CENWAT Commissioning and Fault Finding

Question 1

During the commissioning of a sealed central heating system, what should the cold fill pressure typically be set to?

Accepted Answer

1.0-1.5 bar

Answer

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?

Accepted Answer

Incomplete combustion, possibly due to insufficient air supply or burner fault

Answer

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?

Accepted Answer

Below 57 degrees C

Answer

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?

Accepted Answer

The expansion vessel has lost its air charge or the diaphragm has failed

Answer

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?

Accepted Answer

A manometer or digital pressure gauge connected to the burner pressure test point

Answer

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.

ACS CENWAT Practice Test

ACS CENWAT Practice Test

CENWAT Commissioning and Fault Finding