Gas Safe ACS - Accredited Certification Scheme Burner Pressures and Gas Rates Questions and Answers — Questions and Answers
Question 1: An engineer is commissioning a new boiler with a manufacturer's stated gross heat input of 24 kW. Using a metric meter, they time the gas consumption and record that it takes 135 seconds to burn 0.1 m³. The declared calorific value (CV) of the natural gas is 40.0 MJ/m³. How does the calculated heat input compare to the manufacturer's specification?
- It is significantly above the acceptable tolerance of +/- 5%. (Correct answer)
- It is within the acceptable tolerance of +/- 5%.
- It is significantly below the acceptable tolerance of +/- 5%.
- The calculation cannot be completed without the net heat input value.
Correct answer: It is significantly above the acceptable tolerance of +/- 5%.
First, calculate the gas rate in m³/hr: (0.1 m³ ÷ 135 s) × 3600 s/hr = 2.667 m³/hr. Next, calculate the heat input in kW using the formula: Heat Input (kW) = (Gas Rate × CV) ÷ 3.6. So, (2.667 × 40.0) ÷ 3.6 = 29.63 kW. The acceptable tolerance is ±5% of 24 kW, which is a range of 22.8 kW to 25.2 kW. The calculated heat input of 29.63 kW is well above this range, indicating the appliance is over-gassed.
Question 2: What is the primary purpose of an engineer measuring the burner operating pressure at an appliance's test point during servicing?
- To ensure the gas is being delivered to the burner at the correct pressure for safe and efficient combustion. (Correct answer)
- To confirm the standing pressure at the meter is adequate for the installation.
- To verify that the gas rate is within the manufacturer's specified tolerance.
- To check that the appliance gas valve is not letting-by when closed.
Correct answer: To ensure the gas is being delivered to the burner at the correct pressure for safe and efficient combustion.
The burner operating pressure is a critical measurement set by the appliance manufacturer to ensure the correct volume of gas passes through the injector. This achieves the proper air-to-gas ratio, which is essential for complete, safe, and efficient combustion. While it is related to the gas rate, its direct purpose is to confirm the conditions at the point of combustion are correct.
Question 3: An engineer is checking the gas rate of an appliance and finds it to be significantly lower than the manufacturer's stated heat input. Which of the following is the most likely cause?
- Inadequate working pressure at the appliance inlet. (Correct answer)
- The appliance has been fitted with an injector for LPG instead of Natural Gas.
- The calorific value of the gas is higher than the standard value.
- The gas meter is oversized for the property's total load.
Correct answer: Inadequate working pressure at the appliance inlet.
Low working pressure at the appliance, often due to undersized pipework or a faulty meter regulator, restricts the flow of gas, leading to a low gas rate and under-gassing of the appliance. An LPG injector has a smaller orifice and would also cause a low gas rate, but inadequate working pressure is a more common installation-wide issue. High CV or an oversized meter would not cause a low gas rate.
Question 4: While servicing a room-sealed boiler, an engineer measures the burner operating pressure. The manufacturer's data plate states the pressure should be 14.0 mbar ± 1.0 mbar. The engineer records a stable reading of 10.5 mbar and the flame picture is visibly poor. In accordance with the Gas Industry Unsafe Situations Procedure (GIUSP), how should this be classified?
- Immediately Dangerous (ID)
- At Risk (AR) (Correct answer)
- Not to Current Standards (NCS)
- Requires Verbal Advice (RVA)
Correct answer: At Risk (AR)
A burner pressure significantly outside the manufacturer's specified tolerance (the minimum acceptable pressure is 13.0 mbar) indicates incorrect and potentially unsafe combustion. This presents a potential danger to life or property and must be classified as 'At Risk' (AR). The engineer must turn the appliance off (with permission), apply a 'Warning/Do Not Use' label, and inform the responsible person in writing.
Question 5: When an engineer performs a gas rate check on a domestic appliance, the calculated heat input must be compared against the manufacturer's stated figure. What is the generally accepted industry tolerance for this check?
- ± 15%
- ± 10%
- ± 5% (Correct answer)
- ± 2.5%
Correct answer: ± 5%
According to UK gas industry best practice and guidance, the calculated gross heat input should be within ± 5% of the manufacturer's stated gross heat input to be considered acceptable. A reading outside this tolerance requires further investigation as it could indicate a fault with the appliance or installation.
Question 6: During a service, an engineer finds the burner pressure of a gas fire is significantly higher than specified in the manufacturer's instructions, although the working pressure at the appliance inlet is correct. What is the most probable cause of this fault?
- The meter regulator is failing and causing a pressure surge.
- The wrong type of injector has been fitted (one with a larger orifice).
- A partial blockage in the flue is causing back pressure.
- The burner injector is partially blocked with debris. (Correct answer)
Correct answer: The burner injector is partially blocked with debris.
A partial blockage in the injector orifice acts as a restriction. This causes the pressure to build up at the measurement point (which is before the restriction), leading to a high burner pressure reading. However, the actual volume of gas passing through the restricted orifice will be reduced, resulting in a low gas rate. This is a common fault found during servicing.
Gas Safe ACS - Accredited Certification Scheme Burner Pressures and Gas Rates Questions and Answers
An engineer is commissioning a new boiler with a manufacturer's stated gross heat input of 24 kW.
Using a metric meter, they time the gas consumption and record that it takes 135 seconds to burn 0.1 m³.
The declared calorific value (CV) of the natural gas is 40.0 MJ/m³.
How does the calculated heat input compare to the manufacturer's specification?