Expansion Tank Troubleshooting

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What an Undersized or Waterlogged Tank Actually Does

On a closed system the heater is a sealed bottle. Heat it and pressure climbs. A working expansion tank takes that climb and parks it in compressed air. When the tank cannot, three things show up in the house.

T&P valve dripping on recovery

Not a random drip at 2 a.m. A dribble that starts when the burner or element kicks in, fills a cup in the pan, and stops when the heater is satisfied. That is expansion finding the 150 psi relief because the tank is missing, waterlogged, or too small. A T&P that weeps continuously, even with a cold heater, is more often a tired valve or a thermostat that is overshooting. Replace the valve if it weeps at rest; do not stop there if it only weeps on heat-up. Heater-side diagnosis that is not expansion: water heater troubleshooting.

Pressure gauge spikes

A house gauge that sits at the PRV setpoint (say 55 psi) with nothing running, then climbs through 80, 100, 120 while the heater recovers, then slowly bleeds back when someone opens a faucet, is the textbook closed-system plot. If the gauge never leaves 55 even during a long recovery, either the system is still open to the main or the tank is doing its job. If it slams into the T&P every cycle, the tank is not.

Banging pipes

Expansion pressure can slam checks and make copper tick. So can water hammer from a fast-closing washer valve. Hammer is a millisecond spike when a fixture slams shut. Expansion banging is slower, tied to the heater cycle, and often paired with the T&P drip. Arresters at washing machines do not fix a missing expansion tank. An expansion tank does not fix a solenoid that hammers 3/4 in copper.

How to Check Precharge with a Tire Gauge

The air side of a potable expansion tank is a Schrader valve, same family as a car tire. The number on that gauge, with the tank isolated and empty of water, should match house static fill — the same fill the calculator used (min of incoming and PRV).

1. Turn off the heater. Close the cold-inlet valve to the heater / tank tee so the expansion tank is isolated. Open a nearby hot faucet so the tank is not trapped at high pressure.
2. Drain the expansion tank at the drain/boiler drain or by cracking the tee until it stops pushing water. You are checking air against an empty wet side, not against a tank that is still full of house pressure.
3. Put a tire pressure gauge on the Schrader. Read it. 50 psig house should read about 50. 20 psig on a 50 psi house means the tank was already flooded with water on day one.
4. If the reading is low, add air with a bicycle pump or compressor to the fill pressure. If you cannot build pressure, or if water comes out of the Schrader, the bladder is gone.
5. Reopen valves, restore the heater, watch the house gauge through one recovery. It should rise a little and stop well short of 150.

Check precharge once a year, or any time the T&P starts a new drip. Tanks in a hot closet die faster. A tank you cannot reach with a gauge is a tank you will replace by symptom, which is how people cap T&P valves.

Failed Tank vs Sized Wrong

Those are different repairs. Mixing them is how you hang a second 2.0 gal tank on a system that needed 4.4 gal, or replace a good tank because the PRV is set to 80.

Failed / waterlogged. Precharge is 0, or water exits the Schrader. The tank feels heavy (full of water) when you lift it. House pressure still climbs on recovery after you “added air” because there is no air chamber left. Replace. Do not keep pumping air into a wet Schrader.

Wrong precharge, bladder still good. Gauge reads 20 on a 55 psi house, no water from the valve, tank is light. Add air to 55 with the tank drained. One recovery later the T&P should stop if size was adequate.

Sized wrong, tank healthy. Precharge matches fill, Schrader is dry, tank is light, and the house gauge still walks into the T&P on a long recovery. The air cushion is too small for this heater, this rise, and this fill. Run the calculator. An 80 gal at 80 psig fill is not an ST-5 job. Buy the next class; do not add a second undersized tank in parallel unless the manufacturer says that configuration is listed.

A T&P that weeps because the thermostat is running 180°F is not a tank problem. Turn the setpoint down, confirm a mixing valve if you store hot, and stop blaming expansion for a scald setting.

Banging: Expansion, Hammer, or a Loose Nipple

Heater recoveries that make the cold line tick in time with the burner are often the tank bladder moving, or copper growing. That can be normal at a low volume. Bangs that throw the T&P are not.

Washer and dishwasher solenoids cause classic water hammer: one slam at shutoff, arresters at the valve, not at the heater. If the only bang in the house is during heater recovery and the gauge is climbing, size and service the expansion tank before you nail arresters to every tee.

A loose dielectric or a tank that is hung by the tee with no support will bang when the bladder moves. Strap the tank. The calculator does not hang it for you.

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