The short answer for tankless owners
An expansion tank is a small pressurized vessel on the cold supply line that absorbs the extra volume water takes up when it is heated. Georgia's state minimum plumbing code, the International Plumbing Code with Georgia amendments, requires one for a storage water heater when its cold supply passes through a check valve, pressure reducing valve or backflow preventer.
That sentence says storage water heater. It does not say tankless, so no one should tell you that Georgia code forces an expansion tank onto a tankless unit. What decides it for your house is the installation manual that came with the heater and the way your plumbing is set up.
Why heated water needs somewhere to go
Water expands as it warms, and in a gallon-scale system that expansion is small but real. In an open system the extra volume pushes back into the city main and nobody notices.
A check valve, backflow preventer or pressure reducing valve stops that backward flow and closes the system. With nowhere to go, the extra volume raises pressure in the pipes, and the weak points start to show.
Symptoms of a closed system with no expansion room
These signs point to pressure that spikes when hot water is made. They can come from other problems too, so treat them as reasons to measure, not proof.
Relief valve drips after a shower
A temperature and pressure relief valve that weeps right after the heater runs can mean pressure rose past its setting.
Banging when a faucet shuts
Water hammer is a pressure shock, and a closed system with high static pressure makes it worse.
Faucet drips that start suddenly
Washers and cartridges wear faster under constant high pressure.
Pressure that climbs overnight
A gauge on an outdoor spigot that reads higher in the morning than at noon suggests a closed loop with no relief.
Storage tank versus tankless on the expansion question
The two heater types hold very different amounts of water, and that changes how the question is handled.
| Setup | Water held in the heater | What to do about expansion |
|---|---|---|
| Storage tank on a closed system | About 40 to 80 gallons | Georgia code calls for a thermal expansion control device, usually an expansion tank |
| Tankless on a closed system | A small volume in the heat exchanger | Follow the manufacturer's manual, ask a plumber to measure pressure |
| Tankless with a recirculation or buffer tank | A stored volume in the buffer | Treat the storage portion like a tank and ask the installer |
| Any heater on an open system | Varies | Expansion can flow back into the main, so a tank may not be needed |
How to find out if your house is closed
Look at the water line where it enters the house. A pressure reducing valve is a bell-shaped brass fitting, often near the main shutoff, and a backflow preventer is a boxy valve with test ports. If either is present, the system is probably closed, and a plumber can confirm it.
A plumber can confirm with a pressure gauge on an outdoor spigot. Take one reading with no water running, then another after the heater has made hot water for a while, and compare them. A climb between the two is the clearest sign that expansion has nowhere to go.
Where an expansion tank goes
Georgia's amended code says a thermal expansion tank must sit at least 18 inches in developed length from the heater's cold-water inlet fitting. That is the length of pipe, not the straight-line distance.
It also means the tank needs room on the supply line, which matters in a tight tankless install on a garage wall.
Suwanee note: who owns the valves that close your system
Gwinnett County Water Resources says the home pressure reducing valve is the homeowner's responsibility, and the department does not send personnel to investigate or repair it. The county also lists the backflow preventer as part of the plumbing system that belongs to the homeowner. If your house has either valve, the question of expansion control sits with you and your plumber, not the utility.
The county pressure page does not give a typical supply pressure in psi, so do not rely on a neighbor's number. A reading at your own spigot is the only figure that applies to your house.
The City of Suwanee building permits page lists plumbing changes among the work that needs a building permit. If an expansion tank is added during a new tankless install, ask your installer how the permit and inspection will be handled, and call the city inspections line at 770-945-8996 for questions.
What affects the cost of adding expansion control
An expansion tank is a modest part, and the labor around it is the larger piece. Here is what changes the line on a quote.
Tank size
Sizing depends on the water volume and the pressure, and a manufacturer's manual or plumber can specify it.
Space
A cramped wall with little vertical clearance can require extra fittings or a different mounting.
Pressure work
If static pressure is too high, the pressure reducing valve may need adjusting or replacing first.
Install timing
Adding the tank during a tankless install is quicker than opening finished pipe later.
Permit handling
Ask whether the permit is listed on the quote or billed separately.
Homeowners insurance usually covers sudden water damage from a burst pipe, not the cost of adding a part that was missing. Review your policy before assuming coverage.
Want your real number?
Describe the job on the phone and get a clear price before any work starts.
Gas safety on tankless installs
Most tankless heaters in homes burn natural gas, and a tankless unit needs a larger gas supply than a storage tank. Georgia's plumbing code packet says gas-fired water heaters and their venting fall under the International Fuel Gas Code, so the gas line and vent are checked as their own part of the job.
If you smell gas at any time, leave the building and call the gas utility, for most homes Atlanta Gas Light at 877.427.4321, or 911, from a safe distance. Do not use switches, flames or a phone inside. Call a plumber after the gas utility says it is safe.
Related reading and where we work
The tankless water heater page covers sizing and installation questions in more depth. If you are weighing a swap from tank to tankless, water heater installation explains the connections involved.
Neighbors in Duluth and Lawrenceville can call the same line for tankless questions.
According to the Georgia amendments to the International Plumbing Code, a thermal expansion control device is required where a storage water heater is supplied with cold water that passes through a check valve, pressure reducing valve or backflow preventer.
Tankless expansion tank questions
The code language in Georgia's plumbing amendments is written for storage water heaters. For a tankless unit, follow the manufacturer's installation manual and ask your plumber about your specific system.
It is a house where a check valve, backflow preventer or pressure reducing valve keeps heated water from pushing back into the city main. Expansion then has nowhere to go but into your pipes.
Gwinnett County Water Resources says the home pressure reducing valve is the homeowner's responsibility. The county does not send staff to investigate or repair it.
Ask a plumber to attach a gauge to an outdoor spigot and read it with no water running. Read it again after hot water has been used, and compare the two numbers.
It can, but a worn valve or a failing heater can also cause dripping. A pressure reading separates the causes before you buy a part.