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Thermal Expansion Awareness

Thermal expansion occurs when water is heated by a hot water heater or other device. When heated, water density decreases and volume increases/expands. For example, water heated from 32º C to a thermostat setting of 60º C in a 150-litre hot water heater will expand by almost two litres. The extra volume created by this expansion sometimes overcomes the incoming supply pressure and backs out through the service line into the public water supply.

During no-flow periods in the system, backflow preventers, pressure reducing valves, and other one-way valves create a “closed system,” thus eliminating a path for expanded water to flow back into the public water supply.
Thermal expansion of water in a closed plumbing system can create irritating and potentially dangerous problems. These include: the build-up of high pressure in a system causing pressure surges, and the chronic or continuous dripping of a temperature and pressure relief valve. Thermal expansion also causes dripping faucets and leaking toilet tank ball cock fill valves.

More serious problems can also occur due to thermal expansion. When dangerous pressures are built up in a water heater, internal parts may fail such as the internal flues, fittings or water connections. If a flue way collapses, it can lead to the potential release of toxic gases, such as carbon monoxide, into living spaces. Thermal expansion can also lead to a ruptured or distorted hot water heating tank and may void the manufacturer’s warranty.

According to the National Plumbing Code 2010 (Section 6.1.11), “When a backflow preventer, pressure reducing valve, or a check valve is required, protection against thermal expansion may be required. To accommodate the increase in pressure caused by thermal expansion within a closed water distribution system, one of the following should be installed:

  1. a suitable sized diaphragm expansion tank designed for use within a potable water system; 
  2. an auxiliary thermal expansion relief valve (T.E.R. valve) conforming to CAN/CSA- B125, “Plumbing Fittings,” set at a pressure of 550 kPa or less and designed for repeated use; or 
  3. other means acceptable to authority having jurisdiction.”

To prevent any damage to private property caused by thermal expansion you are responsible to have a licensed plumber inspect your plumbing system to determine if there is a closed plumbing system. If so, you should have the licensed plumber install an appropriate device to eliminate the problems that can occur because of thermal expansion. Failure to address this potential problem within your premises may cause damage to your water heater and/or plumbing system. If any plumbing changes are completed to a plumbing system and plumbing permit is required.