Expansion tank

The expansion vessel (or expansion tank) absorbs the expansion of the fluid as its temperature rises in order to stabilize the pressure in a heating, chilled water, or domestic hot water system. Without it, the pressure would rise rapidly: blown valves, cavitation of cold-start pumps, air leaks due to negative pressure, or damage to components. The vessel creates a compressible buffer (a nitrogen/air cushion behind a membrane) that absorbs the expanded volume and maintains the pressure between Pmin and Pmax.

Expansion vessel: principle, types and key settings

Two families:

1. Closed membrane systems (the standard in HVAC): steel tank, membrane (butyl/EPDM) separating the fluid from the pre-charged gas (P0). P0 is set to ≈ Hstat + margin (Hstat = water height in bar, ~0.1 bar per meter), then the usable volume is sized according to the expansion (heating >> cooling) and the desired pressure range.
   
   
2. Open (old): tray in the upper part with vent; now not recommended in tertiary (corrosion, air inlets).

Practical guidelines (closed networks):

- Position : connect to the hydraulic neutral point (often return on the manifold where the ΔP reference is taken), suction side of the main pump to limit the variation of NPSH.

- Pressures :

• P0 (nitrogen pre-inflation) ≈ Hstat + 0.3 bar (min);
• Pmin in service ≥ P0 + 0.2–0.3 bar;
• Pmax < valve setting (e.g. 3 bar heating).

- Sizing : take into account the coefficient of expansion (water vs glycol), the maximum temperature, the total volume of water and the acceptance efficiency of the vessel.

- DHW : use a sanitary vessel (qualified for drinking water) after the safety group to absorb expansion during heating.

Advantages, limitations and points to consider regarding an expansion tank

Interests

• Pressure safety: protects heat exchangers, pumps, piping; prevents premature valve blow-out.
• Stability: smoothed pressure, facilitated degassing, cavitation avoided in the pump.
• Operation: simple parameterization, quick control (P0 at idle, network pressure, valve).

Boundaries

• Undersized = large pressure variations → tripping, noise, wear.
• Incorrect pre-inflation (too low/high) = lost usable volume, "dead" reservoir.
• Aging membrane (permeation, cracks) → loss of nitrogen, water on the gas side.

Points to consider

• Calibration: check Hstat (m water height), define P0, then Pmin/Pmax consistent with valves and equipment.
• Water quality: descaling, inhibitor; in DHW, stainless steel/coated vessel conforming to drinking water standards.
• Glycol: increased expansion and different compressibility → larger vessel.
• Instrumentation: readable pressure gauge, low pressure contact, isolation valve + drain at the vessel connection.
• Maintenance: annual P0 check (isolated vessel and depressurized water side), valve test, membrane inspection; replace vessels "full of water".
• Installation: connect without unnecessary pressure losses (direct tapping on manifold), insulate in heating, anti-vibration if necessary.
• Domestic hot water safety: the safety group (often 7 bar) remains mandatory; the sanitary vessel limits its discharges but does not replace it.

Anecdote — “Pressure has calmed down in Tours”

In Tours, an office building was experiencing a 3-bar pressure relief valve that would blow out with every restart. The expansion vessel was undersized and pre-pressurized to 0.5 bar for a static head of 10 m (it should have been around 1.3 bar). The system was replaced with a larger vessel, the P0 pressure setting was adjusted, a connection was made to the neutral point on the return side, and the domestic hot water safety group was checked, with the addition of a sanitary expansion vessel. The result: stable pressure across the entire range, zero blow-out, and the pumps free of morning cavitation. The building manager commented: "A little more nitrogen and volume... and far fewer problems."