Water Heater Thermal Expansion

When heat is applied, the nature and behavior of water tends to change. As water is heated it immediately starts to expand. As long as the service pipe between street main and water heater remains unobstructed to reverse flow, the water will move, as it expands, back towards the street main with no measurable increase in pressure. However, should the flow in the service line be limited to flow only towards the service outlets, by the use of a check valve or pressure regulator, or should a manually operated valve be closed, water, as it expands while heating, cannot escape from the system and a damaging pressure could quickly follow. Water (unlike air) cannot be compressed appreciably. The confinement of water results in what is called a “closed system.” Water expands at a rate of approximately 0.00023 pct. for each degree of temperature rise.

If all of the water in a 30 gallon water heater were raised from 60 to 140 F, a temperature rise of 80 degrees, it would increase the original volume to 30.55 gallons, an increase of .55 gallons. Water confined in a storage tank or piping system will, when subjected to a temperature rise of 10 degrees (increasing from 75 to 85 degrees) increases pressure from 50-250psi.

The above phenomenon occurs regularly in high-pressure areas. Many people and even some plumbers are not aware that excessive pressure is accumulating in their water piping systems, even though they are protected from the water company pressure by a regulator. Anytime the water heater ignites, pressure begins to build in the system as water expands. Simply flushing a toilet, or opening a tap, will relieve and normalize the pressure until the water heater relights to keep the water at the heater’s temperature setting. The only way the average person knows when this is happening is when the pressure exceeds 150psi and the water heater T&P relief valve kicks off. Unfortunately, water pressure between 80-150psi has a deleterious effect on the entire plumbing system and causes rapid and unnecessary wear on all its components. To solve this problem it is necessary to install a heat expansion tank at the water heater, which will absorb the expanded water and keep the pressure at a normal setting. A simple test is required to determine if this condition exists.

We recommend that anyone living in a high-pressure area implement the test and make the necessary correction.