Boiler Losing Pressure: Causes, Diagnosis, and Fixes

Quick Answer: A boiler that keeps losing pressure has one of four root causes: a system leak (most common), a failed or flat expansion vessel, a leaking pressure relief valve (PRV), or a passing filling loop. Diagnose by first establishing the rate of pressure loss, then systematically checking the expansion vessel charge, the PRV discharge pipe, all visible pipework and fittings, and the filling loop valves.

Summary

Pressure loss on a sealed central heating system is one of the most common callouts in UK domestic heating. Normal operating pressure is between 1.0 and 1.5 bar when cold, rising to around 1.8-2.0 bar when hot. A sealed system that requires top-up more than once or twice a year has an underlying fault that must be identified and resolved -- simply re-pressurising the system each time masks the problem and risks long-term damage to the boiler and system components. The four primary causes -- system leaks, expansion vessel failure, PRV discharge, and passing filling loops -- each have distinct diagnostic signatures that allow an experienced engineer to narrow down the fault quickly on site. This guide provides a structured decision tree and detailed procedures for each fault path.

Key Facts

Diagnostic Decision Tree

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Step 1: Establish the symptom pattern

Is the pressure DROPPING or RISING uncontrollably?
  |
  +-- DROPPING --> Go to Step 2 (Pressure Loss Path)
  |
  +-- RISING (pressure creeps up without topping up) -->
        |
        Filling loop passing (mains water leaking into system).
        Go to "How do I diagnose a passing filling loop?" below.

Step 2: How fast is the pressure dropping?

How quickly does pressure fall from 1.5 bar to 0 bar?
  |
  +-- SLOWLY (days to weeks) -->
  |     |
  |     Does pressure drop when the system is COLD and OFF?
  |       |
  |       +-- YES --> System leak or passing PRV. Go to Step 3.
  |       |
  |       +-- NO (only drops after heating runs, recovers when cold) -->
  |             Expansion vessel fault. Vessel cannot absorb
  |             expansion, pressure spikes when hot, PRV dumps
  |             water, pressure is low when cold.
  |             Go to "How do I check the expansion vessel?" below.
  |
  +-- RAPIDLY (hours) -->
        |
        Major leak or PRV stuck open.
        Check PRV discharge pipe FIRST (is water flowing?).
          |
          +-- YES --> PRV fault or expansion vessel failure
          |           causing overpressure. Go to PRV section below.
          |
          +-- NO  --> Major system leak. Go to leak detection below.

Step 3: Narrow down -- leak vs PRV

Check the PRV discharge pipe (small-bore copper pipe exiting
through the wall, usually near the boiler flue).
  |
  Is there water dripping, staining, or limescale at the outlet?
    |
    +-- YES --> PRV is discharging. But WHY?
    |     |
    |     Check expansion vessel charge (depressurise first).
    |       |
    |       +-- Vessel flat or waterlogged --> Recharge or replace vessel.
    |       |   The vessel failure caused overpressure which opened the PRV.
    |       |
    |       +-- Vessel charge OK --> PRV itself is faulty (failed to reseat
    |           after a previous activation). Replace the PRV.
    |
    +-- NO  --> System leak. Go to leak detection section below.

Detailed Guidance

How fast is the pressure dropping?

Rate of Loss Typical Cause First Check
Loses 0.2-0.5 bar over weeks Micro-leak at a fitting, radiator valve spindle, or slow expansion vessel air loss Visual inspection of all joints; check EV charge
Loses 0.5-1.0 bar over days Expansion vessel flat/failed, PRV weeping, or moderate leak Check PRV discharge pipe; check EV Schrader valve
Drops to zero overnight Major leak (burst pipe, blown fitting) or PRV stuck open Immediate visual check for water; check PRV pipe
Drops only after heating runs, recovers when cold Classic expansion vessel failure -- vessel cannot absorb expansion, PRV dumps excess, pressure is lower each cycle Depressurise and check EV charge; inspect tundish
Pressure creeps UP without topping up Passing filling loop -- not a pressure loss fault but will trigger PRV and then appear as pressure loss Check filling loop valves; disconnect external loop

How do I check the expansion vessel?

The expansion vessel is a steel dome (typically red, white, or grey) inside or mounted externally on the boiler. It contains a rubber diaphragm separating a gas charge (nitrogen or dry air) from the system water. When the diaphragm fails or the charge leaks away, the vessel fills with water and can no longer absorb the expansion of heated water.

Step-by-step diagnosis:

  1. Turn off the boiler and let the system cool to room temperature.
  2. Depressurise the system to 0 bar by opening a drain valve or bleed valve until the pressure gauge reads zero. This step is essential -- you cannot get an accurate charge reading with system pressure present.
  3. Locate the Schrader valve on the expansion vessel (looks identical to a car tyre valve, usually accessible on the underside or side of the vessel).
  4. Press the Schrader valve pin briefly:
    • Air comes out = vessel has charge. Measure with a tyre pressure gauge. Compare to manufacturer spec (see table below).
    • Water comes out = diaphragm has failed. Vessel must be replaced. Do not attempt to recharge -- it will not hold.
    • Nothing comes out = charge has leaked away completely. Attempt recharge; if it does not hold, replace the vessel.
  5. If air comes out but pressure is low, recharge using a foot pump or hand pump with a Schrader adapter to the correct pre-charge pressure.
  6. Re-fill the system to 1.0-1.5 bar cold, run the heating, and monitor. Pressure should rise no more than 0.5 bar when hot.

Typical pre-charge pressures by manufacturer:

Manufacturer Typical Pre-charge (bar) Notes
Vaillant (ecoTEC range) 0.75 bar Vaillant specify 0.75-0.9 bar for integrated vessels
Worcester Bosch (Greenstar range) 1.0 bar Some older models specify 0.5 bar; check the data plate
Baxi (Duo-tec, Platinum, etc.) 0.75 bar Integrated vessels; external vessels may be 1.5 bar
Ideal (Logic, Vogue, etc.) 0.75-1.0 bar Check specific model installation manual
Glow-worm (Energy, Betacom) 0.75 bar Same parent company as Vaillant; similar specs
Potterton (Assure, Titanium) 1.0 bar Check model-specific documentation
External vessels (Flamco, Zilmet, etc.) 1.5 bar Factory pre-charge; adjust to match static head if needed

When to replace vs recharge:

How do I check if the PRV is leaking?

The pressure relief valve (PRV) is a spring-loaded safety device set to open at 3 bar. It connects via a small-bore copper pipe through the wall to discharge outside. On some installations, a tundish (visible air break) is fitted internally between the PRV outlet and the discharge pipe.

Diagnosis procedure:

  1. Check the external discharge pipe. Go outside and locate the small copper pipe (usually near the boiler flue outlet). Look for:

    • Active dripping
    • Limescale deposits or green copper staining at the pipe end
    • Damp patch on the wall below
    • Any of these = PRV has been discharging

  2. Check the tundish (if fitted). Look inside, usually mounted on the wall near the boiler. Any water sitting in the tundish or drip marks below it = PRV is leaking.

  3. Feel the PRV discharge pipe. With the boiler running and up to temperature, feel the copper pipe downstream of the PRV. If it is warm or hot, water is passing through -- the PRV is not fully sealed.

  4. Establish the root cause before replacing the PRV:

    • If the expansion vessel is flat or failed, it is causing overpressure which forces the PRV open. Fix the vessel first; the PRV may reseat once pressure is controlled.
    • If the expansion vessel is fine and system pressure is below 3 bar, the PRV itself has failed to reseat (common after it has been activated). Replace the PRV.
    • If a passing filling loop is pushing pressure above 3 bar, that is the root cause -- fix the filling loop.

  5. After replacing a PRV, always investigate why it opened in the first place. A PRV that has activated is a symptom, not a root cause.

Key point: Never cap, plug, or block a PRV discharge pipe. It is a critical safety device. If it is discharging, find out why -- do not silence it.

How do I find a system leak?

Visual inspection (always do this first):

  1. Check all radiator valve connections -- TRV heads, lockshields, and tail connections to the radiator. Run your finger along the underside of the fitting; even tiny weeps will leave a residue.
  2. Check all compression fittings in visible pipework, especially in the airing cupboard, under the boiler, and at manifold connections.
  3. Check soldered (capillary) joints -- look for green verdigris staining which indicates a slow copper weep.
  4. Check the boiler itself -- remove the bottom panel and look for drips inside. Common internal leak points: heat exchanger seals, auto air vent, pump connections, and internal PRV weeps.
  5. Check around press-fit connections (Hep2O, JG Speedfit) -- push-fit demountable fittings under floorboards are a common leak source.
  6. Check beneath all flexible filling loop hoses for drips.

Pressure test procedure (when no visible leak found):

  1. Isolate the boiler from the system (close the service valves on flow and return).
  2. Repressurise the system to 1.5 bar.
  3. Mark the exact gauge position or use a digital gauge.
  4. Leave for 24 hours minimum (48 hours preferred). Do not run the heating.
  5. If pressure drops: leak is in the system pipework, not in the boiler.
  6. If pressure holds: re-open boiler valves, repressurise, and leave again. If it drops now, the leak is inside the boiler.

If the leak is in the system but not visible (underfloor or buried pipework):

Common leak locations ranked by frequency:

  1. Radiator valve connections (TRV tail, lockshield gland)
  2. Compression fittings under floors
  3. Soldered joints (especially older systems)
  4. Push-fit connections (Hep2O, Speedfit)
  5. Boiler internal components (heat exchanger, AAV, pump seals)
  6. Underfloor heating manifolds and pipe joints
  7. Towel radiator connections (often overlooked)

What about the filling loop?

A filling loop connects the mains cold water supply to the sealed heating circuit, allowing the system to be topped up. There are two main types:

External filling loop (braided hose with two isolation valves):

Internal (built-in) filling loop:

Passing filling loop consequences:

When should I recommend a magnetic filter?

Sludge (magnetite -- black iron oxide) does not directly cause pressure loss, but it is a significant related factor:

Recommend a magnetic filter when:

How sludge relates to pressure problems:

Filter installation:

Frequently Asked Questions

Can a boiler lose pressure without a visible leak?

Yes. The three most common causes of pressure loss with no visible leak are: (1) a failed expansion vessel that causes pressure spikes when hot, triggering the PRV to dump water outside -- the "leak" is at the discharge pipe which is often out of sight; (2) a micro-leak under the floor or behind a wall that is absorbed by building materials before it becomes visible; (3) a leak inside the boiler itself (heat exchanger, auto air vent, pump seal) where water drips onto internal components and evaporates from the residual heat. Always check the PRV discharge pipe and inside the boiler before concluding there is a hidden pipework leak.

What pressure should a boiler be set to?

Most UK domestic boiler manufacturers recommend a cold fill pressure of 1.0-1.5 bar, with 1.0-1.2 bar being the most commonly specified. When the system heats up, pressure will rise by 0.3-0.5 bar due to thermal expansion -- this is normal. If pressure rises above 2.5 bar when hot, the expansion vessel is likely not functioning correctly. The PRV will open at 3.0 bar. Never exceed 1.5 bar on initial cold fill; this leaves headroom for expansion.

How often should the expansion vessel be checked?

The expansion vessel pre-charge should be checked at every annual boiler service. This requires depressurising the system to 0 bar first, which most engineers should be doing as part of a thorough service. Many engineers skip this step -- it should be standard practice. On boilers over 8 years old, expansion vessel failure is one of the most common causes of repeat callouts for low pressure.

Is it safe to keep topping up the boiler?

As a temporary measure, yes -- the system will function. But repeatedly topping up introduces fresh oxygenated mains water, which accelerates internal corrosion, generates more magnetite sludge, and can breach water regulations if the filling loop remains connected. It also masks the underlying fault. Advise the customer that topping up is a stopgap, not a solution, and that the root cause must be investigated.

Regulations & Standards