An expansion vessel, or expansion tank, is a device in a heating or cooling system that accommodates the extra volume of water produced when the system water is heated or cooled.

It is important to note that the responsibility for establishing and administering a cross-connection control program is with the individual municipality or water purveyor (authorities). For this reason there may be differences between the types of cross connection program that different authorities may have implemented. For more information on individual programs you should check with your respective municipality or supplier of water.

The expansion vessel provides the necessary ‘give’ in the system to handle this extra volume, preventing potential over-pressurisation damage.

3 bar (300 kPa / 43.5 psi) is commonly used for this (recommended by CIBSE) but the manufacturer’s documentation is typically the best source for this information.

Ideally, they should be placed in a cooler section of the system, for instance, the return pipe close to the boiler or chiller.

Backflow is a flowing back of water or reversal of the normal direction of flow. Backflow may occur due to either back siphonage or back pressure.

Consider a system with a water volume of 500L, an initial temperature (T_initial) of 10°C, and a maximum operating temperature (T_max) of 80°C.

"The software offers precision through detailed result outputs and advanced options for efficient pipe sizing, allowing heating engineers to optimise based on parameters like maximum velocity and pressure drop."

CASE STUDIES Award-Winning Heating Design "Using h2x was pivotal, allowing for precise heat loss calculations, pipe sizing and flow rates for each room. This level of detail ensured that the heating load accurately matched each space's requirements, minimising energy waste and maximising comfort. The strategic placement of heat pumps and careful sizing of pipework were crucial in maintaining minimal pressure drops over an 18-metre distance." Read the full case study

A water purveyor should ensure that a proper backflow preventer is installed and maintained at the water service connection to each system or premise that poses a hazard to the public water system (premise isolation). Generally, this would include water service connections to each dedicated fire protection system or irrigation piping system; premises with an auxiliary or reclaimed water system; industrial, medical, laboratory, marine or other facilities where objectionable substances are handled in a way that could cause pollution or contamination of the public water system; premises exempt from Part 7 (Plumbing) of the Ontario Building Code and premises where an internal backflow preventer required under the Ontario Building Code is not properly installed or maintained; classified or restricted facilities; and tall buildings. Zone protection within a facility may be required in addition to premise isolation, a requirement in order to protect users of the facility.

Moreover, to avoid the issue of pump cavitation, installing the vessel on the suction side of the recirculation pump is recommended.

When the repair is completed and the regular pressure is restored everything starts to flow in the proper direction. Any contaminants that had a chance to enter the water supply will start to flow towards any point of use (plumbing fixtures) in homes or businesses. The degree of hazard to health will be dependent on the type and amount of contaminant, the amount of time the situation goes unnoticed and whether or not a protective device is in place.

The critical process of accurately calculating and correctly positioning an expansion vessel is crucial in designing and operating heating and cooling systems.

This number will vary based on your geographical location and the associated climate but will likely be around 5°C (41°F) in the winter months.

Why do water purveyors (supplier) need to control cross-connections and protect their public water systems against backflow?

With the right software, like h2x Engineering’s Heating Design Software, you can drive efficiency and accuracy in your heating system projects.

The transformation of industries by the digital revolution is a story told many times over, and heating system design is no exception.

CASE STUDIES Greengate, UK "I would estimate over the length of this project, I have spent 50% less time designing than I would on our previous software. The ability to output the design straight into Revit assisted clash detection and coordination." Read the full case study

The responsibility for establishing and administering a cross-connection control program is with the individual municipality or water purveyor (authorities). Therefore, there may be differences between the types of cross connection programs that different authorities may have implemented. Please check with the respective municipality or supplier of water for program information.

Many people are unaware that backflow conditions can jeopardize system water quality, posing a health risk to consumers drinking water from the system.

These vital components, also known as expansion tanks, are pivotal in maintaining system safety and operational efficiency.

Expansion vessels are required in systems where a closed circuit is subjected to a fluctuation in temperature e.g. heating and cooling systems.

A cross-connection is any actual or potential connection between a potable (drinking) water system and any source of pollution or contamination.

"I would estimate over the length of this project, I have spent 50% less time designing than I would on our previous software.

Mechanical backflow preventers have internal seats, springs and moving parts that are subject to fouling, wear or fatigue. These mechanical devices and air gaps can also be by-passed. Therefore, some backflow preventers have to be tested periodically to ensure that they are functioning properly and others such as an air gap require a visual check to ensure that they are still in place. The testable backflow preventers must be tested by someone that has had proper training and is qualified to do so. Refer to the training and certification pages for more information on this.

We would like to acknowledge that the work of the OWWA Community takes place within the ancestral and traditional territories of the Anishinabek, Haudenosaunee, Algonquin, Mississauga, Delaware and Cree.

Back pressure is pressure that is greater than the municipal water system supply pressure. It can happen when there is a connection to a non-potable supply operating at a higher pressure than the water distribution system. Increases in pressure can be created by booster pumps, temperature increases in boilers, interconnections with systems operating at higher pressures, and elevated piping (e.g., 30 feet above finished grade).

Simply put, a backflow preventer is a device or assembly that prevents backflow. Some types of backflow preventers require testing to ensure that they will work as intended and there are some that do not require testing.

The maximum allowable pressure is the peak pressure that a system can safely tolerate without causing damage to its components or risking a system failure.

Embracing the digital revolution in the form of heating design software, like that offered by h2x Engineering, is crucial for professionals who wish to stay competitive in the industry.

They counterbalance changes in fluid volume, and consequently pressure fluctuations, which occur due to variations in temperature.

Backflow into a public water system can pollute or contaminate the water in that system. The hazard created by backflow is generally identified in three groups: severe or high, moderate and minor. A severe hazard is likely to result in serious injury or death while a minor hazard may simply affect the colour, odour and/or taste of the water with little or no health effects. Each water purveyor has a responsibility to provide water that is usable and safe to drink under all foreseeable circumstances. Further, the expectation of the public is that water quality supplied by the water purveyor is potable and safe to use and it will remain so regardless of its use. For these reasons, each water purveyor must take reasonable precautions to protect its public water system against backflow.

Back siphonage is backflow caused by a negative pressure (i.e., a vacuum or partial vacuum) in a public water system. The effect is similar to drinking water through a straw. Back siphonage can occur when there is a stoppage of water supply due to nearby firefighting, a break in a water main, high velocities in pipe lines, line repair or break that is lower than a service point, lowered main pressure due to high water withdrawal rate such as firefighting or water main flushing or reduced supply pressure on the suction side of the booster pump.

In Chapter 7 of the Report of the Walkerton Inquiry, A Strategy for Safe Drinking Water, the Hon. Dennis O’Connor states that “as part of their comprehensive distribution system program, water providers should have active programs, working together with building inspectors and public health agencies, to detect and deter cross-contamination”.

The strategic placement of heat pumps and careful sizing of pipework were crucial in maintaining minimal pressure drops over an 18-metre distance."

The simplest way is to eliminate the cross connection from the water distribution (plumbing) system. Where this is not possible, a backflow preventer (BFP) must be installed. A BFP is a means or mechanism to prevent backflow. The basic means of preventing backflow is an air gap, which either eliminates a cross-connection or provides a barrier to backflow. An air gap is easily circumvented however is not always the most appropriate type of BFP to use. The basic mechanism for preventing backflow is a mechanical backflow preventer, which provides a physical barrier to backflow. The principal types of mechanical backflow preventer are testable devices such as the reduced-pressure principle assembly, the pressure vacuum breaker assembly, and the double check valve assembly. There are other types of BFP’s available for secondary types of protection that are not testable.

– Incorrect positioning: this can disrupt the vessel’s primary function of accommodating fluid volume changes, potentially leading to dangerous pressure build-ups or operational inefficiencies.

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As the water heats up, it expands, and without a place to go, this increase in volume can cause a dramatic rise in pressure.

CASE STUDIES Salon Republic, USA "The quality of the designs saw significant improvement with the adoption of h2x." "The software offers precision through detailed result outputs and advanced options for efficient pipe sizing, allowing heating engineers to optimise based on parameters like maximum velocity and pressure drop." Read the full case study

This data can usually be found within the system’s design specifications. In cases where it’s not readily available, a pressure gauge might be needed to measure this pressure.

Gain a comprehensive understanding of expansion vessels inside this informative blog. Includes a calculation example for practical insights.

A common example is a garden hose connected to a hose bibb at one end and the other end of the hose lying in a pool, puddle or any other source of non-potable water. Another example would be the makeup water for a hot water heating boiler. The water in these systems can be rusty and oily and could pose a health threat if consumed.

An example is when there is a water main break and the area must be isolated and repaired. When the valves around the repair site are closed, the flow of water is stopped to all points of use such as homes and businesses and begins to flow backwards towards the repair. This is back siphonage and if there are cross connections, contaminants can be drawn into the water system.

This level of detail ensured that the heating load accurately matched each space's requirements, minimising energy waste and maximising comfort.

Having understood the crucial parameters, we now venture into the specifics of calculating the size of an expansion vessel.