Cross Connection

Thermal Expansion

You must install protection against Thermal Expansion any time a water system is contained. A contained system is any water system that has a backflow protection device installed on the incoming water supply of the building. A simple device called a Thermal Expansion Tank can be installed on the incoming cold water pipe serving the water heater of the building to ensure that water has a place to expand and not risk the breaking of water lines and/or collapsing of water heater flues.

Thermal Expansion Danger

Most homes are supplied with hot water from an electric or gas heated tank. Until the heating element stops working, and one is faced with a cold shower, the water heater is usually taken for granted. However, if not properly maintained, a water heater may become a safety hazard.

Water expands in volume as its temperature rises. The extra volume caused by thermal expansion must go somewhere. If not, the heated water creates an increase in pressure. This is the principle of a steam engine. The temperature and pressure in the water heater is reduced when hot water is withdrawn from a faucet and cold water enters the tank. The increase in pressure from thermal expansion can also be reduced by water flowing back into the public water system. However, when a check valve, pressure-reducing valve or backflow preventer is installed in the service pipe a “closed system” is created. Provisions must be made for thermal expansion in these cases.

The thermostat of the water heater normally maintains the water temperature at about 130º F (54º C). However, if the thermostat fails to shut off the heater, the temperature of the water will continue to increase. If the water temperature increases to more than 212º F (100º C), the water within the tank becomes “super-heated”. When this super-heated water is suddenly exposed to the atmosphere when a faucet is opened, it instantly flashes into steam and a violent reaction may result. As the pressure within the tank continues to build up under super-heated conditions, the tank may explode.

Protection from Thermal Expansion

Protection from thermal expansion is provided in a plumbing system by the installation of a thermal expansion tank in the hot water system piping downstream of the hot water tank and a temperature and pressure relief valve (T & P Valve) at the top of the tank.

The thermal expansion tank controls the increased pressure generated within the normal operating temperature range of the water heater. The small tank with a sealed compressible air cushion provides a space to store and hold the additional expanded water volume. The T & P Valve is the primary safety feature for the water heater. The temperature portion of the T & P Valve is designed to open and vent water to the atmosphere whenever the water temperature within the tank reaches approximately 210º F (99º C). Venting allows cold water to enter the tank.

The pressure portion of a T & P Valve is designed to open and vent to the atmosphere whenever water pressure within the tank exceeds the pressure setting on the valve. The T & P Valve is normally pre-set at 125 psi or 150 psi. Water heaters installed in compliance with the current plumbing code will have the required T & P Valve and thermal expansion tank. For public health protection, the water purveyor may require the installation of a check valve or backflow preventer downstream of the water meter. In these situations, it is essential that a T & P Valve and thermal expansion tank be properly installed and maintained in the plumbing system.

What the Homeowner Should Do to Ensure Protection from Thermal Expansion.

  • The homeowner should check to see that an expansion tank and T & P Valve are in place. If there is any doubt, the homeowner should contact a licensed plumber.
  • The T & P Valve should be periodically inspected to ensure that it is properly operating. T & P Valves are equipped with a test level. Manually lifting the lever unseats the valve, allowing water to discharge. If water continues to leak from the T & P Valve after closing, the valve may need to be replaced. A drain line must be installed to avoid water damage and scalding injury when the valve operates.

T & P Valves are equipped with a test level. Manually lifting the lever unseats the valve, allowing water to discharge. If water continues to leak from the T & P Valve after closing, the valve may need to be replaced. A drain line must be installed to avoid water damage and scalding injury when the valve operates.

  • The T & P Valve should be periodically removed and visually inspected for corrosion deposits and to insure it has not been improperly altered or repaired.
  • The above work can best be done by a licensed plumber.

tandp-value-locationClick image to enlarge.

‘WARNING: Installation of a backflow prevention device could cause thermal expansion resulting in potential serious bodily injury and/or property damage.  When installing a backflow prevention device, you must consult with a professional plumber to protect against thermal expansion AND ENSURE ALL NECESSARY PRECAUTIONS ARE TAKEN.’