Expansion Tank Placement for Water Heaters in Forest Grove

What the Tank Does Inside Your Plumbing

Water expands when it heats. A fifty-gallon heater that warms a full tank pushes out a measurable amount of extra water. In an open system that water backs up into the city main and nobody notices. In a closed system it has nowhere to go, so pressure climbs.

The expansion tank gives it somewhere to go. Inside is a rubber diaphragm or bladder with a cushion of air on one side. As pressure rises, the water pushes into the tank and compresses that air. When the heater cycles off and the water cools, the air pushes it back out.

Without that cushion, the heater's pressure relief valve may start releasing water. Repeated pressure spikes also stress fixtures, supply connectors and the heater itself, which is why the tank is a small, cheap part that protects expensive ones.

How a System Becomes Closed

A system becomes closed when something stops water from flowing backward toward the street. That can be a check valve, a backflow preventer or a pressure reducing valve with a built-in check. Many homes have one without the owner knowing.

This is easy to miss. A house with no expansion tank for twenty years may suddenly start dripping from the relief valve after a plumbing change or a new device on the line.

Signs to watch for include a relief valve that drips, water hammer that shakes the pipes when the heater fires, and faucets that weep. According to EPA WaterSense, ideal home pressure runs 45 to 60 psi, and a simple pressure gauge on a hose bibb will show you the number.

Where on the Pipe the Tank Goes

The tank connects to the cold water supply, on the pipe feeding the heater. A tee fitting goes in between the cold shutoff valve and the heater's cold inlet, and the tank hangs off the tee. Mount it as close to the heater as space allows, so the heated water has the shortest trip to reach it.

Keep it off the hot side unless the manufacturer's directions say otherwise. Putting the tee after the shutoff valve also lets you close the valve and work on the tank without draining the house.

Hanging It Vertically or Supporting It Sideways

Expansion tanks range from a small canister to a larger cylinder. Weight is the concern, because a full tank hanging off a pipe fitting puts stress on the joint.

In general terms, a small tank can hang from the pipe in the upright position with the connection at the top, while a heavier tank needs a bracket or strap that carries part of the load. A horizontal tank almost always needs its own support. Read the installation sheet for your tank, because the rules vary by model.

The air valve on the tank also needs to stay reachable. If the valve is tucked behind the heater or against a wall, the plumber cannot check the charge during later service.

Setting the Air Charge Correctly

A new tank usually ships with a factory air charge. That charge must match the pressure in your supply line. A tank set too low fills with water and goes soft. A tank set too high will not take any expansion at all. Both leave you with the original problem.

1. Close the cold shutoff and relieve pressure at a nearby faucet.
2. Check the tank's air charge at its valve with a tire gauge.
3. Compare it to the house pressure measured at a hose bibb.
4. Add or release air so the two numbers match.
5. Open the shutoff and check for leaks at the new fittings.

A tank that feels heavy, or that squirts water when you press the air valve, has failed. The diaphragm has split. Replace the tank rather than trying to refill it. A tank that only lost air, with no water at the valve, can often be recharged to match the line pressure.

The Relief Valve and Its Discharge Pipe

The temperature and pressure relief valve is the heater's last line of defense. If pressure or temperature exceeds a safe limit, it opens and sends water down a discharge pipe. EPA WaterSense says to check that valve at least once a year, and DOE guidance suggests every six months, so you know it moves.

The Oregon Building Codes Division treats thermal expansion protection and relief valve termination as code items when a water heater is replaced. That means the discharge pipe has to end in a safe place, and the system has to deal with expansion. Do not cap the discharge pipe or plug the valve to stop a drip.

A relief valve that drips on every heating cycle is often reporting thermal expansion, and a new or replaced tank can stop it. One that drips constantly may be failing. If yours is dripping now, call (503) 974-3329.

Cost and Permit Facts

Angi reports an average of $325 ($150 to $450) to install an expansion tank. If the tank goes in with a new heater, the cost is folded into the installation. If your pressure is too high, a pressure reducing valve may be the real issue, and Angi puts a PRV replacement at an average of $400 ($200 to $700).

A water heater replacement needs a permit in Forest Grove, and thermal expansion protection is one of the code items that apply when a heater is replaced. Permits come from Community Development at 503-992-3229. Oregon also expects water heaters to be anchored or strapped. See our water heater replacement and water heater repair pages for what a job covers.