CENWAT Commissioning and Fault Finding — Questions and Answers
Question 1: During the commissioning of a sealed central heating system, what should the cold fill pressure typically be set to?
- 0.5 bar
- 1.0-1.5 bar (Correct answer)
- 2.5 bar
- 3.5 bar
Correct answer: 1.0-1.5 bar
The cold fill pressure for a sealed domestic central heating system should typically be set between 1.0 and 1.5 bar when the system is cold. This provides sufficient pressure for operation while allowing headroom for expansion when the water heats up, typically rising to 1.5-2.0 bar.
Question 2: What does a high CO/CO2 ratio reading during combustion analysis typically indicate?
- Overgassing of the appliance
- Incomplete combustion, possibly due to insufficient air supply or burner fault (Correct answer)
- The flue is too long
- The condensate trap is blocked
Correct answer: Incomplete combustion, possibly due to insufficient air supply or burner fault
A high CO/CO2 ratio indicates incomplete combustion where the gas is not burning fully and producing excess carbon monoxide instead of carbon dioxide. Common causes include insufficient air supply, a dirty or damaged burner, incorrect gas pressure, or a blocked heat exchanger.
Question 3: When commissioning a condensing boiler, what is the expected flue gas temperature when the boiler is operating in condensing mode?
- Over 150 degrees C
- Between 100-130 degrees C
- Below 57 degrees C (Correct answer)
- Exactly 80 degrees C
Correct answer: Below 57 degrees C
When a condensing boiler is operating in full condensing mode, the flue gas temperature should be below the dew point of approximately 57 degrees C. At this temperature, water vapour in the combustion products condenses on the secondary heat exchanger, releasing latent heat.
Question 4: What is the most likely cause of a sealed central heating system that keeps losing pressure over time with no visible leaks?
- The expansion vessel has lost its air charge or the diaphragm has failed (Correct answer)
- The boiler thermostat is set too high
- The room thermostat battery is flat
- The pump speed is too high
Correct answer: The expansion vessel has lost its air charge or the diaphragm has failed
When a sealed system loses pressure without visible leaks, the most common cause is a failed expansion vessel. Without proper expansion absorption, the pressure relief valve operates when the system heats up, discharging small amounts of water each cycle. The system then shows low pressure when cold.
Question 5: What instrument is used to measure the gas burner pressure (working pressure) at a boiler?
- A multimeter
- A manometer or digital pressure gauge connected to the burner pressure test point (Correct answer)
- A gas detector
- A thermometer
Correct answer: A manometer or digital pressure gauge connected to the burner pressure test point
A manometer or calibrated digital pressure gauge is connected to the burner pressure test point on the gas valve to measure the working pressure. For natural gas, the normal burner pressure is typically 10-14 mbar depending on the manufacturer.
Question 6: During balancing of a central heating system, which radiator should be balanced first?
- The largest radiator
- The radiator nearest to the boiler (Correct answer)
- The radiator furthest from the boiler
- The radiator in the coldest room
Correct answer: The radiator nearest to the boiler
When balancing a heating system, you start with the radiator nearest to the boiler by partially closing its lockshield valve, then work progressively outward to the furthest radiator which should have its lockshield valve fully open. The aim is to achieve a consistent temperature drop of typically 11-12 degrees C across each radiator.
CENWAT Commissioning and Fault Finding
During the commissioning of a sealed central heating system, what should the cold fill pressure typically be set to?