Glossary
| Term | Meaning |
|---|---|
| Acceptance factor | Fraction of an expansion vessel's volume that can take water; 1 − pᵢ/pf in absolute pressure |
| Air vent (automatic) | Valve at a high point that releases trapped air from the water circuit |
| Anti-vibration (AV) mount | Rubber or spring support that stops vibration passing to the structure |
| Brazing | Joining metals above 450 °C with a filler rod that melts below the base metal |
| COP | Coefficient of performance: heat delivered ÷ electricity used |
| dB(A) | A-weighted sound level, close to how the human ear hears loudness |
| Decay test | Watching vacuum after isolating the pump to check for leaks or moisture |
| Evacuation | Removing air and moisture from a refrigerant circuit with a vacuum pump |
| Expansion vessel | Diaphragm tank that absorbs water expansion in a closed circuit |
| Flexible connector | Braided or rubber pipe section that absorbs vibration and movement |
| Isolation valve | Valve (often a ball valve) used to shut off a section for service |
| MCB | Miniature circuit breaker: protects a circuit from overload and short circuit |
| Micron gauge | Instrument that measures deep vacuum in microns of mercury |
| Non-return valve (NRV) | Valve that allows flow in one direction only |
| OFN | Oxygen-free (dry) nitrogen used for purging and pressure testing |
| P&ID | Piping and instrumentation diagram: schematic of pipes, valves and instruments with tags |
| Phase sequence | Order of the three phases (R-Y-B); sets the rotation direction of three-phase motors |
| Pre-charge | Air or nitrogen pressure set in an expansion vessel before filling |
| PT | Pressure transmitter or pressure test point |
| RCD / RCCB | Residual current device: trips when current leaks to earth |
| Recirculation | Discharge air from the unit flowing back into its own inlet |
| Safety valve | Valve that opens automatically above a set pressure |
| Site survey | Visit to check location, structure, power, water and access before quoting |
| Static deflection | How far a mount compresses under the unit's weight |
| TDS | Total dissolved solids in water, in ppm |
| Transmissibility | Fraction of vibration force that passes through a mount |
| TT | Temperature transmitter or temperature test point |
| Velocity (pipe) | Speed of water in a pipe, in m/s; typically 0.7–1.5 m/s for heat pump circuits |
| Y-strainer | Y-shaped fitting with a mesh that catches dirt in the water flow |
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