Thermal Expansion Tanks: Why Your TPR Is Dripping (and How to Stop It Safely)
Walk past your water heater and notice a little puddle at the end of that copper pipe? That pipe is connected to the TPR valve (Temperature & Pressure Relief). When it drips, your first thought might be, “Is my water heater dying?” Sometimes—yes. But in many Idaho homes, the real culprit is thermal expansion in a closed plumbing system, and the quiet hero that fixes it is a properly sized and properly charged thermal expansion tank.
This guide explains what thermal expansion is, why closed systems make it worse, how to test your tank in 5 minutes, and the simple, safe steps to stop the drip—for good. When you’re ready, A+ Drain Cleaning & Plumbing can test, tune, or replace your expansion tank and set your water heater up for a long, quiet life.
First, what’s thermal expansion—and why is my TPR involved?
Water expands when heated. In an open system, that tiny expansion can push back into the city main without spiking your home pressure. But most modern Idaho homes have a PRV (pressure-reducing valve) on the main line. A PRV creates a closed system, which means heated water has nowhere to expand.
Enter the TPR valve on your water heater. It’s a safety device designed to open if either temperature or pressure climbs too high. If your system pressure rises every time the tank heats, the TPR does its job and drips—little bits at first, then more often. It’s not misbehaving; it’s protecting you. But constant TPR discharge wastes water, stresses the valve, and is usually a sign that your expansion control needs attention.
The expansion tank: small part, big job
A thermal expansion tank looks like a mini white or gray cylinder mounted near the water heater on the cold-water line. Inside is a rubber bladder. One side holds air; the other side accepts expanding water. When the tank is properly charged to match your home’s water pressure, it absorbs expansion instead of letting your system pressure spike. Result: quiet heat cycles, no TPR drip, and fewer plumbing headaches.
Clear signs you have a thermal expansion problem
Intermittent TPR drips—often after showers or dishwasher cycles
Short pressure surges (hard spray, brief hammer) when a fixture closes
Toilet fill valves “chatter” or don’t fully shut off
Frequent water-heater cycling and occasional “rumble/pops” if sediment is also present
PRV + no (or failing) expansion tank in the same home
Two or more? It’s time to look at expansion—not just the heater.
Quick 5-minute expansion tank test (DIY)
You’ll need a tire gauge and, ideally, a small bicycle pump:
Turn off water to the water heater (cold-water shutoff on the inlet line).
Open a hot tap nearby to relieve system pressure.
Find the air valve (Schrader valve) on the expansion tank—usually a cap on the bottom.
Check air pressure with the tire gauge.
Compare to your home’s water pressure (if you have a hose-bibb gauge). A properly set tank should be charged to the same pressure your house runs (the mid-range most homes prefer).
If air pressure is zero or water comes out of the Schrader valve, the bladder is ruptured—time for a new tank.
Re-pressurize the tank with the pump to the correct value, close the hot tap, reopen cold supply, and watch the TPR line over the next few cycles.
Don’t have a gauge or not sure on house pressure? We’ll test static and working pressure for you, match the tank charge, and leave the numbers in writing.
Sizing and placement: the two mistakes we fix most
Undersized tank
Family of five, big tub, dishwasher runs nightly… and the smallest expansion tank in the aisle. If the bladder is too small for your heater’s volume and your real-world demand, it will saturate and lose effectiveness quickly. We size by heater capacity, temperature rise, and pressure—not by guesswork.Installed on the wrong side or unsupported
Expansion tanks belong on the cold-water inlet to the heater, typically after the PRV in a closed system. The tank needs solid support; a water-filled cylinder hanging off thin pipe will strain fittings. We add bracketing and proper orientation so it lasts.
“My TPR is still dripping—what else could it be?”
Even with a perfect expansion tank, TPRs can drip for other reasons:
High street pressure + tired PRV: If the PRV is drifting, pressure spikes can still hit the heater. We verify and reset/replace the PRV.
Sediment in the TPR seat: Minerals can prevent a clean seal. We’ll evaluate and replace the valve if necessary (and flush the tank).
Overheating or faulty controls: A stuck thermostat or control issue can overheat. We test temperatures and safeties as part of the visit.
No drain air gap / improper discharge routing: Sometimes what looks like “drip” is condensation or backflow issues. We make sure the discharge is safe, visible, and to code.
The A+ diagnostic flow (so we fix it the first time)
Pressure check – Static and dynamic readings at a hose bib and near the heater.
TPR evaluation – Inspect the valve, discharge piping, and test operation safely.
Expansion tank test – Verify charge, bladder integrity, and size vs. system.
PRV health – Check for drift, swings, and proper setpoint.
Water-heater service check – Flush if due, inspect anode and controls, verify gas/electric operation.
Proof cycle – Heat the tank, watch pressure behavior, and confirm no TPR discharge.
Leave numbers + plan – Final setpoint, tank charge, and any recommendations for maintenance.
Most homes are back to “no drip” during the same appointment.
Safety notes you shouldn’t skip
Never cap or plug a TPR line. That valve is a safety device; capping it creates a dangerous condition.
Discharge must be visible and correctly routed. You should be able to see if it opens. The pipe should terminate in an approved location (no upward loops) and be the correct diameter.
Beware of “open flame fixes.” Do not use torches or unsafe heat sources to “dry” anything around the heater. If water is present near electrical components, cut power and call us.
If you have water actively discharging or signs of overheating, treat it as urgent.
Add internal link when you publish: Emergency Plumbing.
How thermal expansion ruins more than your patience
Left alone, pressure spikes from expansion can:
Shorten fixture life (fill valves, faucet cartridges)
Stress braided hoses on washers and toilets
Trip leak sensors and damage flooring around the heater
Mask hidden leaks elsewhere by raising baseline pressure
A $100–$300 problem today can snowball into a multi-room repair later. Expansion control is cheap insurance.
Annual “no-drip” maintenance you can actually stick to
Test expansion tank charge at the start of heating season (we can add it to your yearly service).
Flush the tank annually to reduce sediment and keep recovery time normal.
Verify PRV setpoint and tweak as needed after fixture changes or a new heater.
Check leak sensors and swap batteries—put one under the heater and one by the washer.
Glance at the TPR line monthly—no wet spots, no mystery drips.
We’re happy to bundle all of this into a quick Water Heater Service visit so you don’t have to juggle gauges and pumps.
FAQs
Do all homes with a PRV need an expansion tank?
In almost every case, yes. A PRV makes your system “closed,” so expansion has nowhere to go without a tank.
How long does an expansion tank last?
Depends on water quality and pressure. We see healthy ranges from a few years to a decade. Yearly checks catch low charge before the TPR starts dripping again.
Can I set the tank to any air pressure?
No. The tank air charge should match your home’s water pressure. Too low and it saturates; too high and it won’t accept water.
Is a dripping TPR always OK to ignore?
No. It’s a safety device signaling a condition. We need to find out why it’s opening (expansion, high pressure, overheating, debris) and fix the cause.
What if my expansion tank is water-logged?
Replace it. If water comes out of the Schrader valve, the bladder has failed.
A quick plan if your TPR is dripping right now
Look at the discharge line—is it warm or actively dripping after heat cycles?
Turn the heater to “Vacation/Low” to reduce immediate expansion.
Do not cap the line.
Call A+ Drain Cleaning & Plumbing. Tell us if you have a PRV and whether you see an expansion tank. We’ll arrive with gauges, a replacement tank (if needed), and parts to test/replace the TPR safely.
Ready to stop the drip—safely?
Whether you need a quick tank recharge, a correctly sized replacement, or a PRV tune, we can get your system balanced in one visit—and leave you with the actual pressure numbers. If water is pooling or you see steam, hit Emergency Plumbing; otherwise, book a Water Heaters Service appointment and we’ll button it up before the next bill cycle.