How To Find A Leak In Central Heating System

Finding a leak in your central heating system involves checking for dropping boiler pressure (below 1 bar), inspecting visible pipework for damp patches, isolating different zones to narrow down the location, and using methods like pressure testing or UV dye to pinpoint hidden leaks. Most leaks occur at joints, radiator valves, or in older copper pipes under floors – and ignoring them costs an average of £1,500 in water damage within six months.

Signs You've Got a Central Heating Leak (Not Just Boiler Issues)

Let's be clear about what actually indicates a leak versus normal system behaviour. I've had customers panic over condensation that's perfectly normal, whilst others ignore genuine leaks destroying their joists.

The crucial distinction? True leaks cause pressure loss whether the heating's on or off. If pressure only drops when the system's hot, you're likely looking at expansion vessel failure, not a leak. This single fact saves homeowners thousands in unnecessary leak detection.

Step 2: Zone Isolation Testing (1-2 hours)

This is where we get clever. Instead of guessing, we systematically eliminate sections of your system. Here's how:

1. Record starting pressure (write it down – memory fails)
2. Turn off all radiator valves (both flow and return sides)
3. Re-pressurise to 1.5 bar
4. Wait 2 hours – pressure stable? Leak's in a radiator circuit
5. Open radiators one by one – pressure drops? Found your zone

Step 3: The Pressure Test (Most Accurate DIY Method)

If you're serious about finding that leak yourself, invest £30 in a pressure testing kit. Here's the professional method simplified:

Finding Hidden Leaks Under Floors (Without Destruction)

This is where most DIY attempts fail and professionals earn their money. But there are methods you can try before calling in the cavalry.

The UV Dye Method (£15 Investment)

Add fluorescent dye (typically fluorescein) to your system, run it for 24 hours, then use a UV torch to trace the leak. Sounds simple? Here's what they don't tell you:

• Only works if you can access the leak area
• Dye can stain if leak is behind plasterboard
• Some boiler manufacturers void warranties if dye is used
• Won't work through concrete floors

Better for accessible pipework than truly hidden leaks, but I've seen it work brilliantly for finding leaks inside walls where there's access from below.

Testing the Expansion Vessel

Since 40% of pressure loss calls are actually expansion vessel failures, here's how to test yours properly:

1. Turn off boiler and drain system pressure to zero
2. Find the Schrader valve on expansion vessel (like a car tyre valve)
3. Test with tyre pressure gauge – should read 1-1.5 bar
4. If water comes out instead of air – vessel's failed, needs replacing
5. If pressure's low – pump up with foot pump to correct pressure

When Professional Detection Becomes Essential

After exhausting DIY methods, here's when you need specialist help and what we actually do:

Professional Detection Arsenal

Thermal Imaging: Our thermal cameras see temperature differences through floors. Hot water leaks show as warm patches, making them visible without lifting a single board. Last week, found three leaks in a Bodmin bungalow in 20 minutes – owner had already lifted half the laminate searching.

Tracer Gas Detection

We drain your system, pressurise with nitrogen/hydrogen mix (5% hydrogen, 95% nitrogen – safe, non-flammable), then use sensitive detectors to find where gas escapes. The beauty? It works through concrete, wood, even tiles. Our tracer gas leak detection finds leaks others miss.

Acoustic Leak Detection

Using ground microphones, we listen for the distinctive sound of escaping water. Different from DIY stethoscopes – our equipment filters background noise and amplifies leak signatures. Particularly effective for detecting larger leaks under solid floors.

Combi Boiler Leak Detection (Special Considerations)

Combi boilers add complexity because they're connected to both mains water and the sealed heating circuit. Here's how to determine which side is leaking:

Underfloor Heating Leak Detection

UFH leaks are my speciality – they're complex but follow patterns. After investigating hundreds, here's what you need to know:

Manifold Testing (Always Start Here)

1. Turn off all loops at manifold
2. Pressurise system to 2 bar
3. Open loops one at a time
4. Watch pressure gauge – drops indicate leaking loop
5. Mark faulty loop for detailed testing

Found a leak in St Austell last month where only one loop of twelve was failing. Saved the customer from replacing the entire system – just one loop repair needed. This method is crucial for underfloor heating leak detection.

Warning Signs Specific to UFH

• Cold spots on floor: Dead giveaway of loop failure
• Manifold constantly dripping: Often air, not always leak
• One room never heats properly: That loop's compromised
• Calcium deposits at manifold: Indicates slow weep

For comprehensive UFH troubleshooting, see our guide on underfloor heating leaks: what happens and what to do.

Older Systems: F&E Tanks and Gravity-Fed Circuits

Pre-1990s houses often have vented systems with feed and expansion tanks in the loft. These present unique challenges:

F&E Tank Constantly Filling?

This usually means:

• Leak in the system (water escaping)
• Faulty coil in hot water cylinder (mixing with domestic water)
• Ballcock valve not shutting properly

The Test: Mark water level in F&E tank with tape. Turn off mains supply to tank. Level drops = system leak. Level rises = cylinder coil failure.

Microbore Systems (8mm/10mm Pipes)

Popular in the 1970s-80s, these thin-wall copper pipes are now reaching failure age. Common issues include:

• Pinhole leaks from corrosion – especially at manifolds
• Small diameter means rapid pressure loss – even tiny leaks cause problems
• Difficult to detect – requires high-sensitivity equipment
• Often run in floor screed – expensive to access

Why Leaks Appear in Autumn (And What It Means)

September's my busiest month. Here's why heating systems that worked fine all summer suddenly leak:

1. Thermal expansion/contraction: Summer heat expands joints, autumn cooling contracts them
2. Corrosion acceleration: Oxygen entering during summer non-use causes rust
3. Dried seals: Pump and valve seals dry out without water circulation
4. Pressure changes: First heating cycle stresses weakened components

This is why I recommend running your heating monthly, even in summer – just 30 minutes prevents most autumn failures. If your boiler pressure keeps dropping after summer shutdown, check pump seals first.

The Real Cost of Ignoring Central Heating Leaks

Let me paint you a picture from last month's job in Newton Abbot. Customer ignored a "small" leak for eight months. Here's the final bill:

The kicker? Early detection would have saved thousands. The repair? A corroded 15mm elbow joint – £3 part, 30-minute fix. But eight months of water had destroyed everything around it.

Prevention: Stop Leaks Before They Start

After three decades, I can predict which systems will leak. Here's how to prevent yours joining the statistics:

Systems with proper inhibitor levels last 20+ years. Without it? I've replaced 8-year-old systems riddled with pinhole leaks. That £20 annual inhibitor dose saves £8,000 in premature replacement.

The Truth About Leak Sealers (From Someone Who's Seen The Aftermath)

Leak sealers are controversial. Here's my honest assessment after seeing hundreds of "sealed" systems:

When They Work:

• Tiny pinhole leaks in radiators
• Minor weeps at compression joints
• Emergency temporary fix only
• Systems being replaced soon anyway

When They Destroy Your System:

• Block thermostatic radiator valves (£50 each to replace)
• Clog pump impellers (£300 replacement)
• Coat heat exchanger (£800 replacement)
• Make future leak detection impossible

Insurance and Trace & Access Coverage

Most UK home insurance includes "trace and access" coverage – typically £5,000-10,000 for finding and accessing leaks. But here's what insurers don't advertise:

• Only covers investigation and access – not the repair itself
• Requires evidence of sudden damage – gradual leaks often rejected
• Professional report usually required – DIY detection not accepted
• Excess applies – typically £100-350

Our trace and access service provides insurance-compliant reports with thermal imaging evidence. Worth knowing: insurers prefer non-destructive detection – it reduces their reinstatement costs. For more details, see our guide on what trace and access coverage actually means.

When to Stop DIY and Call Professionals

Pride costs money. Here's when to admit defeat and call in expertise:

The average UK household wastes £300 annually on undetected leaks. Professional detection prevents thousands in damage. When you factor in water bills, energy waste, and structural damage, early professional detection pays for itself within months. Always ensure any work on gas appliances is done by Gas Safe registered engineers.

Your Action Plan: What to Do Right Now

If you're reading this with dropping pressure or suspicious damp patches, here's your immediate action plan:

1. Document everything: Photo damage, note pressure readings, record dates
2. Isolate the problem: Use zone testing method above
3. Check insurance policy: Look for trace and access coverage
4. Attempt visual detection: 30 minutes could save hundreds
5. Set a deadline: If not found in 48 hours, call professionals

Remember, every day a leak continues adds to repair costs. Water doesn't improve with time – it finds new places to cause damage. That "small" leak you're ignoring? It's costing you £5-20 daily in water alone, plus energy waste heating water that's escaping.

The difference between a £300 repair and £10,000 disaster? Acting quickly when you first notice the signs. After 30 years finding leaks others missed, I can tell you this: the leak's always there, waiting to be found. Whether you find it before it destroys your home is up to you.