An expansion tank prevents pressure increases due to thermal expansion of your water. As your water is heated from 50° F to 120° F, it expands by approximately 2%.
If you have a “closed system”, which means there is a backflow preventer or other device that prevents your water from flowing back into the water main, thermal expansion will cause very rapid pressure increases in your plumbing system.
Common questions about water heater expansion tanks include:
What is a check valve?
The check valve is the mechanism that prevents backflow of thermally expanded water in the wrong direction. Without an expansion tank, increased pressure can cause unnecessary stress on your plumbing fixtures, including the water heater, and cut down on their working life.
Does the expansion tank have to be installed on the cold side?
Although expansion tanks can be installed on the hot side, we strongly recommend they be installed on the cold line, downstream of the shutoff valve.
Where should an expansion tank be installed in relation to the hot water heater?
Your expansion tank can be anywhere on your plumbing system and does not need to be installed in close proximity to the water heater. It is most commonly installed using a “T” at the cold inlet to the water heater. But, functionally, it can be installed anywhere on the cold inlet line.
Can the expansion tank be installed at any angle?
The expansion tank can be installed in any attitude. This will conflict with info that comes with any expansion tank you buy at a retail store. The installation instructions that come with retail expansion tanks say the tank must be installed in a hanging vertical position.
Retail expansion tanks come with a “Saddle Fitting”, which allows the installation into the system without any soldering, thereby making installation easier for the average homeowner. The saddle fitting is a device that clamps around the pipe and has female threads that will accept the expansion tank. A small hole is first drilled in the pipe, then the clamp is placed so the hole lines up with the expansion tank inlet. We do not recommend using such a fitting. They are unreliable, and allow placement of the expansion tank in the above mentioned vertical hanging position only.
We recommend using galvanized fittings or soldering the appropriate fittings into the system.