Boiler Pressure Too High: PRV Operation, Expansion Vessel Fault & How to Reduce

Summary

Boiler overpressure is the opposite problem from losing pressure, but both symptoms point to the same component: the expansion vessel. In a sealed central heating system, water expands as it heats up — a closed system has nowhere for that extra volume to go unless an expansion vessel (a membrane-type pressure vessel) absorbs it.

When the expansion vessel fails (most commonly because the pre-charge pressure is too low, or the membrane ruptures), the system pressure rises sharply when heating runs. If pressure reaches the PRV set point (typically 3 bar), the PRV opens and discharges water from the system. After the system cools, pressure drops — and customers often re-pressurise from the filling loop, making the problem worse over time.

Understanding this cycle is essential: a customer who fills up to 1 bar every month and doesn't know why, or whose PRV keeps weeping, has an expansion vessel problem until proven otherwise.

Key Facts

• Normal cold pressure — 1.0–1.5 bar (varies by system; check label on boiler or commissioning document)
• Normal hot pressure — 1.5–2.5 bar (should rise roughly 0.3–0.5 bar from cold pressure when system is at operating temperature)
• PRV set point — typically 3 bar; opens and discharges water to prevent higher pressure
• Expansion vessel function — absorbs water expansion as system heats; diaphragm separates water side from nitrogen pre-charge side
• Pre-charge pressure — nitrogen gas pressure in expansion vessel; typically 1.0 bar for domestic systems; must match static head of the system
• Failed expansion vessel signs — pressure rises sharply when heating; PRV weeping or discharging; pressure drops to less than 0.3 bar when cold after PRV discharge; Schrader valve on vessel discharges water not air
• Membrane rupture — if the rubber diaphragm fails, the nitrogen side fills with water; the vessel loses its function; test by depressing Schrader valve — water instead of air = failed membrane
• External expansion vessel — standard in older systems; can be re-pressurised or replaced independently
• Internal expansion vessel — fitted inside boiler casing on modern combi and system boilers; may require boiler front removal to access
• Filling loop — the mechanism to add water to a sealed system; should not be used routinely; frequent use indicates a leak or expansion vessel failure
• Inhibitor — must be checked after every system drain-down and water loss event
• PRV discharge pipe — must terminate safely; typically in tundish to drains; wet patch under boiler or pipe indicates recent PRV discharge
• Static head calculation — for systems taller than 10m, the expansion vessel pre-charge must equal the height of water above the vessel (0.1 bar per metre)

Quick Reference Table

Detailed Guidance

Diagnosing an Overpressure Fault

Step 1: Record pressures carefully.

• Cold pressure (system off, water cold for minimum 1 hour): should be 1.0–1.5 bar
• Hot pressure (system running at operating temperature, 15–30 minutes): should be 1.5–2.5 bar
• Note how much the pressure rises: from cold to hot, normal rise is 0.3–0.5 bar

Step 2: Check for PRV discharge.

• Look for a wet patch under the boiler, on the boiler casing, or at the tundish/drain connection
• Check whether the PRV shows signs of calcium deposits around the body (repeated discharge deposits limescale)
• A PRV that has discharged should be tested for closure: gently operate the manual test lever (if fitted) and allow to reseat — if it continues to weep, the PRV seat may be damaged

Step 3: Test the expansion vessel.

• Isolate and drain the system to zero pressure (open a drain cock, open bleed valves)
• Depress the Schrader valve on the expansion vessel (looks like a tyre valve; may be under a cap)
• If water comes out: membrane is ruptured; vessel must be replaced
• If air comes out: membrane is intact; check the pre-charge pressure on a calibrated pressure gauge

Step 4: Check and re-pressurise the vessel (if membrane intact).

• With system at zero pressure, use a car tyre pump or nitrogen cylinder to add air to the Schrader valve
• Pre-charge should be 1.0 bar for most domestic systems (or slightly less than cold fill pressure)
• For systems with more than 10m of static head above the vessel: pre-charge = system height in metres × 0.1 bar
• After re-pressurising the vessel, refill the system to 1.0–1.5 bar, run the system, and check hot pressure does not exceed 2.5 bar

Step 5: Replace vessel if membrane is ruptured.

• For external vessels: drain down, isolate, remove and replace. Size must be at least equal to the original (typically 8–18 litres for domestic). Calculate correct size based on system water volume and temperature range.
• For internal vessels (inside boiler): check manufacturer instructions; may require replacing the entire internal assembly or removing the boiler front panel.

PRV Replacement

If the PRV has been repeatedly opening, it may no longer reseat cleanly. The PRV is a safety device — a PRV that weeps continuously reduces system pressure slightly over time and eventually the system loses water. A weeping PRV must be replaced.

PRV replacement procedure:

1. Isolate and drain the system to below the PRV level
2. Unscrew the PRV (3/4" BSP thread is standard for most domestic boilers)
3. Fit new PRV of the same pressure rating (typically 3 bar; confirm on the original)
4. PTFE tape or hemp on threads; hand tighten plus 1.5 turns with wrench; do not overtighten
5. Ensure discharge pipe is reconnected and terminates safely

Never increase the PRV set pressure to prevent discharge — this is dangerous. If the system pressure exceeds the PRV rating, the PRV is the last line of defence against catastrophic failure.

Expansion Vessel Sizing

If replacing or adding an expansion vessel to a system, correct sizing is important. Undersized vessels lead to overpressure recurrence.

Simplified sizing method:

1. Estimate system water volume: boiler manual specifies vessel size for typical system; or calculate from radiator outputs (typically 12–15 litres per kW of boiler output for a fully loaded system)
2. Use expansion vessel sizing charts or online calculators
3. For a typical domestic system (15kW, 10 radiators): expansion vessel of 12–18 litres
4. For large systems or underfloor heating (more water volume): 18–35 litres

The vessel must be installed on the cold side of the heat source (before the pump and boiler, on the return), in an accessible location for future servicing.

Overpressure Due to Filling Loop Left Connected

A common cause of persistent overpressure in combi boilers is a filling loop left connected or partially open. Mains water pressure (typically 2–5 bar) can bypass through a worn filling loop valve into the system, which is at lower pressure — forcing system pressure up continuously.

Check: Is the flexible filling loop connected at both ends? Both valves should be fully closed when not in use, and the flexible hose disconnected entirely if a permanent filling connection is used (Worcestershire Heat Systems and others now require Combi-Fill or FlexFill-type permanent filling connections with double-check valve).

Documenting and Advising the Customer

After repair:

• Explain what has been done and why
• Advise the correct cold pressure range for their system (write on a label stuck near the pressure gauge)
• Explain that the filling loop should only be used if pressure drops below 1 bar and NOT as routine maintenance
• Note the system inhibitor level and service the system

Frequently Asked Questions

Can I reduce boiler pressure by bleeding radiators?

Yes — opening bleed valves releases air and some water, reducing system pressure temporarily. But this does not fix the underlying fault. If pressure is consistently too high when cold, bleeding will only provide temporary relief. Find and fix the root cause (expansion vessel).

My customer's PRV is weeping slightly — is this an emergency?

Not immediately — but it is a fault that must be repaired. A weeping PRV will eventually result in the system losing water and low pressure, and the PRV seat may deteriorate further, leading to full discharge. Schedule repair promptly. In the interim, check the pressure regularly.

How often should the expansion vessel pre-charge be checked?

At every boiler service. It takes 30 seconds to check with the Schrader valve once the system is depressurised, and finding a low pre-charge before it causes repeated PRV discharge saves a callout.

Regulations & Standards

• Gas Safety (Installation and Use) Regulations 1998 — boiler installation and safety

• Building Regulations Part L — Boiler Plus requirements; heating system commissioning

• BS 7074 — Application, selection and installation of expansion vessels

• BS EN 12828 — Design of sealed hot water heating systems

• Boiler Plus scheme — commissioning Benchmark document requirements

• HHIC Boiler Guidance — Heating and Hotwater Industry Council technical guidance

• Worcester Bosch Expansion Vessel Guide — Manufacturer guidance

• CORGI Hometeam Technical Advice — Sealed system diagnosis

• OFTEC Technical Bulletins — Sealed heating system guidance

• low pressure — Boiler losing pressure: diagnosis and repair

• system design — Sealed vs open vent systems; expansion vessel function

• expansion vessels — Expansion vessel sizing, pre-charge, and replacement

• powerflush — When overpressure occurs alongside poor circulation