Check your expansion tank
Annual · typically April
Why it matters
Water expands when it heats. In a closed system — one with a pressure regulator or check valve between you and the street — that expansion has nowhere to go, so codes put a small tank with an air bladder near the water heater to absorb it. When that bladder fails, the tank fills solid with water ("waterlogged") and every heating cycle turns into a pressure spike.
The classic symptom is the temperature-and-pressure relief valve on the water heater weeping a little puddle — which is your TPR valve doing its job (that's the same valve your annual TPR test card exercises) while the real culprit hangs dead on the pipe above. Plenty of water heaters get condemned by techs for a symptom the $60 tank caused.
What skipping it costs
$300–$2,500
A waterlogged expansion tank leaves thermal expansion nowhere to go, so pressure spikes every heating cycle. The relief valve weeps, fittings strain, and the water heater tank itself flexes until something gives — a $50 part failure that retires an $1,800 water heater years early.
Two ways to get it done
Do it yourself
You'll need: Tire pressure gauge, Your knuckles.
- Find it: a small propane-tank-looking cylinder (usually 2–5 gallons) teed into the cold line near the water heater. No tank and no regulator/check valve? You may genuinely not need one — note it and move on.
- Knuckle test: rap the top and the bottom. Top should ring hollow (air), bottom should thud (water). Thud all the way up = waterlogged = replace.
- Schrader test: there's a valve stem under a cap on the air end. Press it briefly — a hiss of air is healthy; water spitting out means the bladder is torn. Replace.
- If it passes both, a tire gauge on that stem (with house pressure relieved) should roughly match your home's water pressure — the number from your water-pressure card. Big mismatch is worth a pro visit.
Hire it out
$150–$300National fair range — local labor moves this number.
A replacement runs $150–$300 installed — a $40–$80 part, a drain-down, and one threaded fitting, but it's live plumbing at the water heater and a torn-bladder tank can be deceptively heavy with water. Ask for the precharge to be set to your actual house pressure before it goes in; a tank charged wrong on day one fails young.
The honest nuance cuts both ways: on a closed system, an expansion tank isn't an upsell — it's physics, and it protects the water heater you already paid for. On an open system it does nothing. The difference is one check valve, and a straight plumber will show it to you.
Ask the tech
- What's my incoming pressure, and what did you precharge the new tank to? Those two numbers should match.
- Can you show me the check valve or regulator that makes this a closed system?
- Is the tank sized right for my water heater, or just the one on the truck?
Red flags you're being upsold
- An expansion tank pitched on an open system — no check valve, no pressure regulator, no backflow device means the street absorbs your expansion and the tank is decoration.
- Quotes over $450 for an accessible swap. The tank is a $40–$80 part threaded onto one fitting.
- A water heater replacement pitch when the symptom is a weeping relief valve — that's routinely the expansion tank (or pressure), a tenth of the price.
Second opinion time: Any diagnosis that jumps from 'dripping relief valve' to 'new water heater' without a pressure reading in between.
The call
DIY the check — it's a knuckle rap and a tire gauge. Hire the swap; it's one fitting but it's on a live water line.