Methods and system for controlling a combination boiler
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Solution Overview
Problem
Current combination boiler systems face challenges in quickly reaching desired temperatures for domestic hot water demands, managing low flow draws, and allowing hot water circulation or recirculation, due to minimum flow rate requirements and frequent firing cycles that lead to thermal cycling and reduced boiler lifespan.
Innovation Solution
A combination boiler system with a primary and secondary heat exchanger, and a controller that monitors temperatures to initiate pre-heat operations by circulating heated water without firing the burner when temperatures are below set thresholds, and fires the burner only when necessary to replenish heat, allowing for efficient temperature management and circulation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a flow switch with minimum flow rate setting is used to detect DHW draw, then the boiler can be triggered to fire, but low flow draws below the minimum setting cannot be detected and the boiler will not fire
Solution Approach 1:
The system changes the detection parameter from flow rate (which has a minimum threshold) to temperature difference across the heat exchanger. By monitoring the temperature drop of water passing through the heat exchanger, the system can detect even very low flow rates that would otherwise go undetected by a flow switch with minimum flow setting.
2Speed
If the boiler fires frequently to meet DHW demands, then hot water can be provided quickly, but thermal cycling occurs and boiler lifespan is reduced
Solution Approach 1:
The system performs preliminary heating of water in the heat exchanger before a DHW demand occurs. By continuously circulating and heating water in the heat exchanger when space heating is active, the system pre-prepares hot water that can be quickly delivered when a DHW draw is detected, avoiding the need for frequent burner firing and thermal cycling.
Solution Approach 2:
The system maintains continuous useful action by using the space heating burner to simultaneously heat water for potential DHW demands. The burner operates continuously for space heating while also heating the water in the heat exchanger, so when a DHW draw occurs, hot water is already available without requiring an additional firing cycle.
3Speed
If hot water circulation is implemented, then hot water can be delivered faster to fixtures, but the circulation flow cannot pass through the combination boiler due to minimum flow rate constraints
Solution Approach 1:
The system introduces an intermediary storage tank that receives heated water from the heat exchanger and stores it. The circulation pump then circulates this pre-heated water through the domestic water loop. This intermediary storage tank allows circulation to occur without requiring the circulation flow to pass through the combination boiler, bypassing the minimum flow rate constraint.
4Loss of time
If pre-heat operations are implemented to reduce time to reach desired temperature, then DHW demand response time is improved, but the system may trigger frequent firing cycles
Solution Approach 1:
The system performs preliminary heating of water in the heat exchanger before a DHW demand occurs. By continuously circulating and heating water in the heat exchanger when space heating is active, the system pre-prepares hot water that can be quickly delivered when a DHW draw is detected, avoiding the need for frequent burner firing and thermal cycling.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution reduces the time to reach desired temperatures, manages low flow conditions, and extends boiler lifespan by minimizing thermal cycling and optimizing energy use.
Implementation Method 1
a primary heat exchanger configured to be connected to the boiler loop and a burner configured to provide heat to the primary heat exchanger
Implementation Method 2
a secondary heat exchanger configured to transfer heat energy from the boiler loop to the domestic water loop
Implementation Method 3
initiate a pre-heat operation of the combination boiler responsive to a low temperature condition by circulating heated water from the primary heat exchanger to the secondary heat exchanger
Data Source
Figure 1
Figure 2
Figure 3
AI summary
A combination boiler provides heated water to a boiler loop and domestic hot water (DHW) to a domestic water loop. The combination boiler includes a primary heat exchanger (PHE) connected to the boiler loop and a burner to provide heat to the primary heat exchanger. A secondary heat exchanger (SHE) transfers heat energy from the boiler loop to the domestic water loop. A controller monitors a PHE inlet temperature and a DHW output temperature, obtains a pre-heat initialization temperature threshold and a pre-heat cancellation temperature threshold, and detects a low temperature condition. A pre-heat operation is initiated responsive to the low temperature condition by circulating heated water from the PHE to the SHE. The burner is selectively fired at least in part according to an outlet temperature of the PHE.