Plumbing tips · November 13, 2026
The small tank above the water heater: is it still doing its job?
An expansion tank gives hot water somewhere to expand. It has no moving parts you can see and no light that comes on, so when it fails nobody notices until the relief valve starts dripping.
What is inside it
The tank is split in two by a rubber bladder. The top half holds water from the house; the bottom half holds a cushion of air. As the water heater warms the water, the extra volume pushes into the tank and squeezes the air instead of straining the pipes.
How it fails
Over time the air cushion leaks away, or the bladder tears and the whole tank fills with water. Either way it stops absorbing anything, and the pressure from heating goes back into the plumbing.
The tap test
Tap the tank with your knuckle, top then bottom. A working tank sounds hollow at the bottom, where the air is. A tank that sounds solid and dull all the way down is full of water. A tank that feels very heavy for its size is waterlogged too.
The valve on the bottom
Most tanks have a small air valve like a tyre valve. Pressing the pin should let out air. If water comes out instead, the bladder has failed and the tank needs replacing; it cannot be repaired.
Questions we get asked
What are the signs it has failed?
A water heater relief valve that drips after the heater has run, pressure that rises when hot water is being heated, and faucets or toilet fill valves wearing out sooner than they should.Can I just add air to it?
If the bladder is good and the air has simply leaked down, it can be recharged to match the house water pressure, with the water off and the tank drained of pressure. If water comes out of the air valve, it needs replacing.Does every water heater need one?
Where the house has a pressure reducing valve or a backflow device on the main line, heated water cannot push back to the street, so an expansion tank is needed. Most homes with those valves have one.Plumbing questions we answer
Where we go
Parker, Wise and Tarrant County, out of the shop in Springtown. Springtown, Weatherford, Azle, Boyd, Decatur, Newark, Millsap, Fort Worth and the country in between.
Call 817-487-8866