That small tank hanging above or beside your water heater is easy to ignore — until your TPR valve starts dripping and nobody can explain why. The expansion tank is usually the explanation. Here is what it does, in plain English.
What it does, in plain English
Water expands when heated. In a closed plumbing system — which you have if there is a pressure regulator or check valve on your supply — that extra volume has nowhere to go, so pressure spikes with every heating cycle. The expansion tank absorbs it: incoming water pushes against a rubber bladder, and the air cushion behind the bladder compresses to make room. No drama, no spikes.
How they fail
Two ways. The rubber bladder ruptures, letting water flood the air side — the tank goes ‘waterlogged’ and stops absorbing anything. Or the air charge slowly leaks out over the years, leaving the bladder with no cushion to push against. Either way the result is the same: pressure spikes return, the TPR valve starts weeping to relieve them, and every joint in the house takes a beating.
The TPR connection
This is the diagnostic shortcut: a TPR valve that drips after every heating cycle, with system pressure testing high, usually means a dead expansion tank — not a bad TPR valve. Replacing the valve without checking the tank just moves the drip to a new valve. Always check both.
How to spot a failed one
Tap it with your knuckle. A healthy tank rings hollow at the top (air) and dull at the bottom (water). A failed, waterlogged tank thuds solid all over and feels heavy. That is a thirty-second test anyone can do. Replacement is quick — but the detail that matters is the pre-charge: the new tank’s air pressure must match your home’s static pressure (normal is 40–60 psi), set while the system is depressurized.
Worth knowing
Expansion tanks do not last forever, and a waterlogged one can also leak from its own fittings. If your TPR keeps dripping and the tank thuds solid, call (833) 713-2101 — this is one of the quickest, most satisfying fixes we do, because the mystery drip just stops.