Thermal Expansion
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The purpose of this page is to inform you about the potential for thermal expansion in certain properties, in order to help you avoid possible damage to your plumbing system.

Pinellas County, along with individual municipalities making use of reclaimed water, adopted a cross-connection ordinance and began an active program to control cross-connections in accordance with the Florida Department of Environmental Protection. As part of the program, water meters are routinely installed with check valves, and certain high-risk premises are isolated by backflow prevention assemblies. Backflow prevention assemblies are intended to protect the public health by preventing the reversal of water flow from a private plumbing system back into the public water system.

The water heater in your building goes through a "recovery process" each time hot water is used. Normally, this process occurs several times daily depending on how often hot water is demanded. When cold water is heated, it expands. This is known as "thermal expansion". A check valve, backflow assembly, or pressure reducing valve can make the plumbing system a "closed" system that prevents the heated/expanded water from being forced back into the public distribution system. Since the public system does not provide a "cushion" to absorb the pressure build-up within the private plumbing/water heater, excessive pressure can occur in the private plumbing system.

Thermal expansion, if not controlled, can damage or reduce the life of plumbing fixtures and appliances. Thermal expansion may be evident by dripping from hot water faucets, or intermittent discharge from the temperature and pressure (T&P) relief valve on the water heater. Some experts even warn that repeated uncontrolled thermal expansion could collapse the center flue in a gas-fired water heater, creating a hazardous presence of carbon monoxide gas or even a water heater explosion.

Historically, water heaters have been equipped with a T&P relief valve even before the advent of cross-connection control measures. However, this valve is an emergency relief valve designed to prevent explosion of the tank and is not intended to compensate for thermal expansion during the normal heating and cooling cycles.

Thermal expansion can easily be contained by the use of a properly sized Thermal Expansion Tank installed on the cold water supply to the water heater. Modern plumbing codes have required them since 1991. These small tanks (about 4 gallons) incorporate a sealed-in air chamber, pre-pressurized and separated from system water by a flexible diaphragm, installed to prevent the air cushion from being absorbed into the system water. The tank allows thermal expansion to occur, but without the dangerous increase in pressure that would cause the T&P relief valve to operate.

You are encouraged to have a licensed plumber inspect your plumbing system to determine if it is a “closed” system. If it is, you should have a Thermal Expansion Tank installed by a licensed plumber. The code requires installation of a Thermal Expansion Tank if you replace your existing water heater. The plumber may also determine if the T&P relief valve is in working order. Taking this simple step may prevent damage to your plumbing system.