Boiler Losing Pressure?

1. Leaking Radiator Valve

The most common and easiest-to-find leak. Radiator valves — especially thermostatic radiator valves (TRVs) — develop small leaks at the gland nut or valve stem over time.

• How to check: Look for water stains, rust, or dampness around every radiator valve. Run a dry tissue around the valve body and nut.
• Fix: Sometimes tightening the gland nut stops it. If not, the valve needs replacement — $100-$200 per valve including labor.
• DIY? You can tighten the gland nut gently. Valve replacement requires draining the system — call us.

2. Leaking Pressure Relief Valve (PRV)

The PRV is a safety device that opens at ~3 bar to prevent overpressure. If it's leaking, you'll see water dripping from the copper discharge pipe outside your house.

• Cause: Expansion vessel failure (most common), filling loop left open, or the PRV itself has failed (didn't reseat after opening).
• Fix: Diagnose the root cause first (usually expansion vessel). Don't just replace the PRV — it's a symptom, not the cause. $300-$600 for expansion vessel replacement.
• Note: If the PRV has opened even once, it may not seal properly again and should be replaced regardless.

3. Failed Expansion Vessel

The expansion vessel contains a rubber diaphragm with an air charge on one side and system water on the other. When the diaphragm ruptures or loses its air charge, water has nowhere to expand when heated, causing pressure to spike, the PRV to open, and system pressure to drop when cold.

• Symptoms: Pressure rises dramatically when heating (above 2.5 bar), then drops to near zero when cold. You hear water sloshing in the expansion vessel when tapped.
• Fix: Repressurize if diaphragm intact ($100-$200). Replace if ruptured ($400-$700).

4. Leaking Pipe Joint or Fitting

Copper pipe joints, compression fittings, and push-fit connectors can develop slow leaks over time — especially if the system was installed with flux residue that corroded the joint.

• How to check: Look for green or white deposits on copper joints, damp patches on walls or ceilings below pipe runs, or musty smells near pipework.
• Fix: Individual joint repair $150-$300. Multiple joints may indicate systemic corrosion — power flush and inhibitor treatment recommended.

5. Heat Exchanger Leak (Internal)

The most serious — and expensive — cause. A cracked or corroded heat exchanger leaks water internally, often into the combustion chamber where it evaporates. You may not see water, but you'll see pressure dropping.

• Symptoms: Pressure drops with no visible external leak. Boiler may produce steam from the flue. Combustion analysis shows unusual readings.
• Fix: Heat exchanger replacement if the boiler is under 8 years old and still under warranty. If over 10 years and out of warranty, boiler replacement is usually more cost-effective.
• Cost: $1,500-$3,000 for heat exchanger replacement. $6,500-$12,000 for complete boiler replacement.

6. Filling Loop Left Open or Passing

The filling loop connects your boiler to the mains water supply. If it's left slightly open, or the valve is passing water, mains pressure can over-pressurize your system and cause the PRV to open.

• How to check: Ensure the filling loop handle is perpendicular to the pipe (closed). If the valve feels loose or doesn't close fully, it may be passing.
• Fix: Replace the filling loop valve — $100-$200. Simple fix, but often overlooked.

Frequently Asked Questions