Water Heater Expansion Tank: Why a New Heater Sometimes Needs One
A water heater expansion tank is a small potable-water pressure vessel on the cold water line at the water heater that provides space for the extra volume created as a storage water heater heats its contents. An air cushion behind a rubber bladder inside the tank compresses as water enters. When the plumbing arrangement prevents that expansion from moving back toward the supply, the installation may need a properly sized expansion tank, or another approved means of thermal expansion control, under the applicable requirements.
Quick answer. If a check valve, a backflow preventer or a pressure reducing assembly on the incoming line stops expanded water from moving back toward the supply, the plumbing is a closed system and the code requires thermal expansion control; an expansion tank is the usual answer and the one you will almost always be quoted, though the code allows another approved device. The devices, the piping layout and the water heater determine the answer, so have a licensed C-36 plumber identify them. A dripping temperature and pressure relief valve needs an assessment: thermal expansion is one possible cause, high incoming pressure, excessive temperature or a valve fault are others. Never cap, plug or obstruct the valve or its discharge pipe, and keep clear of hot discharge.
This usually comes up in one of two ways. Either a plumber quoting a water heater replacement includes an expansion tank and the homeowner wonders whether it is a requirement or an add-on, or someone has a relief valve that drips and cannot work out why. Both lead to the same question.
What Is a Closed System, and Do You Have One?
An open system lets water move both ways. When water in the heater expands, a small amount pushes back out toward the main, and nothing much happens. A closed system has a one-way device on the incoming line: water can come in but cannot go back out, so when the heater fires and the water expands, that expansion is trapped inside the plumbing and the only thing that can absorb it is pressure.
| Device | Why it is there | Where to look |
|---|---|---|
| Device Pressure reducing valve | Why it is there Street pressure is higher than the house should see | Where to look Where the main enters, often near the meter or in a crawl space |
| Device Backflow preventer | Why it is there Required by the water district to protect the public supply | Where to look Near the meter, sometimes only on an irrigation connection |
| Device Check valve | Why it is there Prevents reverse flow, sometimes fitted at the meter | Where to look At or just after the meter |
Location and design matter. A backflow device serving only the irrigation does not necessarily close the house's domestic supply, and some pressure reducing assemblies have bypass arrangements whose behavior depends on the upstream pressure. Have the plumber identify the actual flow path and the devices on it, and measure pressure under appropriate operating conditions; a single gauge reading, or a glance at the meter, does not establish the complete arrangement.
The practical difficulty is that homeowners are rarely told when one of these gets installed. Water districts add backflow prevention at the meter as a matter of policy, and a house can become a closed system without anyone mentioning it. A water heater that ran for years without an expansion tank can start having pressure problems because something changed at the street.
How Much Does Water Expand When Heated?
By a small fraction of its volume, on the order of a couple of quarts on a typical residential tank heated from supply temperature to its setpoint. That sounds trivial, and it is not, because a small increase in water volume can produce a substantial pressure rise when the expansion is constrained. A closed system with no expansion control can climb from normal operating pressure to well above the relief valve's setting during a single heating cycle.
The expansion tank gives that volume somewhere to go. Air compresses where water does not, so the extra volume pushes into the tank, the air cushion takes the pressure, and the system stays within its normal range.
What Are the Symptoms of Not Having One?
| Symptom | What it can mean |
|---|---|
| Symptom Relief valve drips, especially after hot water use | What it can mean Pressure exceeding the valve's setting during heating cycles, or another relief valve cause |
| Symptom Banging or knocking when a tap closes | What it can mean High static pressure making water hammer worse |
| Symptom Toilet fill valves failing repeatedly | What it can mean Pressure cycling wearing the internal seals |
| Symptom Faucet cartridges and supply lines failing early | What it can mean Same cause, different fixture |
| Symptom Water heater fittings weeping | What it can mean Repeated pressure cycling on the tank connections |
| Symptom Pressure gauge reads high after the heater runs | What it can mean The direct measurement, under the right conditions |
The relief valve is the one to take seriously. A temperature and pressure relief valve that discharges is telling you something, and the mistake people make is replacing the valve without finding out what. It may be doing its job against thermal expansion in a closed system. It can also discharge because static pressure in the house is too high, because the water temperature is too high, because debris is holding the seat open, or because the valve is damaged or worn. Excessive temperature in particular is the condition the valve exists to protect you from.
So treat a dripping relief valve as a reason to have the system assessed rather than as a self-diagnosis. Never cap, plug or obstruct the valve or its discharge pipe, keep clear of hot discharge, and do not simply swap the valve. The fixture symptoms cost money quietly: if you have replaced toilet fill valves in more than one bathroom in a short period, the common factor may be the pressure, and we cover the toilet side in a toilet that keeps running.
How Is It Diagnosed?
With a pressure gauge and a look at the incoming line. The assessment should distinguish incoming pressure that is simply too high, which calls for a pressure reducing valve, from pressure that climbs while the water heater runs, which points at thermal expansion in a closed system, and it should check the relief valve and the water temperature where relevant. Sometimes more than one problem is present. Guessing between them wastes money.
What Is Involved in Installing One?
Modest work, and it usually belongs with a water heater replacement rather than as its own project, though failed or missing expansion control should not wait for the next heater.
If an expansion tank is specified, it must be approved for potable water, sized for the system (the heater's capacity, the temperatures and the static pressure all go into that), and installed with suitable support, because a tank full of water is much heavier than the empty one people picture when they see it mounted. Its air precharge must be checked and adjusted under the manufacturer's specified conditions to match the house's static pressure. A factory setting should not be assumed to match the house, and an incorrect charge reduces the tank's usable capacity and can make it unsuitable for the duty.
Do not press the air valve, tap the vessel or loosen connections as a homeowner test. A technician can check the tank's condition and its air charge using the prescribed service procedure, which is a few minutes' work on a service visit. Expansion tanks do wear out: the bladder can fail, and corrosion or vessel failure are also possible, so returning pressure symptoms or visible damage are reasons for a service check.
Is It the Same as a Pressure Regulator?
No, and the three devices people conflate do different jobs. An expansion tank accommodates the volume created by heating water. A pressure reducing valve controls excessive supply pressure coming in from the street. A water hammer arrestor addresses the pressure shock from sudden changes in flow, the bang when a valve snaps shut. One is not a substitute for another, and some houses need more than one of them.
Is It Required, or Is It an Upsell?
Where a closed system exists, thermal expansion control is a code requirement, and an inspector on a permitted water heater replacement will look for it. Where the system is genuinely open, it is not required, and a contractor including a tank anyway should be able to explain why. The California Plumbing Code sets the requirement, adopted through the California Building Standards Commission as part of Title 24 and based on the Uniform Plumbing Code developed by IAPMO; the contractor should be able to cite the section that applies to your installation.
So the question to ask is "is my system closed, and how did you determine that." A good answer names the device: there is a pressure reducing valve at the meter, or the district has a backflow preventer on the domestic service. A vague answer should be pressed on.
Ongaro & Sons is a family-owned company based in Petaluma, serving Marin, Sonoma and Napa County since 1932, licensed C-36 plumbing, C-20 HVAC and C-10 electrical. You can verify any California contractor's license and classification on the Contractors State License Board site.
What Else Should Be Handled at the Same Time?
- Seismic restraint. Required by California law on new, replacement and existing residential water heaters, and certified to a buyer when you sell. We cover it in water heater seismic strapping.
- The relief valve discharge pipe. It should run down toward the floor or outside, unobstructed and uncapped. A missing or capped discharge is a serious defect.
- The flexible gas connector. If it is old, corroded or kinked, replace it while the heater is out.
- Water quality. Scale shortens a water heater's life, and mineral content varies across this service area. We cover the local picture in hard water in Marin County.
Frequently Asked Questions
Do I need a water heater expansion tank?
Have the plumbing arrangement and the water heater assessed. Where thermal expansion control is required, an appropriately sized potable-water expansion tank is the common solution, and another approved device may be suitable. The requirement is for effective control, not automatically one particular product.
How do I know if I have a closed system?
Look at where the water main enters the house and at the meter for a pressure reducing valve, a check valve or a backflow preventer on the domestic supply, and note whether a backflow device serves only the irrigation. A plumber confirms it by identifying the flow path and measuring pressure under operating conditions; a single reading does not establish the whole arrangement.
Why does my water heater relief valve keep dripping?
Thermal expansion in a closed system is one possibility. Excessive supply pressure, a temperature problem, debris on the seat or a defective valve can also cause discharge. Have the cause checked, never cap or obstruct the discharge, and do not simply replace the valve without finding out why.
How can I tell whether the expansion tank has failed?
Returning pressure symptoms or visible damage justify a service check. Sound from tapping the tank is not a reliable diagnosis. A technician inspects it and tests the pressure and the precharge under the manufacturer's specified conditions.
Is an expansion tank required by code?
Where a closed system exists, thermal expansion control is required by the plumbing code, and an inspector on a permitted water heater replacement will look for it. Where the system is open it is not required, which is why the question to ask a contractor is how they determined your system is closed.
Can I install an expansion tank myself?
The plumbing connections are within reach of a capable homeowner, but the sizing, the support and the air precharge, set under the manufacturer's specified conditions to match the house's measured static pressure, are what make it work. If you are not going to size it and set the charge properly, have it done.
Will an expansion tank fix high water pressure?
No. An expansion tank accommodates the pressure increase from heating water. Excessive incoming street pressure needs a pressure reducing valve, and a gauge distinguishes the two. Some homes need both.
Where to Start
Look at your incoming water line where it enters the house and at the meter. If you find a pressure reducing valve or a backflow preventer on the domestic supply, you probably have a closed system and the question of expansion control applies. Then look at your water heater's relief valve: if it drips at all, even occasionally, that is the symptom, and it is worth an assessment rather than a new valve.
If you would like the pressure measured and the system assessed, you can request a free online estimate. Our water heater page covers service and replacement.
