Water Heaters
What Is Expansion Tank?
Definition
An expansion tank is a small secondary tank installed on a water heater's supply line to absorb the extra volume water creates as it heats and expands, relieving pressure that would otherwise build up in a closed plumbing system (one with a check valve or pressure-reducing valve preventing backflow to the street).
Why it matters for your home
Many jurisdictions require an expansion tank whenever a check valve or pressure-reducing valve makes the plumbing system "closed" — skipping it on a closed system can shorten the water heater's life and stress pipe joints over time.
What to do about it
Ask whether your home's plumbing is a closed system and, if so, confirm an expansion tank is part of your water heater replacement quote.
Related terms
This definition is pending licensed-trade technical review. It's sourced from our own published service and guide content, not invented — but hasn't yet been signed off by a licensed HVAC reviewer.
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