A small tank with a big job
Water takes up more room when it's hot. Not much more: water warmed from about 50 to 120 degrees grows by a little over one percent, going by standard water density tables. On a 50 gallon tank that adds up to about half a gallon of extra water, and more at a higher setting, that has to go somewhere every time the heater fires.
In an old-fashioned plumbing system it went back out into the city main, pushed against the supply pressure. Nobody noticed. Plumbing that has a way for water to flow only one direction doesn't have that escape route anymore, and then the extra gallon has nowhere to go. The expansion tank is how it gets one.
What a closed system is
Your house is a closed system if something between the street and the heater stops water flowing backward. The usual culprits are a pressure-reducing valve near the main shut-off, a check valve, or a backflow preventer. Plenty of homes have one, and it's common in newer construction, where a pressure-reducing valve keeps the street pressure from overworking the fixtures.
A quick way to tell is to look near where the water line enters the house, often by the main shut-off or the meter. A bell-shaped brass valve, usually with an adjusting screw on top, is a pressure-reducing valve. If you find one, you almost certainly have a closed system. A plumber can confirm it in a minute.
What happens without one
When the water in a closed system heats, it has nowhere to expand, so the pressure rises. It rises until something gives. The first thing to give is usually the temperature and pressure relief valve on the heater, which is built to open at 150 psi and let water out. That's the valve doing its job, but a valve that opens every day isn't the plan. It will start to drip, and it may stop sealing properly.
Before it reaches that point, you may get spikes in pressure that strain the fittings and fixtures, the supply connectors on the faucets, the fill valves in the toilets, the water heater itself. None of it fails the first day. It's the sort of damage that shortens the life of everything attached to the system and shows up years later as a leak somewhere you didn't expect.
How the tank works
An expansion tank looks like a small, fat canister, usually blue or gray, a couple of gallons in size, hung off the cold line just above the heater. Inside there's a rubber diaphragm. One side is full of air, the other connects to the plumbing. When the heated water expands, it pushes into the tank and squeezes the air. When the heater is idle and the water cools, the air pushes it back out. It's a spring made of air.
The air side is charged at the factory, and the charge has to match the water pressure in the house. If the tank is charged lower than the house, it fills up with water and does nothing. If it's charged higher, the water never gets in. The right way to install one is to measure the house pressure with a gauge and set the tank to match before it goes on the line.
How a plumber sets one up
The job is short but the order matters. The water is shut off and a little is drained from the line so the old tank, if there is one, can come off. House pressure is read with a gauge on a hose bib or the heater's drain, and that number decides the air charge.
With the tank off the line and no water in it, the air side is checked with a tire gauge at its valve, like the valve on a bicycle tire. If it reads too low or too high, air is added or released until it matches the house pressure. Only then is it fitted to the cold line above the heater, on a tee, with a support so its weight doesn't hang from the pipe.
Last, the water is turned back on, the joints are checked for drips, and the heater is run through a heating cycle to see that the pressure holds steady and the relief valve stays dry. That's the whole job, and it takes under an hour on a straightforward install.
Signs you need one, or need a new one
- The relief valve drips after the heater has been running.
- The pressure at a hose bib rises noticeably after the heater fires.
- A hammering or knocking sound in the pipes after a tap closes.
- The water heater was replaced and the new installation has no expansion tank, but the house has a pressure-reducing valve.
- You tap the existing expansion tank and it sounds dull and heavy instead of hollow. That's a tank full of water and a failed diaphragm.
Any of those is worth a look. A drip from the relief valve is the one people tend to ignore, and it's often the first clue.
What pressure spikes do to everything else
A water heater isn't the only thing in the house that suffers when pressure climbs. Every faucet, toilet fill valve, washing machine hose and dishwasher inlet is built for a working pressure, and repeated spikes shorten their lives. A faucet that starts dripping, a toilet that runs by itself, or a washing machine hose that bulges can all be signs.
The pressure-reducing valve at the main is there to protect all of those. When it's installed, it closes the system and the water heater needs an expansion tank to complete the picture. Think of them as a pair: one keeps the pressure down, the other keeps the heater from pushing it back up.
How long they last and what a replacement involves
An expansion tank typically lasts about five to ten years. The diaphragm tires, or the air charge leaks away, and the tank stops working without telling anyone. Replacing it is a short job: shut the water off, drain a little line, unscrew the old tank, measure the house pressure, set the new tank, tighten it up and test. It's one of the cheaper ways of protecting the plumbing.
Because it hangs off the pipe, the tank needs support. A heavy expansion tank dangling from a copper line with no bracket is a leak waiting for a bump. Look for a strap or a bracket when you check.
A note on doing it yourself
An expansion tank is within the skill of a confident homeowner on their own homestead, and some people do fit one. The two mistakes to avoid are skipping the air-charge check, which is the whole point of the device, and hanging it unsupported from copper. If you aren't sure how to isolate the water, drain the line and test the joints, it's an easy job to get wrong with water everywhere. A plumber doing it as part of a heater swap adds very little to the visit.
Why it's part of nearly every replacement
When a plumber fits a new heater into a closed system, adding an expansion tank is part of doing the job to current code, and a good quote lists it as a line. Older installs often lack one because the rule was less strictly followed when the house was built. It's the first thing we look for when we see a relief valve with a damp mark underneath.
If your heater is being replaced anyway, it's the easiest time to add one. If it isn't, a tank can be added on its own at any time, and the expansion tank installation page covers that job. The replacement page shows where it fits in the full swap.
How big does it need to be
For a typical 40 to 50 gallon heater on ordinary house pressure, a tank of around two gallons is the usual choice. Larger heaters, from about 65 gallons up, or systems with high pressure, call for a bigger one, often around four to five gallons. A larger tank holds more of the extra water for the same pressure rise.
Maker's tables set the exact size from the heater's capacity, the house pressure and the water temperature. A plumber reads the table and picks the right one. Undersizing is the usual error, because a too-small tank fills up and gives only part of the protection.
It's worth knowing that the sizing is based on the pressure at the house, so a house with high pressure may need both a bigger tank and a pressure-reducing valve to bring the pressure down. The two are a pair.
A note on pressure
House water pressure is commonly somewhere between 50 and 80 psi. Higher pressure puts more strain on everything, and if your readings at a hose bib are high, a pressure-reducing valve and a properly set expansion tank are the right answer together. That isn't something to guess at. A gauge costs little and tells you straight away. For a closer look at what else gives a heater trouble, see the post on signs of a failing water heater.