Water Heater Expansion Tank: Do You Need One?

What an expansion tank does

Water expands when it heats. Watts' guide gives the example of a 40-gallon water heater making about a half gallon of extra water as it heats. In an open system, that extra water just pushes back into the city main. In a closed system, it can't, so the pressure in the pipes climbs until something gives. Usually that's the T&P relief valve on the heater, which starts dripping.

An expansion tank has a rubber diaphragm or bladder with pressurized air on one side and house water on the other. As the water heats and expands, the extra volume squeezes into the tank and compresses the air. Watts describes its tanks as absorbing that excess water so the relief valve doesn't trigger.

Do you really need one?

You need one when the system is closed. The City of Boise's install guide puts it plainly: an expansion tank must be installed if a check valve is on the cold-water side or a pressure reducing valve is present on the supply. Many water utilities install check valves or backflow preventers at the meter, which closes the system even if you didn't add anything.

Signs your system is closed and needs a tank:

• The T&P valve drips after the heater runs, then stops. Watts says relief caused by thermal expansion typically releases about one cup of water for each 10 gallons of tank capacity.
• Water pressure jumps after a long run of the heater, then drops when someone opens a tap.
• Toilets refill or hiss on their own.
• A plumber or utility tells you there's a check valve or PRV at the meter.

Check the house water pressure too. A.O. Smith says most codes allow a maximum incoming pressure of 80 psi and recommends staying around 50 to 60 psi. High static pressure is a separate problem that an expansion tank won't fix; that's a pressure reducing valve job.

Code requirements vary by town. Check with your local building department, especially when you replace a water heater, since that's when inspectors usually look for one.

Sizing an expansion tank

A.O. Smith says the tank must be sized according to the water heater's tank capacity and pressurized to match the home's incoming water pressure. Tank makers publish sizing charts based on those two numbers: heater gallons and supply pressure. The higher your pressure, the larger the tank needs to be for the same heater.

For a typical residential tank heater, the answer is usually one of the small potable-water tanks, but use the chart on the tank you're buying. Buy one labeled for potable water, not a boiler (hydronic) expansion tank. They look alike, but the boiler version isn't built for drinking water.

How to install a hot water expansion tank

Tools and parts

• A potable-water thermal expansion tank, sized from the maker's chart.
• A tee fitting for the cold supply line, plus fittings to match your pipe.
• A tire pressure gauge and a bicycle pump or small compressor.
• A water pressure gauge that threads onto a hose bib.
• A strap or bracket to support the tank.
• Thread sealant rated for potable water.

Steps

Once a year, it's worth checking that the relief valve isn't dripping and the tank isn't waterlogged.

How to tell if your expansion tank has failed

Tap the tank. A good tank sounds hollow on the air side and solid on the water side. If it sounds solid all the way up, the air charge is gone or the diaphragm has failed. Another test: press the air valve stem briefly. Air should come out. Water means the bladder has ruptured and the tank needs replacing.

A failed tank puts you back where you started, with a dripping relief valve. See water heater T&P relief valve for that side of the problem.

Does a tankless water heater need an expansion tank?

The same closed-system logic applies, but tankless heaters hold very little water, so the amount of expansion is much smaller. Whether you need one depends on the heater maker's instructions and your local code. Read the tankless manual and ask the building department rather than guessing.

When it's a pro job

If you have copper or steel pipe and don't solder or thread pipe, this is a short job for a plumber. It also makes sense to add the tank when you replace the water heater, since the plumber has the water off anyway. HomeGuide puts expansion tank replacement at $300 to $800 for materials and labor.