An HVAC technician was dealing with an unusual failure report after a customer said an expansion tank had blown off the system — and mentioned finding a spring.

The mystery made more sense once photos arrived, and the tech shared one on Reddit with the caption, "Never seen anything like this before."

Here's what to know

In a Reddit thread, the photo showed what looked like a metal coil where the tank connection gave way. The OP said the image came after a customer called to report that the expansion tank had blown off the system and had mentioned a spring.

For some users, the first issue was how the tank was mounted. 

One person said, "Most likely wasn't supported properly. Expansion tanks need independent support. You cannot hang them off of existing piping, no matter how small they are."

Other users focused less on the mounting and more on the mangled metal in the photo. One user said, "That spring is the threads from the female adaptor. Bad fitting from factory," and the OP responded that the same setup "uses this exact tank with no support" and leaves it "just hanging off the threads as this one was."

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More background

While the OP said they had "never seen anything like this before," the discussion that followed split across several possible causes, including inadequate support, a faulty fitting, hard water, and water hammer.

Some of the thread also turned to the danger of older plastic piping under added weight. One user recalled fixing a similar tank that was "free hanging off CPVC," prompting another to warn, "You can look at cpvc wrong and snap it when it's old and brittle."

To explain the purpose of an expansion tank, one user offered a simple demonstration: heat a completely filled, sealed milk jug to 130 degrees Fahrenheit and watch the pressure build as the water expands. Their takeaway was that an expansion tank gives that extra volume somewhere to go.

Warranty was another theme. One user said the OP should contact the manufacturer directly, arguing that this was a "low pressure system (boiler expansion tank)" and that "there is no way that tank should have failed under max 30 PSIG." 

What can be done?

If an expansion tank is hanging only "off the threads," said the OP, homeowners may want a qualified pro to check whether the setup matches manufacturer requirements and whether the piping and fittings are carrying too much weight.

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If a part looks suspect, document the damage and ask about warranty coverage. One user specifically said to contact the manufacturer directly because they are "generally really good with warranty," especially if the tank threads failed on a low-pressure boiler system.

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Where can I learn more?

For more on the kinds of problems raised in this thread, these stories look at similar issues with support, fittings, and pressure. They follow homeowners and plumbers as they work through burst tanks, questionable installs, and leak sources.

• A homeowner learned an upside-down install is normal after a burst expansion tank raised alarms.

• In Florida, plumbers warned homeowners to please only use brass for expansion tank fittings.

• After a $500 quote, plumbers said the easy swap needs support to last.

• A plumber told a homeowner that reused push-fit fittings can leak around a new tank.

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