Expansion Vessel: How to Tell the Diaphragm Has Failed (2026)
Updated: 2026-10-01 · ~2 min read
What it does
The expansion vessel (membrane/diaphragm tank) absorbs the water volume change as the loop heats up (~4% over a 60–80 K temperature rise). If it works, pressure stays in band; if the diaphragm fails, water gets on the gas side, the cushion is gone, and the system behaves strangely — usually with pressure drift.
The failure signatures
| Signature | What's happening |
|---|---|
| Pressure falls after cold periods / weekends, recovers oddly when the system runs hot | diaphragm leak: water enters the gas side, air gets pushed out or dissolves — cushion gone |
| Vessel "sloshes" when tapped (sounds wet instead of hollow) | water on the gas side = failed diaphragm; classic confirmation with a hammer tap |
| Water comes out of the vessel's water-side fitting during pressure relief / service | direct proof — replace it |
| Rapid, repeated need for top-ups even after proper bleeding | failed vessel (or a leak) — check both; if no leak is found, vessel first |
Quick verification routine (before ordering anything)
- Cold baseline: note pressure with the pump off and the system cool.
- Tap test: tap the vessel body — a healthy vessel sounds hollow on top; if it sounds full/wet, water has crossed the diaphragm.
- Preload check (if accessible): with the system isolated and bled down, the gas-side pressure should read near preload (often ~1–1.5 bar factory value — see the vessel label). If it reads zero with the water side drained → water got in.
- Decision: any of the above pointing to wetness → replace the vessel. It's a cheap part (tens of euros) and a service visit; don't try to "fix" a diaphragm by re-pressurizing — it holds for a while, then fails again.
Buying the right replacement
- Match volume (original liters), connection size (1/2", 3/4", 1"), and max working pressure of your system (check the loop's relief valve setting).
- Preload must match the static height of your system — ask the installer to set it correctly, not just "add air".
- If your system uses an open-type or different vessel design (some older installs), replace with the same type.
What NOT to confuse it with
- A real leak: if pressure keeps falling even with a healthy-sounding vessel, look for leaks first (see pressure drop guide) — replacing the vessel won't stop a leak.
- Air in the loop: wobble after maintenance is usually air, not a vessel failure — bleed the system and re-test before ordering parts.
Affiliate slot: expansion vessels — to be wired via a marketplace after publisher registration.
Sources
- Expansion vessel failure modes and verification (tap test, preload check); heating-system engineering references; as of October 1, 2026.
- Vessel sizing/preload practice per manufacturer installation specs; as of October 1, 2026.