306A Water Heating Systems 1 — Questions and Answers
Question 1: What is the function of the temperature and pressure (T&P) relief valve on a water heater?
- To regulate the water temperature for the user
- To release water if the temperature exceeds 99°C or pressure exceeds 1035 kPa (150 psi), preventing a catastrophic tank failure (Correct answer)
- To drain sediment from the tank
- To control the gas supply to the burner
Correct answer: To release water if the temperature exceeds 99°C or pressure exceeds 1035 kPa (150 psi), preventing a catastrophic tank failure
The T&P relief valve is a critical safety device that opens automatically if the water temperature exceeds 99°C (210°F) or the pressure exceeds 1035 kPa (150 psi), preventing the tank from rupturing or exploding.
Question 2: Where must the discharge pipe from a T&P relief valve terminate?
- Into the sanitary sewer directly
- Within 150 mm of the floor or to an approved indirect waste receptor, with an air gap (Correct answer)
- Back into the cold water supply
- Outside the building through the wall
Correct answer: Within 150 mm of the floor or to an approved indirect waste receptor, with an air gap
The T&P discharge pipe must terminate within 150 mm (6 inches) of the floor or into an approved indirect waste receptor with an air gap to safely direct any discharge while making it visible to the building occupants.
Question 3: What is the minimum capacity of a residential water heater for a family of four?
- 100 litres (26 gallons)
- 150 litres (40 gallons)
- 190 litres (50 gallons) (Correct answer)
- 284 litres (75 gallons)
Correct answer: 190 litres (50 gallons)
A 190-litre (50-gallon) water heater is generally recommended as the minimum for a family of four to provide adequate hot water for multiple bathrooms, laundry, and kitchen use during peak demand periods.
Question 4: What is the 'recovery rate' of a water heater?
- The speed at which the heater drains
- The number of litres of water the heater can raise by a set temperature in one hour (Correct answer)
- The time it takes for the tank to cool down
- The rate at which the heater fills from the cold supply
Correct answer: The number of litres of water the heater can raise by a set temperature in one hour
Recovery rate measures the volume of water a heater can heat through a specified temperature rise (typically 50°C/90°F) in one hour. A higher recovery rate means faster hot water replenishment after heavy use.
Question 5: What is the purpose of an anode rod in a storage-tank water heater?
- To heat the water electrically
- To sacrificially corrode instead of the tank lining, extending the tank's life (Correct answer)
- To filter sediment from the water
- To regulate water pressure inside the tank
Correct answer: To sacrificially corrode instead of the tank lining, extending the tank's life
The anode rod (typically magnesium or aluminum) is a sacrificial metal that corrodes preferentially, protecting the steel tank lining from rust and corrosion. It should be inspected and replaced every 3 to 5 years.
Question 6: What minimum clearance is required around a gas-fired water heater for combustion air and servicing?
- No clearance is needed
- As specified by the manufacturer, typically 150 mm on sides and 300 mm in front (Correct answer)
- 1 metre on all sides
- Only clearance above the unit is required
Correct answer: As specified by the manufacturer, typically 150 mm on sides and 300 mm in front
The manufacturer's installation instructions specify the required clearances, which typically include at least 150 mm on the sides and rear, and 300 mm in front for servicing access. These clearances also ensure adequate combustion air supply.
306A Water Heating Systems 1
What is the function of the temperature and pressure (T&P) relief valve on a water heater?