Commissioning a monobloc that trips on low pressure in heating mode
A newly installed 12 kW monobloc air-to-water heat pump serves an underfloor heating manifold on the ground floor and four radiators upstairs. On first commissioning in heating mode the unit runs for about four minutes, the low-pressure switch trips, and it locks out. The installer notes the outdoor unit is about 18 m of pipe run from the indoor manifold, with the outdoor unit mounted at first-floor level and the manifold at ground level. The system was pressure-tested at 3 bar and left at 1.5 bar static. The expansion vessel pre-charge was not checked. Flow temperature setpoint is 40 °C. The primary circuit is filled with plain water, no antifreeze.
1. List the most probable causes of the low-pressure trip in this specific installation and how you would confirm each.
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The trip is on the refrigerant low-pressure switch, so focus on refrigerant charge and heat exchange, not the water side.
- Undercharge / leak. An 18 m run with a factory-charged monobloc often needs additional charge; if the pre-charge was calculated for a shorter run the unit will be short of refrigerant. Confirm by checking suction/liquid pressures against the commissioning chart and looking for subcooling/superheat anomalies.
- Airflow / evaporator starvation. Check the outdoor fan is running at correct speed, the coil is clear, and there is no recirculation because the unit is mounted at first-floor level close to a wall or in a recess. Low airflow drops evaporating pressure.
- Low outdoor temperature / defrost fault. If commissioning in cold weather, confirm defrost is initiating; a stuck defrost ices the coil and collapses low-side pressure.
- Water-side flow too low. Restricted flow means poor heat transfer, high condensing pressure and the unit modulating down; check manifold flow rates and pump setting, and confirm the 18 m run has adequate pipe diameter (pressure drop).
Confirm each with gauges, a clamp meter on the fan, flow/temperature readings across the heat exchanger, and a leak test before adding charge.
2. The static pressure was left at 1.5 bar and the expansion vessel pre-charge was never checked. Explain why this matters and what you would do.
Read the worked answer
An unchecked pre-charge is a classic commissioning omission. If the vessel pre-charge is, say, 1.0 bar but the system static is 1.5 bar, the vessel is already partially filled with water at cold, so there is little or no acceptance volume left. As the system heats to 40 °C the water expands, pressure rises sharply, and you get either the safety valve lifting or, on the low side, erratic operation and nuisance trips.
Procedure: isolate and drain the vessel, check pre-charge with a gauge on the Schrader valve, set it to the correct value (typically static + 0.3 bar, so around 1.8 bar here, but follow manufacturer data), then refill to the correct cold fill pressure. Record both figures on the commissioning sheet. Also confirm the vessel size suits the system volume - 18 m of primary plus underfloor and radiators is a substantial volume and the factory vessel may be undersized, in which case add a second vessel. Plain water with no antifreeze is acceptable only if the outdoor unit and exposed pipework cannot freeze; if any risk exists, add inhibited antifreeze and re-check pump and flow settings.