Can A Heat Pump Lose Pressure Without A Leak?

Can A Heat Pump Lose Pressure Without A Leak?

Yes, a heat pump can lose pressure without any visible water leak, and this happens more often than most homeowners expect. If you cannot find any wet patches, dripping valves, or damp pipework but the pressure gauge keeps falling, the cause is almost certainly not a conventional water leak.

Many homeowners spend weeks searching for a leak that does not exist, while the real cause sits within a component they have never thought to check. Each time the system is topped up, the pressure returns for a short period before dropping again, and the cycle repeats.

This guide explains every cause of pressure loss in a heat pump system that does not involve a conventional water leak, and explains what each one looks like in practice.

Expansion Vessel Problems

A failed or incorrectly charged expansion vessel is the single most common cause of unexplained pressure loss in a heat pump system. The expansion vessel is a sealed container, usually painted red or grey and located in the plant room, containing a pressurised air bladder separated from the system water by a rubber diaphragm.

When water heats up it expands. The expansion vessel absorbs this additional volume, preventing the system pressure from rising too high during operation. When the vessel is correctly charged, system pressure fluctuates gently between its cold reading and its operating reading. When the vessel fails or loses its air charge, it can no longer absorb that expansion.

A faulty expansion vessel typically produces one of two recognisable patterns:

• Pressure that rises rapidly to 2.5 bar or above when the heating runs, then falls below 1 bar again when the system cools, because the vessel cannot absorb thermal expansion and the pressure relief valve is discharging water to compensate

• Pressure that gradually falls over days or weeks without any heating activity, because the diaphragm has failed and water has entered the air side, reducing the vessel’s ability to hold system pressure

• The pressure relief valve discharge pipe releasing water outside the building, often only visible in cold or wet conditions when dripping is noticeable

• A persistent need to top up the system that returns within hours or days each time, suggesting active water loss through the PRV rather than a slow leak

The expansion vessel can often be re-pressurised if only the air charge has dropped, which is a straightforward job for a heating engineer. If the diaphragm has ruptured, the vessel needs replacing. Neither issue involves a water leak in the conventional sense, but both cause persistent and recurring pressure loss. Our article on what pressure your heat pump system should be running at explains the normal operating range and how to read the pressure gauge correctly throughout the heating cycle.

Automatic Air Vents

Most heat pump systems include automatic air vents, usually positioned at high points in the pipework circuit where air naturally accumulates. These vents open to release trapped air, and as they do, small amounts of water are discharged alongside it. Each individual release is tiny, but across weeks and months the cumulative water loss is enough to reduce system pressure noticeably.

A similar water discharge mechanism applies to anti-freeze valves on the external pipework. These mechanical valves open when pipe temperature approaches freezing point, releasing a small quantity of water to prevent ice formation. Each activation removes water from the closed circuit. In cold weather with multiple activations, the cumulative water loss can cause a measurable drop in system pressure. Our guide on how anti-freeze valves work and when they discharge explains both valve types and how to distinguish normal discharge from a valve that has developed a fault.

When an anti-freeze valve develops a fault rather than simply activating normally, the water loss can be continuous rather than occasional and can cause significant and recurring pressure drops. Our case study on a property in Suffolk where a faulty anti-freeze valve was causing repeated pressure loss and heat pump lockouts provides a real example of how this develops and what investigating it remotely looks like in practice.

Automatic air vent discharge is particularly pronounced immediately following:

• A recent installation, where the system water is still carrying dissolved air from the initial fill process

• Maintenance work that required draining and refilling any part of the circuit

• Radiator replacements or additions that introduced new pipework and air into the circuit

• Initial commissioning in the first few weeks of operation, when the largest volume of dissolved air is still working its way out of the system

Bleeding Radiators

Every time a radiator is bled, a deliberate release of system water occurs. The process removes trapped air from the top of the radiator by opening the small bleed valve, and water follows the air out until a steady stream replaces the bubbling.

After bleeding a single radiator, the effect on system pressure is usually minimal. After bleeding multiple radiators across a property in a single session, the combined water loss can reduce pressure noticeably.

Finding a lower pressure reading after a bleeding session is entirely normal and does not indicate a problem. Topping up the system using the filling loop to return the pressure to its normal range is the correct response.

The only concern arises if the pressure continues to fall after topping up. That suggests the air has re-entered the system through an as-yet unidentified source, which may indicate a slow leak or an unsealed joint somewhere in the circuit.

Pressure Relief Valve Discharge

Most heat pump systems include a pressure relief valve with a discharge pipe routed externally through the wall of the property. The valve opens automatically if system pressure exceeds a set threshold, typically around 3 bar, releasing water to bring pressure back within safe limits.

Because the discharge goes outside the building, homeowners are frequently unaware it is happening. The first sign is often a recurring need to top up the system, with no visible sign of water loss anywhere inside the property and no fault code to explain it.

If you suspect the PRV may be discharging, check the external discharge pipe on the outside wall of the building, typically a 15mm or 22mm copper or plastic pipe, when the heating is running. Any dripping or water staining below it confirms that the valve has been activating.

The most common reasons the PRV discharges repeatedly are:

• A failed expansion vessel that is no longer absorbing thermal expansion, causing pressure to spike during every heating cycle and force the PRV to open

• A system that has been overfilled during commissioning or after a maintenance top-up, leaving insufficient headroom for normal thermal expansion

• A PRV that has weakened over time and opens at a lower threshold than its rated pressure, which is a component fault rather than a system design issue

Trapped Air Within The System

A newly installed or recently serviced heat pump system will often contain a significant volume of dissolved air introduced during the filling process. As the system heats and cools through its first weeks of operation, this dissolved air gradually comes out of solution and moves as small pockets around the circuit until it reaches an air vent or bleed point and is released.

Each time a pocket of air is released, the overall water volume in the circuit decreases slightly. This is why a single top-up, or sometimes two, is expected and normal in the months following a new installation or a major service. It is not a fault; it is the system settling.

If pressure continues to fall beyond the first three to four months of operation, residual installation air is no longer a plausible explanation and the cause should be investigated more thoroughly.

When Should You Be Concerned?

A one-off or seasonal pressure drop usually has a straightforward explanation from the list above. The following patterns are the ones that indicate something requires proper investigation:

• Pressure falls more than once every few weeks without an obvious explanation such as recent bleeding, maintenance, or known cold weather activation of the anti-freeze valve

• You top up the system and have to top it up again within days, suggesting an active water loss mechanism rather than a gradual drift

• Pressure spikes noticeably above 2.5 bar when the heating runs and then falls back when it cools, which points strongly to an expansion vessel fault

• Water is visible dripping from an external pipe on the outside wall, confirming that the pressure relief valve has been discharging

• Flow-related fault codes begin appearing alongside the pressure drop, which suggests pressure has fallen far enough to affect water circulation through the heat pump

Each of these patterns points to a different specific cause. Our dedicated guide on why a heat pump keeps losing pressure covers each cause in detail. Our case study on a West Yorkshire property where system pressure kept dropping on a large underfloor heating circuit shows what methodical pressure investigation looks like in a real system.

Thinking About A Heat Pump Installation?

Pressure problems, including expansion vessel sizing, pre-charge settings, and anti-freeze valve placement, are among the issues most reliably avoided through a proper system design review. Incorrect expansion vessel sizing relative to the system volume is a frequent commissioning oversight that causes recurring problems for the life of the installation.

Our Pre-Installation Design Review reviews proposed heat pump systems before installation begins, including expansion vessel sizing, pipe volume, system layout, and any design elements that could affect long-term pressure stability.

Need Help Diagnosing A Pressure Problem?

If your heat pump keeps losing pressure and none of the straightforward explanations account for it, our Fix My Heat Pump service can help.

We review your system layout, photographs, pressure readings, fault history, and the pattern of when pressure losses occur to identify the most likely cause and what action needs to be taken before the situation causes further problems.