Why Your Boiler Pressure Keeps Dropping and How to Fix It

If your boiler was working fine yesterday but today your radiators feel lukewarm, you're not alone. Many Toronto homeowners first notice something's off when the boiler starts flashing a low-pressure warning or the heat just doesn't seem to "kick in" the way it normally does.

This problem also tends to show up more often during winter cold snaps, when your heating system is under the most stress. Low boiler pressure is common, but it's still something you'll want to take seriously before it turns into a no-heat situation.

Is It Safe if Boiler Pressure Keeps Dropping?

In most cases, low boiler pressure is not an immediate safety danger, but it is a warning sign that something isn't right in your heating system. A boiler is designed to operate as a sealed system, so if the pressure keeps dropping, it usually means water is escaping somewhere, either through a small leak, a faulty valve, or an issue with the expansion tank.

If your boiler pressure falls too low (often below 1.0 bar), your boiler may shut off to protect itself, leaving you with little or no heat, and in some cases no hot water (especially with combi boilers). While you can often top up the pressure temporarily, repeated refilling is a red flag and can lead to bigger problems over time.

If you notice visible leaking, pressure dropping rapidly, or pressure loss happening repeatedly during Toronto cold snaps, it's best to stop troubleshooting and call a licensed boiler technician before the issue gets worse.

93% of Canadian households report having a primary heater, and even though boilers are used in around 8% of homes, issues like pressure loss can still leave families uncomfortable during extreme cold. (source)

What Is Normal Boiler Pressure?

For most Toronto homes with a hot water boiler system, the pressure gauge should usually sit around 1.0 to 1.5 bar when the system is cold (meaning the heating hasn't been running for a while). This is considered a normal range for many residential boilers, and it gives the system enough pressure to circulate hot water properly through your radiators or baseboard heaters.

When your boiler is actively heating, it's normal for the pressure to rise slightly, often into the 1.5 to 2.0 bar range. This happens because water expands as it heats up. A small increase is expected, and modern boilers are built to handle it.

What's not normal is pressure that consistently drops below 1.0 bar, or pressure that keeps falling over and over again after you've topped it up. If your boiler pressure keeps dropping, it usually means there's an underlying issue that needs attention, not just a one-time adjustment.

Signs Your Boiler Pressure Is Too Low

Sometimes the boiler will make it obvious that pressure is low, but other times the symptoms show up gradually. If your boiler pressure keeps dropping, you may notice one or more of these common warning signs:

Radiators feel lukewarm or don't heat evenly

One of the first signs is that your radiators (or baseboards) aren't getting fully hot. You may feel warmth at the bottom but cooler spots near the top, or certain rooms may stop heating properly.

Your boiler shuts off or won't stay running

Many modern boilers will lock out or stop firing when pressure drops too low. This is a built-in safety feature designed to protect the system.

A low-pressure warning light or error code appears

Some boilers display a warning symbol, while others show a numeric error code. Even if the boiler still runs, a low-pressure warning is not something to ignore.

You hear gurgling or bubbling in radiators

Low pressure can allow air to move through the system more easily, which may cause gurgling sounds or uneven heating.

Hot water becomes inconsistent (especially in combi boilers)

If you have a combi boiler, low pressure can sometimes affect domestic hot water performance as well, not always, but it's a common complaint when pressure is borderline.

You keep needing to top up the pressure

If you've already had to refill the boiler once and the pressure drops again within days or weeks, that's a strong sign that the system is losing water somewhere and needs professional diagnosis.

The Most Common Reasons Boiler Pressure Keeps Dropping

A boiler is meant to operate as a sealed, pressurized system. So if your boiler pressure keeps dropping, it's almost always because water is leaving the system, or the system is struggling to manage pressure properly as it heats and cools.

Here are the most common causes Toronto homeowners run into.

A Small Leak Somewhere in the Heating System

This is the most common reason boiler pressure drops over time.

Even a slow leak, a few drops per hour, can cause the pressure to fall below the safe operating range. Leaks can happen in:

• Radiator valves or bleeder valves
• Pipe joints (especially older connections)
• Baseboard heater connections
• The boiler itself
• Hidden pipes behind walls or under floors

Important: Some leaks evaporate quickly because pipes get warm, so you might not always see a puddle.

The Pressure Relief Valve Is Releasing Water

Boilers have a safety valve called a pressure relief valve (PRV). If system pressure gets too high, the PRV opens to release water and prevent damage.

If the PRV is discharging water, you may notice:

• Drips or moisture near the boiler
• Water coming from a copper pipe that ends near a floor drain
• A damp spot under the discharge pipe

If your PRV is leaking, it may be faulty, but very often it's a symptom of another issue (like an expansion tank problem).

Expansion Tank Issues (Very Common in Winter)

Your boiler's expansion tank is designed to absorb pressure changes when water heats up and expands.

If the expansion tank is failing or has lost its air charge, your boiler may:

• Spike in pressure when heating turns on
• Trigger the pressure relief valve
• Then drop into low pressure afterward once water is released

This is one of the most common "cycle patterns" seen in Toronto homes during cold snaps:
pressure rises > PRV releases water > pressure becomes too low

You Recently Bled Your Radiators

Bleeding radiators removes trapped air, but it also releases a small amount of water. After bleeding, it's normal for boiler pressure to drop slightly.

In many cases, you simply need to:

• Top up the system once
• Re-check pressure afterward

However, if you find yourself bleeding radiators often, or pressure drops quickly afterward, it may point to a deeper issue like a leak or expansion tank problem.

The Filling Loop Was Left Open or Isn't Working Properly

Some boilers have a filling loop used to top up pressure.

If it's left partially open, it can cause pressure issues (including overfilling). If it's not functioning correctly, you may also have trouble refilling the system properly.

This is one of those areas where it's easy to accidentally create a bigger problem, so if you're unsure, it's worth getting a professional to check it.

An Internal Boiler Fault (Less Common, But Possible)

If the system pressure keeps dropping and there's no obvious leak, the issue may be inside the boiler itself. Possible causes include:

• A small internal leak
• A failing heat exchanger
• A sensor or pressure gauge issue
• A component slowly releasing water during operation

These problems usually require proper diagnostic testing, and should be handled by a licensed boiler technician at The Boiler Shoppe.

What You Can Safely Check at Home (No Tools)

If your boiler pressure keeps dropping, there are a few simple things you can check safely before calling for service. The goal here isn't to "fix" a serious issue, it's to gather clues and avoid making the problem worse.

Check the boiler pressure gauge

Look at the pressure reading when the system is cool (before the heat has been running for a while).

• If it's below 1.0 bar, that's usually too low
• If it's around 1.0 to 1.5 bar, that's generally normal
• If it swings dramatically when heating turns on, that may point to an expansion tank issue

Look for visible water around the boiler

Check the floor and the area directly underneath the boiler.

Look for:

• Moisture
• Drips
• Rust stains
• Water marks or mineral buildup

Even a small amount of water matters.

Check the pressure relief discharge pipe area

Most boilers have a pipe that runs down toward a drain or ends near the floor. If you see:

• Dampness under the pipe
• Water staining
• A steady drip

…it may mean the pressure relief valve is releasing water.

Check radiator valves and connections

Walk around and look at the valves on the side of your radiators (or baseboards).

Check for:

• Damp spots
• Staining
• Corrosion
• Water marks on the floor below

These leaks are often small enough that homeowners don't notice them until pressure keeps dropping.

Think about what changed recently

This is one of the most useful "diagnostic clues" you can give a technician.

Ask yourself:

• Did you bleed radiators recently?
• Did the issue start after a cold snap?
• Did the boiler recently shut off with an error code?
• Did anyone adjust the boiler settings or refill the pressure?

Track how fast the pressure drops

This helps determine whether the issue is urgent.

For example:

• Drops in hours: often a leak or PRV discharge
• Drops in days: common with slow leaks
• Drops over weeks: can still be serious, especially in winter

Avoid opening panels or touching internal components

Even if you're handy, avoid removing the boiler casing or trying to access internal parts. Boilers involve gas components, sealed pressure systems, and safety controls, and it's easy to turn a manageable issue into an expensive one.

A quick note before you refill the pressure
If you need to top up your boiler pressure to restore heat, that can be okay as a short-term step, but if the pressure drops again, it's a strong sign you need professional service. A boiler should not need frequent refilling.

How to Repressurize a Boiler

If your boiler pressure has dropped too low, your system may stop heating until the pressure is brought back into a normal range. Many Toronto homeowners end up needing to repressurize their boiler at least once, especially after bleeding radiators or during heavy winter use.

Repressurizing should be a temporary fix, not something you have to do regularly. If the pressure keeps dropping again, there's an underlying problem that needs professional attention.

Step 1: Confirm the pressure is actually low

Look at the pressure gauge on your boiler.

In many systems:

• Anything below 1.0 bar is too low
• A common target is around 1.2 to 1.5 bar when the system is cold

(Your boiler manual may recommend a slightly different range.)

Step 2: Find the filling loop (if your boiler has one)

Most modern boilers have a filling loop, usually a small silver braided hose or a rigid connection with one or two valves. This is what allows you to add water to the sealed system.

If you're not sure where it is, check:

• Under the boiler
• Near the bottom piping
• Your boiler manual

Step 3: Add water slowly and watch the gauge

When you open the valve(s), the pressure should begin to rise.

Important tips:

• Go slowly
• Watch the gauge the entire time
• Stop once the pressure reaches the recommended range
• Do not overfill the system

Overfilling can trigger the pressure relief valve and create a new problem.

Step 4: Close the valve(s) completely

Once the pressure is back in range, make sure the filling loop valves are fully closed. Leaving them even slightly open can cause pressure issues and may damage the system over time.

Step 5: Turn the boiler back on and monitor the pressure

After the boiler starts heating again, you may see a small increase in pressure, this is normal.

However, if:

• The pressure drops again within a short time
• The boiler repeatedly locks out
• You notice water around the boiler or discharge pipe

…it's time to stop refilling and have the system checked.

Important warning:
Do not keep topping up the pressure repeatedly

If you're refilling the boiler every few days or every week, it's a strong sign that:

• Water is escaping somewhere
• A valve is releasing pressure
• The expansion tank may not be working properly

Repeated refilling isn't just inconvenient, it can also increase wear on your boiler and lead to bigger repair costs.

When to Stop Troubleshooting and Call a Licensed Pro

Some boiler issues are simple, but when boiler pressure keeps dropping, it's often a sign of a problem that won't resolve on its own. A one-time pressure drop after bleeding radiators is usually manageable, but repeated pressure loss is not normal and shouldn't be ignored.

Here are the situations where it's best to stop troubleshooting and call a licensed boiler technician.

You can see water leaking anywhere

Even a small drip matters. If you notice moisture around:

• The boiler
• Pipes or fittings
• Radiator valves
• Baseboard connections
• The pressure relief discharge pipe

…it's time to call a professional before the leak worsens.

The pressure drops again soon after repressurizing

If you top up the boiler pressure and it falls again within days (or even hours), the system is losing water somewhere, and refilling repeatedly is only masking the real issue.

The boiler keeps shutting off or locking out

Modern boilers will shut down to protect themselves when pressure gets too low. If your boiler repeatedly stops running, it's a sign that the problem is actively affecting performance.

The pressure swings high when the heat turns on

If you notice the pressure gauge climbing quickly when the boiler fires up (and then dropping later), it may point to an expansion tank issue. This is especially common during winter when the system is cycling more frequently.

You suspect the pressure relief valve is discharging

If the relief pipe is dripping or leaving water stains, it's often a symptom of pressure regulation problems. This is not something to ignore, because it usually gets worse over time.

You have no heat during a Toronto cold snap

If your heat is unreliable during extreme cold, it becomes more than a comfort issue, it can quickly turn into a plumbing and property risk. In Toronto winters, it's worth addressing pressure problems early before they become a full no-heat emergency.

Why Pressure Problems Show Up During Cold Snaps

A lot of Toronto homeowners notice boiler pressure issues for the first time during winter, and that's not a coincidence.

When temperatures drop, your boiler runs longer and cycles more often to keep up with demand. That extra workload puts stress on the entire heating system, and it can expose weak points that weren't obvious during milder weather.

Your heating system is running more often

In winter, your boiler may be firing multiple times per hour, especially in older homes or during overnight temperature drops. More run time means more pressure changes, more expansion and contraction, and more opportunity for small issues to become noticeable.

Pressure changes happen more frequently

Water expands when heated and contracts as it cools. During cold snaps, the system goes through more heating cycles, which means more pressure movement. If the expansion tank is failing or undercharged, these pressure swings can become more extreme.

Small leaks become bigger problems

A slow leak that barely affected your system in the fall can become a real issue in January. When the boiler is running constantly, the system can lose pressure faster, and the boiler may shut down before your home has a chance to recover.

No-heat risk is much higher in Toronto winters

Low pressure doesn't just mean "less efficient heat." It can lead to:

• Boiler lockouts
• No heat
• Frozen pipes in extreme cases
• Emergency repair calls at the worst possible time

That's why it's worth dealing with pressure loss early, even if your boiler still seems to be running.

Fix the Pressure Problem Before It Becomes a No-Heat Call

If your boiler pressure keeps dropping, it's usually a sign that your heating system isn't staying sealed the way it should. Sometimes the cause is minor, like pressure loss after bleeding radiators, but ongoing pressure drops often point to a leak, a faulty pressure relief valve, or an expansion tank issue that won't resolve on its own.

In Toronto winters, it's worth addressing low boiler pressure early, before it turns into a no-heat situation during a cold snap. If you've already topped up the system and the pressure keeps falling again, a licensed boiler technician can diagnose the cause quickly and help prevent bigger repairs down the road.

If you need help troubleshooting boiler pressure loss in Toronto or the GTA, The Boiler Shoppe can inspect the system and get your heat running reliably again.