A plumber is installing a 200-litre domestic hot water tank in a closed plumbing system (with a backflow preventer on the main line). The system pressure is 415 kPa (60 psi). What type and minimum size of thermal expansion tank should be installed according to common practice and code requirements?

Question: A plumber is installing a 200-litre domestic hot water tank in a closed plumbing system (with a backflow preventer on the main line). The system pressure is 415 kPa (60 psi). What type and minimum size of thermal expansion tank should be installed according to common practice and code requirements?

Answer options:

• Potable water (diaphragm type), minimum 8 litres (2 US gallons). ✅ Potable water (diaphragm type), minimum 19 litres (5 US gallons).
• Heating system (bladder type), minimum 8 litres (2 US gallons).
• Heating system (bladder type), minimum 19 litres (5 US gallons).

Correct answer: Potable water (diaphragm type), minimum 19 litres (5 US gallons).

Explanation: For potable water systems, a potable water type expansion tank (often diaphragm type to keep water separated from air) is required. For a 200-litre tank in a closed system, a minimum 19-litre (5 US gallon) expansion tank is typically required by plumbing codes to accommodate thermal expansion and prevent excessive pressure. An 8-litre tank would be too small for a 200-litre water heater. Heating system tanks are not for potable water applications.