Deciphering Combi Boiler Pressure Loss: Two Crucial Tests
When a combi boiler continuously drops pressure down to zero, homeowners and technicians face a common troubleshooting fork in the road: is water escaping through a failed internal component, or is a hidden leak draining the radiators? Before tearing up floorboards, heating engineers rely on two fast, non-invasive diagnostic checks to pinpoint the exact failure mechanism: testing the expansion vessel Schrader valve for fluid bleed and inspecting the exterior Pressure Relief Valve (PRV) copper discharge pipe for weeping.
While these checks investigate different ends of the heating system, they are intimately connected. To understand the wider system dynamics and how these components interact, consult our foundational guide: Combi Boiler Pressure Loss Diagnostics: Identifying Expansion Vessel Failure vs. Pressure Relief Valve Leaks.
Below is a practical, field-tested comparison of how to conduct both diagnostics, what the physical findings reveal, and how cross-referencing their results will tell you whether to recharge a vessel, replace a PRV, or hunt for an underfloor leak.
Diagnostic 1: Schrader Valve Fluid Bleed Testing
The expansion vessel contains a flexible rubber diaphragm dividing two chambers: one filled with system water and the other pre-charged with compressed air or nitrogen (usually between 0.75 and 1.0 bar). At the top or side of the vessel sits a standard Schrader valve—identical in appearance to a car tire valve.
How the Test Is Conducted
- Access the valve: With the boiler powered down and cool, locate the Schrader valve on the expansion vessel (often situated at the top rear of the boiler or inside an internal casing panel).
- Depress the centre pin: Using a small screwdriver, the tip of a key, or a fingernail, momentarily depress the spring-loaded centre pin for a fraction of a second.
- Observe the discharge: Watch and listen closely to what escapes the valve stem.
Interpreting the Results
- Immediate Fluid Discharge (Water): If water squirts, mists, or trickles out of the Schrader valve, the internal rubber diaphragm or bladder has split. Water has migrated into the dry air chamber. Diagnosis: Terminal vessel failure. The entire vessel must be replaced.
- Hiss of Dry Air: A sharp hiss of dry air indicates that the rubber diaphragm is intact and holding a charge. However, if the boiler pressure still spikes during operation, the pre-charge pressure may simply be depleted below nominal specification and needs recharging with a pump and gauge.
- Total Silence (No Air, No Water): If nothing escapes, the vessel is completely flat. The diaphragm may still be intact, or a tiny pinhole may have slowly bled the charge out over years. The vessel must be depressurised on the hydraulic side and recharged to establish if it holds pressure.
Diagnostic 2: Exterior PRV Discharge Pipe Weep Inspection
The Pressure Relief Valve (PRV) is a spring-loaded mechanical safety device set to crack open at 3.0 bar. When boiler pressure exceeds this limit, the PRV lifts to vent boiling water safely outside the property via a 15mm copper pipe (the blow-off or discharge pipe), which terminates through an external wall, typically bent back toward the brickwork.
How the Inspection Is Conducted
- Locate the termination point: Walk the perimeter of the building exterior directly behind or below the boiler’s location to locate the 15mm copper tail.
- Visual and touch inspection: Check the mouth of the pipe for dripping water, mineral crusting, or green copper carbonate staining.
- The “Dry Glove” or “Baggie” Test: Because a slow weep may evaporate on hot masonry before you see it, secure a dry plastic sandwich bag or rubber glove over the end of the copper pipe with an elastic band. Leave it in place over a full 24-hour cycle of central heating operation.
Interpreting the Results
- Active Water Accumulation in Bag: Water is escaping through the safety line. This proves the boiler’s pressure drop is not an invisible leak under the floor—water is leaving via the engineered emergency exit.
- Heavy Discharge During Firing Only: If the pipe discharges water only when the heating is on and the radiators are hot, the expansion vessel has lost its absorption capacity. Thermal expansion pushes pressure past 3.0 bar, forcing the PRV to discharge.
- Constant Slow Drip (Heating On or Off): If the pipe weeps constantly even when the boiler is cold and reading 1.0–1.5 bar, the PRV brass seat has failed, the internal spring has weakened, or central heating grit/magnetite is preventing the valve from fully seating shut.
- Bone Dry: If the pipe and test bag remain dry over several days while the boiler loses pressure, the PRV is exonerated. The loss is occurring on the heating circuit pipework, a radiator valve, or an internal boiler heat exchanger seal.
Cross-Referencing the Results: The Diagnostic Matrix
Neither test should be evaluated in total isolation. By cross-referencing what you find at the internal Schrader valve with what appears at the external copper outlet, you can isolate the exact root cause with high accuracy:
Scenario A: Water from Schrader + Water from Exterior Pipe
The Verdict: Catastrophic expansion vessel failure. The ruptured diaphragm caused thermal expansion to hammer the PRV open. You will need a new expansion vessel, and frequently a new PRV as well, because venting dirty system water often ruins the PRV’s internal rubber washer.
Scenario B: Air/Silence from Schrader + Hot Spurt from Exterior Pipe
The Verdict: Depleted pre-charge or unseated diaphragm. The vessel is physically intact but has lost its air cushion. The system water expands when heated, drives the pressure gauge into the red zone (over 3 bar), and trips the PRV. Recharging the vessel with a stirrup pump back to 1.0 bar (while system water pressure is zeroed) usually resolves the issue.
Scenario C: Dry/Charged Schrader + Constant Cold Weep from Exterior Pipe
The Verdict: Independent PRV mechanical failure. The expansion vessel is functioning normally, but the PRV seal has degraded or captured debris. The PRV needs stripping, cleaning, or outright replacement.
Scenario D: Dry/Charged Schrader + Bone Dry Exterior Pipe
The Verdict: External system leak. Both the expansion vessel and the safety valve are operating as designed. The pressure loss is caused by a leak on the distribution network: weeping radiator valves, pinholed copper beneath the floor, or a failing auto-air vent.
Safety and Professional Intervention
While homeowners can easily monitor the exterior copper pipe with a plastic bag, accessing the expansion vessel Schrader valve often requires removing the boiler’s room-sealed outer case. Under UK Gas Safety (Installation and Use) Regulations, removing the main casing on most modern combi boilers breaches the room seal and requires a Gas Safe registered engineer.
If your exterior inspection confirms water loss or you suspect the expansion vessel has failed, contact a qualified heating professional. They have the specialised equipment—including hydraulic pressure gauges, digital manometers, and system flushing tools—to safely recharge vessels, replace seized PRVs, and verify flue integrity after reassembly.