Boiler keeps losing pressure —
four causes and how
to find which one you have
A sealed heating system should not need regular topping up. Compare the gauge at the same cold condition: a continuing fall can indicate leakage, PRV discharge, repeated air removal, expansion-vessel trouble or a faulty indication. Repeated topping up without diagnosis is not a repair.
Safety scope: This article is for identifying likely water-side sealing and pressure-system symptoms. Do not open combustion chambers, gas-side components, flues or any appliance you suspect is unsafe. Gas work, boiler safety checks and unsafe appliance decisions belong with a competent Gas Safe registered engineer.
How fast it drops tells you where to look
The rate of pressure loss is the first diagnostic clue. A drop of 0.5 bar over a week is a different problem from a drop of 0.5 bar overnight. Both need investigating — but the urgency and the likely causes differ.
Slow drop — weeks or months
- Minor leak at a joint or radiator valve
- PRV discharging occasionally under peak pressure
- Air bleeding from radiators without topping up
- Expansion vessel diaphragm failing gradually
Fast drop — hours or days
- Active leak — visible or hidden
- PRV stuck open or discharging frequently
- Radiators bled without system being refilled
- Major leak at joint or component
Compare like with like: note pressure with the system cold and in the same operating state. A falling baseline is evidence to investigate, but the gauge alone does not prove where water went; check leakage, discharge, air removal, vessel condition and indication accuracy.
The four causes
The most common reason a sealed heating system loses pressure is a leak. This sounds obvious, but leaks in heating systems are frequently small — a weeping radiator valve, a slightly loose compression fitting, a joint that is dripping onto insulation rather than the floor. The leak may not be where the pressure loss points you.
Water from a heating system leaves traces. Look for limescale rings around joints and fittings — a white or brown circular stain on pipework or around a valve body where water has dried repeatedly. Look for rust staining on metal components. Look for damp patches on walls behind radiators or under floor sections where pipes run.
Radiator valves — particularly older thermostatic radiator valves — are a common source of slow weeping. The valve spindle seal deteriorates over time. The drip is tiny, often running down the valve body and evaporating before it reaches the floor. The pressure drops slowly over weeks and nobody can find where the water is going.
Flat face gasket joints — pump unions, boiler connections, heat exchanger fittings — are another common microleak location. A gasket that has been in service for years compresses flat and begins to pass at the edges of the contact ring. No dramatic failure. A slow weep that the pressure gauge records over weeks. Compression fittings on radiator pipework can also pass slowly if the olive has not seated correctly.
How to find it
Inspect every visible joint, valve and radiator connection systematically. Dry each one with tissue — moisture shows before a visible drip forms. Check behind radiators, under floor boards where accessible and around the boiler itself. Limescale staining marks old drip points even if they have dried.
What it needs
The leak location determines the repair. A weeping radiator valve may need a new gland packing or replacement valve. A loose compression fitting needs re-tightening or re-making. A gasket joint needs a new gasket on a clean face. Hidden leaks under floors or in walls need a qualified plumber to locate and repair.
When you bleed a radiator, you open the bleed valve and allow air to escape. Along with the air comes a small amount of system water. When the air is gone and water starts flowing from the bleed valve, you close it. The water that escaped lowers the system pressure slightly.
This is normal. A small pressure drop after bleeding radiators is expected and the system should be topped up to restore pressure to the correct level. It is not a fault — it is how the process works.
Where it becomes a problem is when radiators need bleeding frequently — monthly or more often. Air should not be continuously entering a sealed system. If it is, there is an underlying cause: a microscopic leak drawing air in under negative pressure, an automatic air vent not functioning, or system water chemistry that is promoting air release from dissolved gases.
How to confirm it
Pressure dropped shortly after bleeding radiators — this is expected. If bleeding is needed frequently and pressure keeps dropping between bleeding sessions, the underlying cause of repeated air ingress needs investigating.
What it needs
After bleeding, restore pressure to the specified cold fill level. If bleeding is needed more than occasionally, ask a heating engineer to investigate why air is repeatedly entering the circuit.
The pressure relief valve opens when system pressure exceeds its set point — typically 3 bar on domestic boilers — and discharges water to the overflow pipe. If the system regularly reaches overpressure, the PRV regularly discharges water. Every discharge lowers the system volume. Pressure drops. The homeowner tops it up. It rises to overpressure again. The PRV discharges again.
This is the cause that looks like pressure loss but is actually pressure loss and pressure gain in the same system. The PRV is working. The overpressure condition — usually a failed expansion vessel — is the root cause.
The tell-tale sign is the overflow pipe. The PRV discharge pipe typically terminates outside the property or in a tundish. If this pipe drips or shows water staining after the heating has run, the PRV is discharging. The pressure loss is water leaving through the PRV, not a leak in the circuit.
How to confirm it
Check the PRV overflow pipe for drips or water staining during or after a heating cycle. If the pipe shows evidence of discharge, the PRV is opening. The pressure loss is the PRV doing its job — the cause is upstream overpressure, not a circuit leak.
What it needs
Investigate the cause of overpressure — most commonly a failed expansion vessel. Addressing the overpressure stops the PRV from discharging, which stops the pressure loss. See the expansion vessel section below and the related article on high boiler pressure.
The expansion vessel contains a diaphragm separating an air charge from the system water side. When the system heats up and water expands, the vessel absorbs the expansion. When it cools, the vessel releases that water back into the circuit and pressure stabilises.
When the air pre-charge is too low, usable expansion capacity falls; if the diaphragm ruptures, water can enter the gas side. These are distinct faults and should be tested separately. It can no longer absorb expansion. Pressure spikes on heating — the PRV opens to release the excess. On cooling, the circuit loses the water that discharged through the PRV. Cold pressure is lower than before. You top up. The cycle repeats.
This is one of the subtler pressure loss patterns because the loss happens via the PRV on heating, but is only noticed when the system has cooled and pressure reads low. The homeowner sees low cold pressure, not the high hot pressure that caused the PRV discharge.
How to confirm it
Note cold pressure before running the heating. Run a full heating cycle. Note hot pressure — if it climbs significantly above 2 bar, the vessel may not be absorbing correctly. A competent engineer can isolate and depressurise the water side, verify pre-charge and test the Schrader valve according to the manufacturer's procedure. Do not diagnose pre-charge from an in-service pressure reading alone.
What it needs
Expansion vessel pre-charge top-up or replacement — a qualified heating engineer job. The system must be at zero pressure before the vessel is worked on.
The diagnostic sequence
When pressure keeps dropping, work through these in order:
- Was the system recently bled? A pressure drop immediately after bleeding is normal. Top up to the correct cold fill pressure and monitor. If it drops again without bleeding, one of the other causes is active.
- Is the PRV overflow pipe wet or stained? Yes — the PRV is discharging. Pressure loss is through the valve, not a circuit leak. Investigate overpressure cause — usually expansion vessel.
- Does pressure rise significantly on heating? Hot pressure well above 2 bar while cold pressure is low — expansion vessel likely waterlogged. Water is leaving through the PRV on every heating cycle.
- Is pressure dropping steadily with no heating? Active circuit leak — systematic inspection of all visible joints, valves and radiator connections.
- No visible drip but slow steady drop? Microleak at a gasket or compression joint — bring to working pressure and temperature, systematically dry and check every flat face joint and fitting with tissue. Limescale staining marks previous weeping points.
Pressure does not drop by itself. Water is leaving the circuit — the question is where and how fast.
Drop after bleeding: normal, top up once. PRV overflow wet: expansion vessel causing overpressure, water leaving through PRV — not a circuit leak. Pressure low after each heating cycle: expansion vessel likely waterlogged. Steady slow drop with no PRV activity: system leak, visible or microleak at gasket joint — find it. Topping up repeatedly without finding the cause is not maintenance. It is delay.
FAQ
Why does my boiler keep losing pressure?
A falling cold-pressure baseline usually means water or air has left the sealed circuit, the expansion-vessel condition has changed, or the indication is misleading; compare readings at the same temperature and operating state. Four causes account for many cases: a system leak — ranging from a visible drip to a slow microleak at a gasket or compression joint; a pressure drop after bleeding radiators, which is normal and expected; a failed expansion vessel that allows pressure to spike on heating, causing the PRV to discharge and system volume to drop; and the PRV discharging repeatedly as a symptom of that overpressure. Rate of pressure loss gives the first diagnostic clue — fast drop points to active leak or PRV discharge, slow drop over weeks to a minor leak or gradual vessel failure.
Is it normal for boiler pressure to drop slightly?
A very small drop over a long period — a few tenths of a bar over several months — can occur in sealed systems and may not indicate an active problem. Pressure that drops noticeably between comparable cold readings, or requires repeated topping up, is not normal and should be investigated for leakage, discharge, air removal, vessel condition or indication error.
How often should I need to top up my boiler pressure?
A well-maintained sealed heating system should rarely need topping up — perhaps once or twice a year at most, and only after work has been done on the system such as bleeding radiators. Needing to top up monthly or more frequently is a clear fault signal; find the cause rather than repeatedly adding fresh water and diluting inhibitor.