Boiler Losing Pressure: Expansion Vessel, PRV, Leaks & Microleaks

Summary

Boiler pressure loss is one of the most common heating system faults in the UK. The sealed (pressurised) central heating system installed as standard with modern combi and system boilers requires a stable pressure of 1–1.5 bar when cold. This pressure is maintained by water volume in the system and the nitrogen charge in the expansion vessel. When either changes — through a leak or failed vessel — the pressure gauge reading changes.

The key distinction that many homeowners and some tradespeople miss is between a primary leak (water escaping the system) and an expansion vessel failure (no water escaping, but pressure cycling outside normal range). An expansion vessel failure causes the system to be over-pressured during heat-up (the water expanding during heating has nowhere to go) and to vent through the pressure relief valve (PRV). After venting, the cold pressure drops. The homeowner adds water via the filling loop, overcharges the system, the process repeats, and the PRV discharge pipe becomes permanently damp. Meanwhile, the expansion vessel has not been diagnosed.

Understanding this pressure cycling pattern is essential for correct diagnosis. A system that loses pressure only after it heats up and cools down has a different fault from a system that loses pressure when cold (which indicates a genuine water leak).

Key Facts

• Normal operating pressure — 1–1.5 bar cold; typically rises to 1.8–2.5 bar when at full operating temperature (80°C flow temperature); manufacturer-specific
• Expansion vessel function — contains a nitrogen-filled bladder; as system water expands on heating (~4% volume increase from 10°C to 80°C), water enters the vessel and compresses the nitrogen
• Expansion vessel pre-charge pressure — typically 1 bar; must match the system fill (cold) pressure; check with system cold and depressurised via Schrader valve on the vessel
• Bladder failure — when the bladder ruptures, the vessel fills with water; it can no longer accept expanding water; pressure rises rapidly on heat-up and vents through the PRV
• PRV setting — typically 3 bar for domestic heating systems; opens when system pressure exceeds this value; should discharge to a safe location via a 15mm discharge pipe to outside or drain
• PRV leaking continuously — may indicate PRV is stuck open (mineral deposit on seat), or that system pressure is genuinely exceeding 3 bar (expansion vessel failure)
• Microleaks — pinhole leaks in radiators, pipework, or fittings that are too small to see as drips; often evaporate before reaching the floor; identified by limescale staining or brown rust marks under radiators
• Inhibitor loss — inhibitor is lost along with water when the system loses pressure; top up inhibitor (Fernox F1 or equivalent) after any significant water loss
• Filling loop — the temporary connection between mains cold water and the heating system for repressurising; must be removed after filling (Water Regulations requirement) unless a disconnectable type is fitted
• Combi boiler pressure — combi boilers have the expansion vessel built in; the vessel is sized for the boiler's own internal water content, not for a large system; adding radiators to an existing combi may require an external expansion vessel
• Pressure gauge accuracy — built-in boiler pressure gauges are not precision instruments; a separate test gauge gives a more reliable reading
• Radiator valve bodies — compression fittings on thermostatic radiator valve (TRV) bodies and lockshield valves are common microleak sites; look for white limescale deposits or orange rust staining

Quick Reference Table

Detailed Guidance

Decision Tree

BOILER LOSING PRESSURE — START HERE
               |
               v
WHEN does pressure drop?
               |
       --------+---------
       |                 |
       v                 v
   ALWAYS              ONLY after
   (cold &             heat / cool
   hot)                cycle
       |                 |
       v                 v
System LEAK          Expansion vessel
-- inspect           failure likely
   all joints,       |
   rads, valves      v
       |          Cold pressure OK?
       v          (1–1.5 bar)
   Find leak:         |
   - under floor  YES | NO
   - behind       |    \
     plasterboard |     v
   - under        |  System
     screed       |  overfilled
       |          |  -- bleed some
       v          |  water out
   Thermal        |
   imaging or     v
   specialist     Does pressure rise
   leak           above 2.5 bar
   detection      when hot?
   required           |
                  YES | NO
                      |   \
                      v    v
               Expansion   Normal
               vessel      expansion
               fault       -- continue
               -- test     monitoring
               vessel

========================
EXPANSION VESSEL DIAGNOSIS
========================
               |
               v
1. Let system cool to room temperature
2. Note cold pressure
3. Locate expansion vessel (grey cylinder, typically above/beside boiler)
4. Find Schrader valve on vessel
5. Depressurise system to 0 bar via drain cock or bleeding
6. Press Schrader core -- what comes out?
               |
         ------+------
         |            |
         v            v
      AIR only    WATER (or nothing)
      -- check    -- bladder ruptured
      pressure       REPLACE vessel
         |
         v
      Is pressure at or above system fill pressure (1 bar)?
         |
      YES | NO
         |   \
         v    v
      Vessel  Recharge to 1 bar
      OK --   (nitrogen or dry air)
      look    via Schrader valve
      for     with system empty
      other   |
      cause   Refill system
              |
              Test: pressure should
              hold during heat cycle

Expansion Vessel Testing and Replacement

The expansion vessel is a sealed steel cylinder containing a rubber bladder. On one side of the bladder is the system water; on the other side is nitrogen gas pre-charged to the same pressure as the system fill pressure (typically 1 bar).

Testing the vessel:

1. Allow the system to cool completely.
2. Depressurise the heating system to 0 bar (open a drain cock or bleed a high radiator carefully).
3. Locate the Schrader valve on the expansion vessel — it looks identical to a tyre valve.
4. Press the Schrader core with a tyre pressure gauge or a sharp object.
5. If water comes out, the bladder has ruptured — replace the vessel. If only air comes out, the vessel is intact.
6. Check the air pressure reading. It should be 1 bar (matching the system fill pressure). If lower, recharge using a bicycle pump or compressor to 1 bar.
7. Refill the system and monitor through a heat-up cycle.

Vessel sizing: Expansion vessels are sized for the system water volume. A typical 3-bedroom house system has approximately 80–100 litres of water. The required vessel size depends on the system volume, operating temperature, and fill pressure. For a 100-litre system with 80°C max temperature and 1 bar fill pressure, an 8–12 litre vessel is typical. Undersized vessels are common in older systems where radiators have been added.

Replacing the vessel:

1. Isolate the boiler and allow to cool.
2. The vessel is connected to the system by a 3/4 inch BSP fitting (usually with an isolation valve). Close the isolation valve.
3. Depressurise the vessel side (Schrader valve).
4. Disconnect the water connection and remove the vessel.
5. Fit the new vessel with a new fibre sealing washer. Pre-charge to the system fill pressure before connecting.
6. Open the isolation valve. Refill and pressurise the system.

Finding System Leaks

If the expansion vessel is confirmed healthy and the system is still losing pressure, a water leak is present. Finding it is often the most challenging part of the diagnosis.

Visual inspection first:

• Check all radiator valve bodies (TRV bodies and lockshield valves) for white limescale deposits or orange rust staining, which indicate a past or ongoing leak
• Check all visible pipework joints, particularly push-fit connectors (which can work loose) and compression fittings under floorboards at access points
• Inspect the boiler casing interior — many combi boiler leaks occur at the heat exchanger or plate heat exchanger connections inside the boiler
• Check the PRV discharge pipe — a wet or dripping pipe confirms the PRV is opening

Microleaks under flooring or behind walls: If no visible leak is found and the system is losing 0.2 bar or more per week, a microleak is present in concealed pipework. Options:

• Leak detection fluid — add a system-compatible UV dye (Fernox or similar); check discharge points with a UV torch
• Thermal imaging — an infrared camera can detect damp spots through plasterboard and flooring where water is evaporating
• Specialist leak detection — acoustic detection equipment can identify the sound of water escaping under pressure through concealed pipes

Combi boiler internal leaks: Common internal combi boiler leak points:

• Plate heat exchanger (separates heating and domestic hot water circuits) — a crack causes domestic water to enter the heating circuit; pressure rises when DHW is used
• Pump seal — look for staining below the pump on the boiler back panel
• Motorised valve seals
• Pressure gauge capillary tube connections

If a plate heat exchanger is leaking, the symptoms include: heating pressure dropping when DHW tap is opened, or DHW tasting of inhibitor/system water.

PRV Diagnosis

The pressure relief valve is a safety device — it must not be the primary means of controlling system pressure. If the PRV is dripping or discharging:

1. Check system pressure when cold — if above 2.5 bar cold, the system is overfilled. Open a drain cock slightly to reduce pressure. The PRV opening at 3 bar is normal protection.
2. Check hot pressure — if normal cold pressure (1–1.5 bar) but system reaches above 3 bar when hot, the expansion vessel is undersized or has failed.
3. PRV stuck open — occasionally a PRV fails to reseat after opening, particularly if limescale or debris has lodged on the seat. If the discharge pipe drips at normal system pressure, the PRV needs replacement.

PRVs are not serviceable — replace rather than repair. Use a like-for-like replacement (3 bar, 1/2 inch BSP is standard for most domestic boilers). After replacing, confirm the discharge pipe is correctly routed and unobstructed.

Frequently Asked Questions

How often should I need to top up my boiler pressure?

A properly sealed system with no leaks should not need topping up more than once or twice per year, if at all. Small losses are normal due to the slight permeability of radiator valves and the absorption of dissolved air from the system. If you're topping up more than once per month, investigate further — there is an active leak or expansion vessel fault.

Is it safe to keep topping up the boiler?

Topping up frequently masks a fault. Each time you add fresh water, you add dissolved oxygen and fresh minerals. Dissolved oxygen promotes internal corrosion. Minerals (especially in hard water areas) accelerate scale build-up. The correct approach is to find the cause of the pressure loss, not to continuously add water. Additionally, if the expansion vessel is waterlogged, over-filling accelerates the PRV discharge problem.

The pressure is fine when cold but the boiler cuts out on low pressure when it gets hot — why?

If pressure drops during heat-up (rather than rising), this suggests the expansion vessel charge pressure is too high. As the water attempts to expand into the vessel, it cannot because the vessel pre-charge pressure exceeds the system pressure. The water has nowhere to go — but this should cause pressure to rise, not fall. If pressure genuinely falls during heat-up, suspect a leak that opens under thermal expansion (a microleak in a fitting that seals when cold but weeps when hot and under pressure).

Can I recharge the expansion vessel myself?

Yes — if you have a bicycle pump or compressor, a tyre pressure gauge, and can safely depressurise the heating system first. The key is to depressurise the system side (not just the vessel) before checking and recharging. Checking the vessel pressure with the system full and under pressure gives a false reading. Always depressurise the system to 0 bar before testing the vessel's Schrader valve.

My PRV is discharging to the garden — is this normal?

A brief discharge during initial system fill is normal. Continuous discharge or regular discharge on every heat-up cycle is not normal and indicates either an expansion vessel problem (most likely), an overfilled system, or a PRV that has been damaged by repeated opening. Trace the discharge pipe from the boiler — it should exit to an external or safe location; the discharge area should not be blocked or submerged.

Regulations & Standards

• Gas Safe Register — all work on gas boilers must be carried out by a Gas Safe registered engineer

• BS 7593:2019 — treatment of water in domestic hot water central heating systems; inhibitor requirements, water quality

• BS EN 12828:2012 — heating systems in buildings; design for water-based heating systems including expansion vessel sizing

• Water Supply (Water Fittings) Regulations 1999 — filling loops must be disconnectable; not permanently connected to mains

• Heating and Hotwater Industry Council (HHIC) — boiler system guidance and fault diagnosis

• Gas Safe Register: Boiler pressure advice — safe boiler operation guidance

• Fernox Technical Information — inhibitor dosing, leak detection dyes

• Caleffi: Expansion vessel sizing guide — technical guide for vessel sizing and pre-charge

• low pressure — detailed expansion vessel and PRV testing procedure

• powerflush — system cleaning after repeated pressure loss and inhibitor dilution

• thermostat not working — other causes of boiler lockout

• system design — sealed vs open vent systems, expansion vessel positioning