Expansion Vessel Replacement — Brent
Expansion vessel replacement in Brent
Expansion vessel replacement and pressure diagnosis across Brent — Wembley, Kilburn, Willesden, Harlesden. Internal boiler vessel from £200. External vessel from £250. Vessel nitrogen re-charge from £80. PRV discharge investigation and system pressure diagnosis. Same-day service available.
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What you need to know
Expansion vessel replacement in Brent: what you need to know
What the expansion vessel does
As water in the heating system heats up it expands. The expansion vessel — a steel cylinder containing a rubber diaphragm and a pre-charge of nitrogen at 1 bar — absorbs this expansion. Without a working expansion vessel the system pressure rises above 3 bar when hot and the pressure relief valve (PRV) opens to release water. You find a drip from the PRV discharge pipe on the external wall of the property.
How the diaphragm fails
The rubber diaphragm perforates over time — typical lifespan 8 to 12 years. When the diaphragm fails, water fills the entire vessel and the nitrogen cushion is gone. The vessel becomes waterlogged and can no longer absorb expansion. System pressure then rises to 3 bar or above when hot and the PRV discharges. A waterlogged vessel is diagnosed by pressing the Schrader valve pin on top of the vessel — if water comes out, the diaphragm has failed.
How to diagnose a failed vessel
Check the system pressure when cold — it should be 1.0 to 1.5 bar. Run the boiler until hot — pressure should rise to a maximum of 2.5 bar. If pressure rises above 3 bar or the PRV discharges, the expansion vessel is likely failed. With the system cold and depressurised at the vessel, press the Schrader valve pin. If water comes out, the diaphragm has failed and the vessel must be replaced. If air comes out, the nitrogen charge can be topped up to 1 bar.
Internal and external vessel types
An internal expansion vessel is inside the boiler casing — compact combi boilers have vessels of 8 to 12 litres. Replacement requires opening the boiler. An external expansion vessel is on system boilers and unvented cylinders — sizes from 12 to 80 litres, easier to access and replace independently. The vessel must be correctly sized for the system water volume — an undersized vessel causes PRV discharge even when new.
Vessel re-charge option
If the Schrader valve test shows air rather than water, the diaphragm is intact and the nitrogen pre-charge has simply bled off over time. With the system cold and fully depressurised, the vessel can be re-charged via the Schrader valve to 1 bar using a standard tyre pump or pump with gauge. The system is then repressurised to 1.0 to 1.5 bar and retested. A re-charge is from £80 and avoids vessel replacement if the diaphragm is sound.
Unvented cylinder vessel
Hot water cylinders also have their own expansion vessel for the domestic hot water (DHW) circuit. If the hot water system pressure relief valve discharges — typically at the top of the cylinder or via a discharge pipe outside — the DHW expansion vessel may have failed. This is a different vessel from the heating circuit expansion vessel and is replaced separately. Prestige Engineers assess both heating and DHW expansion vessels.
Services
Expansion vessel services
Internal Boiler Expansion Vessel Replacement
Replacement of the expansion vessel inside the boiler casing on combi and system boilers. Includes isolating gas and water, draining down the boiler, removing and refitting the vessel, refilling, and repressurising to 1.0 to 1.5 bar. From £200.
External Expansion Vessel Replacement
Replacement of an external expansion vessel on system boilers or unvented cylinders. Includes closing isolation valve, draining down vessel, fitting correctly sized replacement vessel, pre-charging nitrogen to 1 bar, repressurising system, and testing. From £250.
Vessel Nitrogen Re-Charge
Re-charging the nitrogen pre-charge on an expansion vessel where the diaphragm is intact but the gas charge has bled off. System depressurised, vessel charged via Schrader valve to 1 bar, system repressurised and retested. From £80.
PRV Discharge Investigation
Investigation of a pressure relief valve that is discharging water at the external discharge pipe. Diagnosis of whether the cause is a failed expansion vessel, an overfilled system, a faulty PRV, or another pressure fault. Diagnosis from £80, redeemable against repair.
System Pressure Diagnosis
Full boiler pressure diagnosis — cold and hot pressure check, expansion vessel Schrader test, PRV test, and system integrity check. Identifies whether pressure loss is caused by a leak, a failed vessel, or a faulty filling loop. From £80.
Unvented Cylinder Expansion Vessel
Replacement of the domestic hot water expansion vessel on an unvented hot water cylinder. Includes isolation of the DHW circuit, vessel replacement with correctly sized unit, pre-charge to manufacturer specification, and functional test. Priced on survey.
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We replace expansion vessels and diagnose PRV discharge across all of Brent.
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Expansion vessel replacement Brent: frequently asked
How much does expansion vessel replacement cost in Brent?
Expansion vessel replacement in Brent starts from £200 for an internal boiler vessel and from £250 for an external vessel on a system boiler or unvented cylinder. Vessel nitrogen re-charge only (where the diaphragm is intact) from £80. Full system pressure check and diagnosis from £80. Unvented cylinder DHW expansion vessel replacement is priced on survey. All prices include labour and materials.
Why does my boiler pressure keep dropping in Brent?
Boiler pressure that drops repeatedly in Brent is caused by one of three things: a leak in the heating system (pipework, radiator valves, or heat exchanger), a failed expansion vessel that is causing the pressure relief valve to discharge water each time the boiler heats up, or a faulty filling loop that is not holding its valve fully closed. If you are topping up the boiler pressure regularly, have the system checked by a heating engineer. A failed expansion vessel causes the PRV to discharge water when hot — the system loses pressure when it cools back down. Prestige Engineers diagnose boiler pressure faults across all of Brent.
Why is my pressure relief valve dripping outside in Brent?
A pressure relief valve (PRV) that drips or discharges water from the external discharge pipe in Brent indicates that the system pressure is exceeding 3 bar when hot. The most common cause is a failed expansion vessel — the rubber diaphragm has perforated, the vessel is waterlogged, and it can no longer absorb the pressure rise when the water in the system expands as it heats. A secondary cause is an overfilled system — if the cold pressure is already above 2 bar, normal thermal expansion will push the pressure above 3 bar. Prestige Engineers diagnose and replace expansion vessels across Brent.
How do I know if my expansion vessel has failed in Brent?
There are two tests for a failed expansion vessel in a Brent property. First, check the system pressure when cold — it should be 1.0 to 1.5 bar — then run the boiler until hot and check again. If the pressure rises above 3 bar or the PRV discharges, the expansion vessel is likely failed. Second, with the system cold and fully depressurised at the vessel (close the isolation valve and press the system drain), press the Schrader valve pin on the top of the vessel — like a tyre valve. If water comes out, the diaphragm has perforated and the vessel must be replaced. If air comes out, the nitrogen charge can be re-topped to 1 bar and the vessel may not need replacing.