Boiler Leaking Water – Causes, Warning Signs, and What to Do

Seeing water around your boiler can be stressful — and for good reason. A boiler leak can range from a minor fitting that’s started weeping to a serious internal failure that needs immediate professional repair.

If you’re in the Denver area, leaks can be especially disruptive in winter when your system is working hardest. The key is knowing what’s safe to do right now and what needs a trained technician.

Quick note before we start

This guide is designed to help you reduce risk and prevent damage. It does not replace professional diagnosis. If you smell gas, hear unusual hissing, or suspect electrical contact with water, stop and call a professional right away.

Is a leaking boiler dangerous?

A leaking boiler is often urgent, even if it looks small.

Here’s why:

• Water damage can spread fast. A slow drip can soak flooring, drywall, and nearby framing over time.
• Water + electricity is a bad mix. Boilers have electrical components (controls, wiring, circulator pumps). If water reaches them, it can create a shock hazard or damage the system. 
• Leaks can signal pressure problems. Some leaks show up only when the system heats up and pressure rises.
• Mold and corrosion risks. Persistent moisture can encourage mold and accelerate rust around the unit and nearby materials. 

Even if the boiler still runs, a leak is usually a sign something isn’t operating correctly.

Common reasons boilers leak water

Boilers leak for a few “usual suspect” reasons. Some are external (valves, fittings), others are internal (components inside the boiler cabinet).

1) Pressure relief valve discharge (pressure problems)

A boiler’s pressure relief valve is designed to release water if system pressure gets too high. If that valve is dripping or discharging, it may be doing its job — but it’s also telling you something is wrong. 

Common underlying reasons include:

• System pressure running too high
• A faulty expansion tank/vessel (can’t absorb pressure changes properly)
• A filling loop/auto-fill valve left feeding (keeps adding water and raising pressure) 

What homeowners usually notice:

• Drips from a copper pipe or outlet that terminates near the floor/drain
• Pressure gauge reading higher than normal when the system is hot

2) Corrosion in an older boiler or system piping

Over time, metal components can corrode—especially in older systems. Corrosion can weaken:

• Boiler sections
• Pipework
• Joints and connections

Rust can eventually create pinhole leaks or cause seals to fail. 

What homeowners usually notice:

• Brownish staining
• Rust around fittings or the base of the unit
• Dampness that keeps returning even after wiping it up

3) Pump seal leaks or failing circulator components

Hydronic systems rely on a circulator pump to move hot water through the system. Pump seals can wear out, and small leaks can form around the pump housing or flanges.

What homeowners usually notice:

• Water directly beneath the pump area
• Dripping that worsens when the boiler is running

4) Loose joints, compression fittings, or valve packing

Sometimes the leak isn’t “the boiler” itself — it’s a connection:

• Near shutoff valves
• At pipe joints
• Around unions/flanges

These leaks can start small and get worse with vibration, heat cycles, or pressure fluctuations.

What homeowners usually notice:

• A steady drip that appears near a fitting
• Mineral deposits or white/greenish buildup around the joint

5) Cracked heat exchanger (more serious)

A cracked heat exchanger/boiler section can allow water to leak from within the unit. This is one of the more serious causes because it can indicate advanced corrosion, thermal stress, or component failure.

Many consumer HVAC references list a cracked heat exchanger as a common culprit behind boiler leaking and advise professional diagnosis and repair. 

What homeowners usually notice:

• Leaking from inside the boiler cabinet
• Repeated shutdowns
• Corrosion patterns or rust streaks originating from internal panels

Because this can involve combustion-side components (depending on system design), it’s not a DIY situation.

Warning signs the leak is serious

Any leak is worth addressing, but these red flags suggest you should stop troubleshooting and call a professional quickly:

• Water pooling on the floor or actively dripping
• Boiler pressure dropping repeatedly (you top it up, but it falls again)
• Rust, corrosion, or mold around the unit or nearby materials
• Boiler shutting down or throwing error codes after leaking starts
• Dripping from the pressure relief discharge pipe (especially if it happens frequently) 
• Water near electrical wiring, controls, or the burner compartment (do not touch—shut down safely) 

If you see any of the above, treat it as a priority repair.

What to do immediately if your boiler is leaking

Here are the safest “first response” steps most homeowners can take. The goal is to reduce risk and prevent further damage — not to repair internal parts.

1) Turn off the boiler’s power

Water and electricity don’t mix. If you see active leaking, switch the boiler off using:

• The boiler’s power switch (if accessible), and/or
• The breaker/fuse that supplies the boiler

Many boiler leak guides recommend powering the unit down first for safety. 

2) If safe, shut off the water supply feeding the boiler/system

If you know where the boiler’s feed valve is (or a system water shutoff), turning it off can reduce ongoing leakage—especially if a valve or fitting has failed.

If you’re unsure which valve controls what, don’t guess—move to the next steps and call a pro.

3) Let the boiler cool down

Avoid touching components while the system is hot. Some guidance suggests letting the unit cool before inspection or cleanup. 

4) Contain the water and protect the area

• Put towels or a shallow pan under the drip
• Mop up standing water to reduce slip risk and protect flooring
• If water is near outlets, cords, or wiring, keep people/pets away

5) Don’t keep resetting the boiler

If the boiler has locked out or shut down, repeated resets can mask symptoms and potentially worsen damage. Many homeowner guides advise prioritizing shutdown + diagnosis over repeated resets when a leak is present. 

6) Take a few quick notes (this helps your technician)

When you call for service, this information speeds up diagnosis:

• Where you see water (under boiler, by pump, from relief pipe)
• Whether it leaks only when heating is on
• Current pressure gauge reading (if visible)
• Any error code or warning light

Why boiler leaks require professional repair

Even when the leak looks “small,” the underlying cause often isn’t.

For example:

• A dripping relief valve may be caused by pressure management issues (expansion tank/vessel, overfilling, regulator problems), not the valve itself. 
• Repeated pressure drops can indicate a hidden system leak—and topping up repeatedly can accelerate corrosion inside the system.
• Internal leaks can damage control boards, sensors, pumps, and other components.

A trained boiler technician can:

• Pressure-test and isolate sections of the system
• Verify expansion tank performance
• Inspect the heat exchanger/boiler sections safely
• Repair or replace failed components correctly

If you’re dealing with a boiler leak, it’s often safest to schedule professional boiler repair in Denver.

Final Thoughts

A leaking boiler is never “normal.” While some leaks start small, they rarely stay that way. Shutting the system down, controlling the water, and getting professional help early can prevent costly repairs and system failure—especially during Denver’s colder months.