water heating · guide · TN LLP #5045
Do I Need an Expansion Tank?
What thermal expansion does to a closed system, and when code requires one.
Short answer
If your house is on a closed system — which most are now, because of a check valve or pressure regulator at the meter — then yes, you need one, and code generally requires it on a water heater replacement. It is a small tank that gives heated water somewhere to expand.
Water expands when it is heated. In an old open system it could push back into the main. Modern systems usually have a check valve or regulator that stops backflow, which means that expansion has nowhere to go and pressure rises every heating cycle.
What the pressure does
On a closed system without an expansion tank, every heating cycle spikes the pressure. The relief valve on the heater opens to release it, which is exactly what it is for — but it was designed as a safety device, not a daily-use valve.
The visible symptom is usually a heater relief valve that drips. People replace the valve, the new one drips too, and the actual cause is never addressed.
What it costs you if you skip it
Repeated pressure spikes shorten the life of the heater, the fixtures, and the supply joints. It is a small part working against your whole plumbing system every time the heater fires.
It also tends to shorten the tank itself, which makes skipping it a poor trade against the cost of the part.
How we handle it
On a replacement we check whether the system is closed and whether an expansion tank is present and actually working — a waterlogged one is as good as absent. If one is needed, it goes in as part of the job and the inspector sees it.
Permits
Some plumbing work requires a permit and an inspection, and some does not. Whether one is required for your job is determined by the Authority Having Jurisdiction — the city or county building and codes office for your address — and it is at their discretion.
Usually requires a permit
- Water heater replacement and tankless conversion
- Whole-house repiping
- Water service line and sewer line replacement
- Gas line installation and repair
- Relocating a fixture to a new position
Usually does not
- Drain cleaning and clearing blockages
- Emergency and repair work
- Like-for-like fixture replacement
- Water heater repair
- Camera inspection and diagnostics
This is general guidance rather than a ruling, and it can differ from one town to the next. The Authority Having Jurisdiction makes the determination. Across the Bradley–McMinn corridor we pull the permit and book the inspection ourselves. Inside Hamilton County and Chattanooga the permit office does not accept an application from our license class, so permit-required jobs there are referred to a licensed partner who does the work and meets the inspector.
Common questions
Do I Need an Expansion Tank — questions
My heater relief valve keeps dripping. Is this why?
How do I know if my system is closed?
Can an expansion tank fail?
Will I know the price before you start?
Are you licensed?
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