Condensing Boiler With Low Gas Rate and High CO/CO2 Ratio
After a routine service on a 24 kW room-sealed condensing boiler, the flue-gas analyser shows CO 680 ppm, CO2 8.8%, CO/CO2 ratio 0.0077, flue temperature 62 °C and ambient CO 0 ppm. The boiler has been running for 15 minutes. The gas meter dial moves 0.055 m³ in 120 seconds. Inlet pressure at the boiler is 19 mbar; at the meter it is 21 mbar. The condensate trap is clear. The homeowner reports the boiler 'sounds rough' and sometimes locks out in windy weather. The manufacturer's data plate states 24 kW input and a maximum CO/CO2 ratio of 0.004. The flue is a horizontal room-sealed terminal on a wall facing an open field.
1. Calculate the gas input and compare it with the boiler rating. Show your reasoning.
Read the worked answer
Gas rate: 0.055 m³ in 120 s = 0.055 × 30 = 1.65 m³/h. Using a natural gas calorific value of about 10.8 kWh/m³, input ≈ 1.65 × 10.8 = 17.8 kW, not 24 kW-about 74% of rating. Check the calorific value on the gas bill; at 11.0 kWh/m³ it is still only about 18.2 kW. A low gas rate with 19 mbar at the boiler and 21 mbar at the meter suggests the boiler is not at full rate, the meter or pipework is undersized, or the gas valve is misadjusted. Confirm the boiler is in heating mode, no other appliances are running, and repeat the gas rate before condemning the appliance.
2. Interpret the combustion readings and list the likely causes.
Read the worked answer
CO 680 ppm and CO/CO2 0.0077 exceed the manufacturer's 0.004 limit, so combustion is incomplete. CO2 at 8.8% is not high, so the mixture is not simply over-gassed. Likely causes include a restricted combustion-air intake or flue, a fouled heat exchanger, an incorrect gas valve setting, low inlet pressure under load, a fan or venturi fault, or wrong injectors. The windy-weather lockout supports a flue or air-supply problem. Do not adjust the gas valve to chase readings before checking the air path, flue and heat exchanger.
3. What immediate actions and classification are appropriate, and what must be done before the boiler is left running?
Read the worked answer
Treat the boiler as at risk or immediately dangerous until proven safe. Turn it off, isolate the gas supply, label it, and tell the homeowner not to use it. Record the readings, gas rate and pressures. Investigate and repair the cause, then repeat combustion analysis and gas rate. Leave it running only if CO/CO2 is within the manufacturer's limit and all other checks pass. If the cause cannot be corrected on site, leave it isolated and issue the appropriate warning notice.