Expansion Tanks

We have recently had several discussions with clients about the relative need and purpose of thermal expansion tanks installed in a plumbing system, and decided to write this blog post to shed some light on these little understood pieces of plumbing hardware.

The residents in a typical home connected to a community water source never really think much about their water. They open a tap and water comes out. Maybe they see and pay attention to the annual reports required of water suppliers, documenting the testing and quality of the water being delivered to the home… or maybe they don’t. They might complain a bit when they pay the water bill, but overall, water systems are taken for granted.

Most community water systems deliver water to homes with a backflow preventer or check valve installed that does not allow the water to flow backwards from a home into the system. It’s a little counterintuitive to understand how that might happen, but suffice it to say that in unusual circumstances it can. That could put everyone on the water system at the mercy of what any one customer in the system might have in their plumbing, and that has cost some cities a lot of money. Most now take steps to prevent that.

The presence of a backflow preventer or check valve means that a home is a closed system… it receives water from the community source but the water cannot flow in the other direction – kind of like a one-way door. Not an obvious problem… until the installed tank-style water heater starts to heat water. Water is unusual in its expansion/contraction… we all know that when water freezes it expands, but that’s because of the crystals that form. As it heats from 32 degrees to about 40 degrees it contracts. Then as it warms from 40 degrees, it starts to expand again. When a tank-style water heater heats water from about 50 degrees to about 120 degrees, the water inside is expanding. 50 gallons of water at 50 degrees will expand to about 50 ¾ gallons at 120 degrees (some sources say it would expand to about 52 gallons). Why is that important? Water will not compress, and that 3 quarts (or 2 gallons) of water has to go somewhere! That can result in increased water pressure in the plumbing system, including on washing machine hoses, dishwasher hoses, and the water heater tank itself. Any metal like the water heater tank made to bend a little over and over again will eventually fail. The solution? A thermal expansion tank.

These little tanks, often a 2 gallon size installed on the cold water line right at the water heater, have a rubber bladder inside. There is water on one side of the bladder and compressed air on the other. A properly installed and maintained tank will have air pressure on the air side approximately equal to the home’s water pressure on the water side. When the home’s closed water system sees an increase in pressure as a result of the water heater doing its job, that bladder will flex, allowing room in the system for a larger volume of water. When a water tap is opened anywhere in the house the increased pressure normalizes, and the rubber bladder moves as needed to accommodate the pressure change in the system. Think of the thermal expansion tank as a plumbing system’s shock absorber.

So are they necessary? From a physical safety standpoint, probably not. If the water heater has a properly installed and functioning temperature/pressure relief valve, dangerous pressures are unlikely. But not having one will take its toll on the components of the plumbing system, likely shortening their life. Water heaters on a closed system without a thermal expansion tank will not likely last as long as they would with one installed. The possibility of the washing machine hoses bursting and flooding the home is increased without a thermal expansion tank.

Could you drive down the street in a car with no shock absorbers? Sure… but would you want to? Don’t you think it would be kinda rough on all of the other parts of your car?

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