A Belgravia apartment’s unvented cylinder kept losing pressure. See how RARE Plumbing identified the expansion vessel fault and restored reliable hot water.
Quick Answer
If an unvented hot water cylinder repeatedly loses pressure, the cause may be an expansion vessel, pressure-relief valve or leak. In this Belgravia case, RARE Plumbing restored stable hot water by diagnosing the pressure-control components and testing the system through a full reheat cycle.
At a Glance
Typical Causes
Lost expansion-vessel charge, pressure-relief valve discharge or a small leak
Average Repair Time
1–3 hours after diagnosis
Emergency?
Prompt attention recommended if water is discharging
Approx. Cost Range
Inspection and repair costs depend on the failed component
Who Should Deal With It
Qualified unvented hot water engineer
Domestic or Commercial
Domestic
Risk Level
Medium
Hot water cylinder pressure loss in Belgravia
A homeowner in Belgravia, SW1, contacted RARE Plumbing after their unvented hot water cylinder repeatedly lost pressure. The property had intermittent hot water, a discharge pipe that occasionally dripped, and a pressure gauge that fell below its normal operating range after heating cycles.
Quick answer: Repeated pressure loss on an unvented cylinder is commonly linked to an incorrectly charged expansion vessel, a pressure-relief valve passing water, or a small leak. It should be assessed by a qualified engineer because unvented cylinders operate under mains pressure.
The brief
The client wanted a discreet diagnosis with minimal disruption to a finished apartment. We isolated the hot water system, checked the visible pipework and reviewed the pressure behaviour during a controlled reheat cycle.
What we found
The cylinder itself was sound. Our checks showed that the expansion vessel had lost its correct air charge, allowing system pressure to rise excessively as the water heated. This caused the pressure-relief valve to discharge and left the system with insufficient pressure once it cooled.
Common Symptoms
Causes
- 1Expansion vessel has lost its air charge
- 2Pressure-relief valve is passing water
- 3Hidden leak on associated pipework
- 4Failed pressure-reducing valve
The solution
We safely recharged the expansion vessel, inspected the pressure-relief valve and verified that the controls, temperature settings and discharge route were operating correctly. The cylinder was then monitored through a full heating cycle to confirm stable pressure and reliable hot water delivery.
How to spot the problem early
- The pressure gauge repeatedly drops after the cylinder reheats.
- Water appears at the tundish or discharge pipe.
- Hot water flow becomes inconsistent at showers or taps.
- The system needs frequent topping up or resetting.
Why specialist diagnosis matters
Unvented hot water systems are safety-critical. RARE Plumbing provides experienced cylinder servicing and repairs for prime London homes, with careful protection of interiors and clear advice on the condition of the system. For related help, see our hot water cylinder service and Megaflo servicing pages.
Outcome
The client regained stable hot water pressure without a cylinder replacement. We also recommended annual servicing to identify wear in safety components before it affects day-to-day comfort.
Engineering Insight
How Our Engineers Diagnose It
Inspect the cylinder, tundish and discharge route
Check static and heated pressure behaviour
Test the expansion vessel charge
Inspect pressure-relief and control valves
Run a controlled reheat cycle
Preventative Maintenance
Frequently Asked Questions
A pressure-control fault, a passing relief valve or a leak can cause repeated pressure loss. An engineer should inspect an unvented cylinder safely.
Talk to a specialist today.
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