Thermal Expansion

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Thermal Expansion
by Rich Grimes

We have covered several topics related to water heating in previous articles and we will continue with the issue of Thermal Expansion. Thermal Expansion will occur whenever there is a heat source and the piping loop is “Closed”. This implies that the piping is operating as a Closed Loop, separated from incoming fresh water by a check valve of a backflow device. Closed Loop systems typically operate at lower system pressures than incoming cold water pressures. A closed loop heating boiler is a good example of such a system. The water in the closed loop of piping is not potable and must be prevented from backflow into the domestic cold water supply. Once a backflow device is installed and the loop is heated, thermal expansion will occur and must be controlled. This is also an issue on domestic water heaters installed on “Open Loop” piping because of code required BFP devices.



Hydronic heating boilers have always required an expansion tank and the sizing is calculated to absorb the system’s thermal expansion. Water heaters were traditionally installed with no check valve on the cold water supply, so the cold water piping would absorb the expanding heated water. Larger commercial systems would utilize a swing check valve to prevent over-heating of the cold water supply. Plain steel expansion tanks could not be used on fresh water systems so the common practice was to drill a ¼” hole in the flapper of the check valve to allow expanding hot water to escape into the cold water supply. This sounds crazy but it was documented in various manufacturer’s literature and did help to alleviate the problem. But it did not fix the problem.

But along came Backflow Prevention and it created an immediate need for Domestic Thermal Expansion Control. The basis of BFP is to prevent cross contamination, but the result was a “Closed” piping loop that experienced thermal expansion just like a boiler! This is similar to Newton’s Law because for each and every action there is an equal and opposite reaction. All of a sudden there were expansion issues that did not exist before!

Manufacturers of expansion tanks provided tanks with internal bladders that could be pressurized and separated the steel tank from the fresh water. The same tank design was being used on fresh water well systems.

Today, backflow prevention is a standard installation practice on domestic cold water systems. There are various BFP’s and Thermal Expansion devices that can be used, depending on local code requirements.



Water cannot be compressed like air so it will expand, creating more volume. This expansion creates a pressure increase that can be entrapped by a check valve or BFP. Water will expand at a rate of .000023 percent for each degree of temperature rise. This may not seem like a lot but if a 30 gallon water heater was heated from 60ºF to 140ºF (80ºF Rise) it would increase in volume by .55 gallons. The additional ½ gallon of water must expand as the volume increases. If there is not a means of expansion control then the expanding water will lift the relief valve to discharge the additional volume and increasing pressure. Temperature and Pressure Relief Valves will discharge with a condition of 210ºF or 150 Psi. Expanding water can easily exceed the 150 Psi T&P valve rating when heating up a “closed” water heater. This is commonly seen at the end of the heating cycle when the relief valve lifts for several seconds. It is also commonly misdiagnosed as a bad relief valve and the replacement relief valve functions just like the “defective” valve, discharging water.

Expansion is a predominately a pressure issue, but temperature accelerates the expansion. Thermal Expansion, expansion caused by heating. New water heaters have a clean heating surface and can expose thermal expansion where the old heater did not display such signs. I have also seen where houses with ½” piping experience more expansion issues than houses piped in ¾”. This is due to the same rate of volume increase with less piping to absorb the expansion. Old heaters that are full of scale have an extended, slower heating cycle that helps to gradually add the expansion.

This will also make you realize how much expansion can be created by large commercial systems with high BTU inputs.



Domestic Thermal Expansion tanks are constructed typically of an epoxy coated steel shell. They have a butyl rubber internal bladder that separates the bare steel from the fresh water. They have a connection for connecting to the cold water supply and an air connection for pre-charging the bladder pressure. The bladder pressure MUST be preset equal to or a little greater than the incoming cold water pressure. This is crucial to installing an expansion tank. A setting of 10 Psi greater than measured cold water pressure is recommended to compensate for varying pressures. For instance, a neighborhood typically has a little less pressure in the morning (heavy use time) than it might at 2:00 PM when the water usage is less. Cold water pressure should be measured with a hose bibb pressure gauge or similar dial-type gauge. Almost every expansion tank comes factory pre-charged to 40 Psi. While 40 Psi may be expected on a well system, pressures of 60 to 80 Psi are common in Florida. A tank that is pre-charged to 40 Psi and is installed on a 60 Psi system will be ineffective. The air in the bladder is pushed all the way up into the tank and it cannot absorb any expansion. An expansion tank must be pre-charged with no water pressure present for the proper setting.

The connection of the expansion tank to the cold water supply is also critical. The expansion tank MUST be installed between the heater and the cold water check valve or BFP. The hot water will try to expand away from the heater towards the cold supply where it is absorbed by the Expansion Tank.

Bladder style expansion tanks can be mounted in the most convenient location and piped over to the system, unlike gravity style tanks which must be located at the highest point of the system. Bladder tanks are also smaller in size to an open gravity tank due to their ability to absorb expansion at a higher volume.

Thermal expansion tanks are sized based on volume of water, incoming water temperature and pressure, stored water temperature and possibly some pressure and expansion factors. There are various sizing programs available in print and online. There are probably ten to twenty manufacturers of domestic thermal expansion tanks to choose from. There are also larger bladder type tanks available for commercial applications.



Domestic Thermal Expansion Tanks are required in most systems today due to backflow prevention devices. These devices provide a solid, positive shut off that will not allow for any thermal expansion. There are also other means of expansion relief, but most involve a self-seating valve that will lift and allow the relief of the additional volume, prior to the heater T & P valve lifting. The Bladder type tanks provide proper expansion protection and also a little protection from water hammer and thermal shock. Systems that experience extreme hammer or thermal shock should be provided with additional protection such as water hammer arrestors or shock absorbers.

It is important to pre-charge the expansion tank and make sure that the connection to the system is located between the water heater and the check valve/BFP. If a tank is existing and has the incorrect pre-charge air pressure, the cold water pressure must be relieved so the tank air pressure can be properly set. Thermal Expansion tanks will prolong the life of water heaters as they absorb the excess volume created by thermal expansion. Backflow prevention is primary to keeping our water supplies safe from cross connection contamination. This technology has created the need for domestic thermal expansion devices that are here to stay.

Thanks and we’ll see you in the next article!


Rich Grimes

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