Thermal Expansion Tanks

Thermal expansion tanks prevent excessive water pressure build up from your hot water heater. This is basic plumbing knowledge if you’re a plumber. But since moving to Lees Summit I find myself frequently explaining this to my plumbing customers. So lets explore it.

What is thermal expansion

Water heaters don’t just heat your water. They also create pressure. When you heat water it expands. When it expands it wants to move backwards through the water pipes. But if you have something blocking this backward water flow the expansion will quickly create pressure. That’s thermal expansion.

What is a thermal expansion tank

A thermal expansion tank is a small tank, usually three to five gallons, that attaches to the cold water inlet pipe of your water heater.

excessive water pressure, leaking water heater, relief valve
Thermal expansion tank cutaway

Inside the tank is a rubber bladder that’s pressurized with air. So on one end of the tank is the rubber air bladder. And on the other end is a water connection. When connected to your hot water heater, water will push against the rubber bladder inside the expansion tank. The water pressure and air pressure should be equal. This will create a static cushion. When thermal expansion occurs it will allow the water room to expand instead of building excessive water pressure.


Do I need a thermal expansion tank

The ONLY time you need a thermal expansion tank on your water heater is when there is a device blocking water from flowing backwards in your water pipes. And even then, these devices could have bypass valves installed to allow for thermal expansion. That’s where the judgement of your plumber comes into play. The only such devices most Lees Summit homeowners will come across are water pressure regulating valves and check valves.

Lees Summit hot water heater, thermal expansion tank
How thermal expansion tanks work

A quality water pressure regulating valve will have a built-in bypass valve. This valve will allow water that exceeds city water pressure to bypass the valve and flow back into the city water supply. But if the city water pressure is too high this bypass won’t help much.

A check valve is sometimes installed on a water meter by the city. They do this to protect the drinking water system from back-siphonage. You’ll definitely need a water heater thermal expansion tank in this circumstance. But there are some symptoms of thermal expansion for which you can check.

Symptoms of thermal expansion

Finding thermal expansion in a home usually isn’t too difficult. The first place I look is the water heater. An intermittently leaking water heater relief valve is a sure sign. The relief valve will eventually stick open and leak all the time.

A sudden surge of faucet water pressure that quickly dissipates is another sign to look for. If you have a solid plumbing system with no leaking fixtures or appliances, pressure builds up. When a faucet or toilet is used this pressure quickly equalizes.

The usual signs of excessive water pressure are also to be expected. This could mean banging pipes, high water velocity noises or plumbing fixtures or appliances wearing out too soon.

Who installs thermal expansion tanks

Plumbers install thermal expansion tanks. You’ll hear air conditioning and heating contractors or handymen advertising for them. But many times they don’t know how they work. You wouldn’t hire a landscaper to work on your car. Only hire a plumber to work on your plumbing. Call your local Lees Summit hot water heater specialists at Advocate Master Plumbing. We not only know how to install plumbing. We know how and why it works.

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