Hydronics & Pressure Control | Module 309
Expansion Tank Precharge Audit
Field Status: Closed-loop hydronic systems require an expansion tank to manage the volume fluctuations caused by water thermal expansion. If the internal diaphragm bladder ruptures or the air precharge is lost, the system loses its ability to absorb volume changes. This leads to massive pressure spikes as the water heats up, forcing the Pressure Relief Valve (PRV) to lift and dump water from the system.
For accurate testing, the expansion tank must be isolated from the system pressure to get a true reading of the air-side precharge pressure. If water comes out of the air valve, the diaphragm has failed.
SYS AUDIT: Pressure Margin Vector
PRESSURE DEVIATION: —
TANK INTEGRITY: —
—
Expansion Dynamics
The expansion tank must be precharged to the exact cold fill pressure of the system. If it is low, the tank fills with water immediately upon filling the system, leaving no room for the water to expand when it heats up.
| Condition | Result | Correction |
|---|---|---|
| Low Precharge | PRV Relief Lift | Drain tank, add air to match cold fill. |
| Bladder Rupture | Waterlogged Tank | Replace entire expansion tank. |
| Correct Precharge | Stable Pressure | Operational performance. |
Diagnostic Gateway Challenge
You are at a building with 12 PSI cold fill pressure. You isolate the expansion tank and test the schrader valve; it reads 12 PSI. However, the system pressure relief valve still leaks every afternoon when the boiler fires. What is the diagnosis?