A boiler that keeps losing pressure without a visible puddle usually has a leak somewhere less obvious, or a component venting small amounts without a drip.
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A gas boiler losing pressure with no visible leak usually has water escaping somewhere that is not easily seen: a hidden pinhole leak inside a wall or under flooring, a slowly weeping relief valve or automatic air vent that evaporates before pooling, a leak at a radiator valve gland hidden behind furniture, or occasionally a leak inside the boiler itself. A technician traces the loss systematically rather than assuming there is no leak just because none is visible.
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Water loss from a hydronic heating system does not always leave a puddle. A slow enough leak, especially one at a warm pipe or fitting, can evaporate as fast as it seeps out, leaving only a faint mineral stain, a slightly damp patch that dries between visits, or nothing visible at all if it is happening behind a wall, under a floor, or inside the boiler's own casing where it drains away unnoticed. Relief valves and automatic air vents can also weep small amounts intermittently, releasing just enough to explain a slow pressure drop without ever producing a puddle a homeowner would notice.
The most productive places to check are the connections and moving parts rather than open runs of pipe, since pipe itself rarely fails without a joint or fitting nearby doing the same job under similar stress. Radiator valve glands, where the valve stem passes through a seal, are a common slow-leak point, particularly on older systems with cast-iron radiators common in many Vancouver-special and character homes. Circulator pump flanges and seals are another frequent source, since the pump vibrates slightly during operation and can work a gasket loose over years. The pressure relief valve and any automatic air vents are worth checking closely, including underneath, since a slow weep there often runs down the outside of the pipe rather than dripping cleanly off the end.
Less commonly, the leak is inside the boiler itself, at an internal gasket or a developing crack in the heat exchanger, in which case any escaping water may drain internally toward the boiler's own condensate path and never appear outside the cabinet at all, which is one reason a technician's internal inspection matters more here than a homeowner's visual walk-around.
Track the pressure gauge reading at the same point in the heating cycle over several days, since a consistent downward trend over time is more informative than a single reading. Check for faint mineral staining, white or greenish crust, or a musty smell near radiator valves, pipe joints, and the boiler's base, since these can indicate a slow weep that has been evaporating rather than pooling. Avoid repeatedly topping up the system pressure yourself without narrowing down the cause first, since doing so just feeds more oxygen-carrying water into the system and can accelerate internal corrosion while masking how fast the underlying leak actually is.
Most pressure loss traces back to a single component, a valve gland, a relief valve, an air vent, that is a straightforward repair or replacement. If a technician finds the loss is happening inside the boiler's heat exchanger itself, that is a more significant finding, since heat exchanger leaks are generally not economical to repair on most residential boilers and point toward replacement, which ballparks $7,500 to $15,000 or more depending on system size, not a quote. Confirming the source before assuming the worst is exactly why proper diagnosis matters here rather than guessing.
Safety note
A relief valve discharging hot water or steam is a safety device functioning correctly, not something to cap or disable. If you ever smell gas near the boiler while investigating a pressure issue, leave the home immediately and call FortisBC at 1-800-663-9911.
A well-sealed system should hold pressure steady between refills for months. Needing to top up more than once every few months, or a clear downward trend within days, points toward a real leak worth investigating.
CANRO does not recommend sealant additives in a hydronic heating system, since they can clog circulator pumps, valves, and heat exchanger passages rather than reliably fixing the leak.
Most residential gas boilers are entirely indoors, so pressure loss almost always traces to something inside the home's piping, the boiler itself, or occasionally an outdoor condensate line rather than an external component.
Occasional topping up per the boiler manual is generally fine short-term, but repeated frequent refills without diagnosis both mask the leak's severity and introduce more oxygen into the system, so it should not go on indefinitely.
Pressure testing, sometimes combined with a dye or UV tracer added to the system water, lets a technician find where pressure escapes even when no visible puddle has formed.
If pressure testing finds no leak, a failing expansion tank or a venting air separator issue can sometimes explain pressure behaviour that looks like a leak, and the technician checks those components next.
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