Combustion Analysis on a Condensing Boiler
You are commissioning a new condensing boiler. The boiler is rated at 30 kW and is room-sealed with a horizontal flue. The flue is 1.5 m long and terminates on a wall facing a busy road. The boiler is set to maximum rate. You perform a combustion analysis using a calibrated flue gas analyser. The readings are: CO = 180 ppm, CO₂ = 9.5%, O₂ = 5.2%, flue temperature = 55°C, ambient temperature = 20°C. The boiler manufacturer specifies a maximum CO/CO₂ ratio of 0.004 and a minimum CO₂ of 8.5% for natural gas. The analyser also shows a NOx reading of 35 ppm. The boiler is condensing, so the flue gas temperature is low. You notice that the flue terminal is close to an openable window (within 300 mm). The window is a fire escape window that opens outwards. The boiler is not yet signed off. The customer is eager to use the boiler.
1. Interpret the combustion readings and determine if the boiler is operating safely and efficiently. Show your calculations.
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First, calculate the CO/CO₂ ratio: CO = 180 ppm = 0.018%, CO₂ = 9.5%. Ratio = 0.018 / 9.5 = 0.00189. This is below the manufacturer's maximum of 0.004, so combustion is acceptable from a CO standpoint. Next, check CO₂: 9.5% is above the minimum of 8.5%, indicating good combustion efficiency. The O₂ level of 5.2% is typical for a condensing boiler at maximum rate. The flue temperature of 55°C is low, indicating condensing mode, which is efficient. However, the flue terminal is within 300 mm of an openable window, which is a safety issue. According to regulations, a flue terminal must be at least 300 mm from an openable window or air vent. Here it is exactly at the limit, but since the window opens outwards, it may be acceptable if it cannot be opened fully? Actually, the distance is measured from the terminal to the window opening; if it's 300 mm, it meets the minimum, but it's borderline. Also, the terminal is on a busy road, so there may be pollution ingress. The NOx reading is 35 ppm, which is low. Overall, combustion is safe and efficient, but the flue terminal position needs to be checked against regulations. If the window can open, products could enter. You should advise the customer not to open the window when the boiler is running, or relocate the flue. Since the boiler is not signed off, you must ensure compliance before commissioning.
2. What actions should you take regarding the flue terminal position and the combustion results before commissioning the boiler?
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The combustion results are satisfactory: CO/CO₂ ratio is well below the limit, CO₂ is above minimum, and efficiency is good. However, the flue terminal is within 300 mm of an openable window. The regulations (e.g., BS 5440-1) require a minimum distance of 300 mm from an openable window, but if the window is a fire escape window that opens outwards, the distance may need to be greater to prevent products entering. You should measure the exact distance. If it is less than 300 mm, you must relocate the flue or the window cannot be opened. Since it is exactly 300 mm, it may be compliant, but you should check the manufacturer's instructions, which may specify a larger distance. If the window is openable and within the limit, you should classify the installation as At Risk (AR) and advise the customer not to use the boiler until the flue is moved. You should not commission the boiler until the flue position is compliant. Additionally, check that the flue terminal is not near any other openings or vents. Document your findings and issue a warning notice if necessary. After rectification, re-test combustion and confirm safety.