Boiler Pressure Dropping: What to Do (Especially in Winter)

When temperatures drop and your heating system runs continuously, it is not uncommon for your boiler pressure to decrease. Don’t worry—most of the time, there are simple solutions to fix the issue and restore your thermal comfort. So what are the right habits and basic checks to adopt? And when should you rely on a professional? Here’s everything you need to know.

How to tell if your boiler pressure is too low

When a boiler lacks pressure, certain signs are unmistakable. The good news is that you can spot them well before the boiler stops working completely!

Symptoms to look out for

Several symptoms indicate insufficient pressure:

• your radiators are lukewarm, or even cold at the top;

• some rooms are not heating as well as usual;

• water struggles to circulate in the system (gurgling or irregular flow noises);

• your boiler frequently goes into safety mode.

By identifying these signals early, you can act quickly without risking damage to your system.

Pressure drop: when is it normal and when should you be concerned?

It is normal for pressure to vary slightly over time. At the start of the heating season, when the boiler is restarted, pressure tends to be lower than usual. The same applies after bleeding radiators. As long as these variations do not exceed 0.3 bar, the system remains within a normal operating range. Beyond that, it may indicate imbalance or malfunction.

However, some situations require closer attention:

• your boiler pressure drops regularly, even after refilling (a drop of more than 0.3 bar in a short time);

• the boiler struggles to restart heating after being turned back on;

• pressure drops occur every night or after long periods without use.

While not necessarily alarming, these situations deserve further investigation. They may indicate a deeper issue (pump problem, expansion vessel issue, invisible leak, etc.) that should be addressed quickly to avoid a complete breakdown.

Why is your boiler pressure dropping? The most common causes

Is your boiler pressure abnormally low or decreasing? Here are the most common explanations.

A water leak (cause no.1, even if invisible)

In most cases, pressure drops are linked to a water leak somewhere in the heating system. Pressure drops without leaks are rare, so even if you cannot immediately see one, it is worth investigating further. In winter, when the system is used more intensively, even a tiny leak (a simple drip) can gradually reduce pressure.

Start by checking sensitive areas:

• the boiler itself, especially the safety valve or automatic air vent;

• the pipework, including connections and joints;

• heat emitters (radiators, underfloor heating systems).

If your system is not visible (e.g. underfloor heating or pipes embedded in walls), look for indirect signs:

• damp areas on floors;

• stains or bubbling on walls;

• marks, mould, or damp smells;

• small unexplained puddles.

If your inspection reveals nothing but pressure loss continues, it is best to call a plumber or heating engineer. Likewise, call a professional if you cannot check your system yourself. Some leaks are difficult to detect and require professional tools.

A clogged or worn pump

Another possible cause is the boiler pump, also known as the circulator.

This essential component ensures hot water circulates throughout the heating system. If it is clogged, blocked, or worn, water no longer flows properly, leading to pressure drops and insufficient heating.

A pump can typically last up to 20 years, but wear may be accelerated by:

• lack of maintenance;

• system clogging;

• limescale build-up, especially in areas with hard water.

A deflated expansion vessel

If no leak or pump issue is identified, the pressure drop may come from the expansion vessel. This component compensates for water expansion (as water heats up, it expands) and maintains stable pressure throughout the system.

If the vessel loses air or becomes “deflated,” it can no longer absorb expansion. Pressure then drops regularly or even suddenly during heating cycles.

Excessive bleeding: an underestimated cause

In winter, when radiators make noise or heat unevenly, it is common to bleed them repeatedly. However, while bleeding removes air from the system, doing it too often can be counterproductive. Each bleed releases water from the system, and repeated operations can eventually cause a lasting drop in pressure.

What to do to increase your boiler pressure

Fix the leak to restore pressure

If you have identified a water leak causing the pressure drop, it must be repaired—either by yourself or by calling a plumber.

Be careful not to overestimate your DIY skills. If the repair seems complex, call a professional immediately. Heating systems involve hot or pressurised water and can be hazardous. If the leak originates from the boiler itself, only a heating engineer can properly assess the situation and determine whether repair or replacement is needed.

Discover our boiler maintenance solutions

Refill the system with water: best practices

If you have ruled out a leak, simply add water to the system to restore proper pressure.

Here’s how to proceed:

1. Locate the filling valve(s): usually under the boiler or near the water supply. If unsure, check the installation manual;
2. Open the valve slowly: turn it as indicated until you hear water entering the system;
3. Monitor the pressure gauge or display: watch the pressure rise gradually until it reaches the recommended range (generally between 1 and 1.5 bar, depending on the size and height of the home);
4. Close the valve firmly once the correct pressure is reached.

Important: this should remain an occasional operation. Frequent refilling means the system is losing water or a component is failing.

Check and/or restart the pump

If the pump (circulator) is stopped or malfunctioning, hot water will not circulate properly. Pressure gradually drops and heating efficiency decreases.

To check:

1. Place a screwdriver near the circulator to listen for the motor;
2. If you hear no noise, the pump is likely blocked or stopped.

In some cases, it can be manually restarted using a screwdriver to unblock it.

However, proceed with caution: this is a sensitive component. To avoid damage—or if in doubt, it is safer to call a heating engineer, especially during the heating season.

What if the expansion vessel is the issue?

If the expansion vessel is deflated, pressure will not remain stable even after refilling.

Unlike simply adding water, checking or re-pressurising an expansion vessel requires specific tools and precise adjustments. It is therefore best handled by a heating engineer, who can test, re-inflate, or replace it if necessary.

When should you contact a professional? Warning signs to watch for

When a simple refill is no longer enough, it is important to involve a professional quickly to limit damage and avoid a complete boiler breakdown—especially in winter.

Contact a professional if:

• pressure drops again shortly after refilling;

• you need to add water more than twice in the same heating season;

• you suspect a micro-leak but cannot locate it;

• the pump is malfunctioning, not restarting, or making unusual noise;

• the expansion vessel appears deflated or is leaking water/gas;

• the boiler repeatedly goes into safety mode;

• the heating system can no longer maintain a comfortable temperature.

Why waiting makes the problem worse when pressure drops

For proper operation, all boilers require sufficient water pressure, regardless of:

• the fuel used (wood, pellets, gas, or oil);

• the technology (standard, low-temperature, or condensing boilers).

Pressure ensures hot water circulates efficiently throughout the system (both horizontally and vertically in multi-storey homes) and supplies radiators or underfloor heating.

When pressure becomes unstable, the system constantly compensates, which strains components and reduces efficiency. Not only does comfort decrease, but also:

• energy consumption increases, as the boiler runs longer to reach the desired temperature;

• energy bills rise;

• key components (pump, expansion vessel, safety valve) wear out faster;

• the risk of breakdown increases during cold periods when heating demand is highest.

The longer you wait, the higher the risk of serious failure—and potentially costly repairs. Preventive action and maintenance are therefore essential.

Maintenance: the long-term solution for stable pressure all winter

A well-maintained boiler operates optimally, and its pressure remains stable for longer even during cold spells.

Annual servicing is mandatory

Don’t forget the mandatory annual maintenance! Since 2009 in France, boilers must be serviced by a certified professional at least once a year. This is an opportunity to assess your equipment and identify the causes of recurring pressure drops.

Everyday maintenance tips

In addition to annual servicing, adopt simple habits:

• regularly check the pressure gauge on your boiler;

• in case of a slight drop, gently open the filling valve until pressure returns to normal;

• avoid bleeding radiators or underfloor heating too frequently.