2 | D4 | DC Control | Will not be used; see modifications below. |
IO Slot 3 | TK | Triple Contact Input | May not be used. |
H Comms Slot | PD | Profibus with D-type Connector | Module is an AH026222U002 (visually identified). |
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.
Controller Logic
Block | Description/Comments |
---|---|
DigAlarm1 | Triggers a non-latching alarm if Call for Heat is active for an extended period without Main Valve (MV) Active. This alarm has no functional impact; it is used to alert the operator only. If the Call for Heat input is not connected, this alarm will never trigger. |
DigAlarm2 | Triggers a manually-latched alarm per a rising edge on Lgc2 1. The alarm remains active until acknowledged. |
Instrument | Provides access to the OP1 and OP2 "beacons" (display indicators) for OP1 and OP2. |
IO.PV | The conditioned RTD measurement is output on PV. If any of the three wires to the RTD are broken/disconnected, SBrkOut is active. |
IO.LgcIO.LA | The Call for Heat input. This is an optional input (pending system wiring revision). It is used to detect a boiler fault, but has no other functional purpose at this time. |
IO.LgcIO.LB | The Main Valve (MV) Active input. |
IO.Mod 2.A | The Stage 2 output relay. |
IO.RlyAA | The Stage 1 output relay. |
Lgc2 1 | Logically ORs the delayed-on Main Valve (MV) Active input signal and the result of the conditioned RTD measurement is less than the low-temperature threshold comparison. A rising edge on the output of this block indicates that the boiler temperature is below the threshold after the Main Valve (MV) (Stage 1) has been active for some period of time. |
Lgc2 2 | Logically ORs the output of Lgc8 1 and the control loop output to produce a Stage 2 control signal. |
Lgc2 4 | The output is active if the conditioned RTD measurement is less than the low-temperature threshold. |
Lgc8 1 | Logically ORs four inputs, all inverted, from Alarm 1, Alarm 2, DigAlarm 2 and the signal break output (SBrkOut) from IO.PV. The output is active unless any of the alarms are triggered or signal break is active. Essentially inhibits operation of Stage 1 and Stage 2 in the event of a fault condition. |
Loop 1 | Operates an on-off control loop with a setpoint (Stage 2, displayed on the controller as "Stg2") and feedback via the conditioned RTD measurement. |
Timer1 | Provides a delayed-on Main Valve (MV) Active input signal. Used to act upon conditions that are only relevant some period of time after the gas has fired (i.e. boiler warm-up). |
UsrVal 1 | The low-temperature threshold. Displayed on the controller as "LowT". |
Definitions
Stage 1
The term "Stage" refers to the amount of heat that the boiler is requested to produce. During "Stage 1", only the lower-output burners are active.
Stage 2
During "Stage 2", both the lower-output and higher-output burners are active. The maximum amount of heating is occurring.