Why does my boiler lose pressure overnight?

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What Boiler Pressure Actually Means

A residential boiler is a sealed, closed-loop system: the same water circulates through the boiler, pipes, and radiators or baseboards over and over. The pressure gauge on the front of the unit tells you how much force that water is exerting on the system, and it should sit within a manufacturer-specified range, commonly around 12-15 PSI when the system is cold, rising a bit once it heats up. Pressure naturally shifts a little with temperature, but a sealed system should hold roughly steady from night to night. When you see the needle noticeably lower every morning, water (or the air that keeps the system pressurized) is leaving the loop somewhere it should not be.

Common Reasons a Boiler Loses Pressure Overnight

Overnight pressure loss usually comes down to one of a handful of culprits. Most involve a slow water leak, though a few are mechanical failures that do not always leave a visible puddle.

• A weeping pressure relief valve (PRV) — the valve is designed to open under excess pressure, but a worn seat can let it seep continuously, especially at night when the system cycles less
• A pinhole leak in a radiator, baseboard, or copper fitting — common in older Metro Vancouver homes where hard water has scaled and thinned pipe walls over years
• A failing or waterlogged expansion tank — if the tank's internal air bladder has lost its charge, the system cannot absorb normal pressure swings and the boiler compensates by venting
• A loose or corroded auto air vent — these bleed trapped air automatically but can also weep water once the seal degrades
• A leaking circulator pump seal — pumps have a shaft seal that wears over time and can drip slowly, often pooling near the pump housing
• Loose unions or fittings that were never fully tightened, which tend to show up as a slow drip rather than a sudden loss

What You Can Safely Check Yourself

Before calling for service, a homeowner can do a quick, safe visual inspection without touching any gas or electrical components.

• Look at the pressure gauge at the same time each morning for several days and write down the readings — a consistent nightly drop versus a one-time dip tells a technician a lot
• Check the floor and walls around the boiler, expansion tank, and any visible pipe runs for damp spots, mineral staining, or small puddles
• Look at the pressure relief valve's discharge pipe for drips or mineral crust, a classic sign of a weeping valve
• Check around the circulator pump for moisture at the pump housing
• Note whether radiators are cooler at the top (often trapped air, not necessarily a leak) versus cooler throughout (can point to low system pressure)

What you should not do yourself: bleed or repack a pressure relief valve, replace an expansion tank, add pressure through the fill valve repeatedly without knowing why it is low, or open any part of the gas train or wiring. In BC, gas fitting work and most boiler component repairs must be carried out by a Technical Safety BC licensed gas fitter, and attempting this work yourself can void warranties and create real safety hazards.

What to Expect From a Professional Visit

A licensed technician diagnosing overnight pressure loss will typically pressure-test the system to isolate whether the leak is in the boiler itself, the piping loop, or a component like the expansion tank or PRV. This often includes dye testing, visual tracing along accessible pipe runs, and checking the expansion tank's air charge with a gauge. In condos and townhomes, the technician will also confirm whether any of the loop runs through walls or under flooring, since access can affect both diagnosis time and repair approach. Once the source is confirmed, the technician can quote the repair, whether that is a valve or seal replacement, a fitting repair, resealing a pinhole leak, or recharging or swapping the expansion tank.

BC Permit and Licensing Notes

Because a boiler is a gas-fired appliance, any work that involves the gas line, burner, or venting requires a Technical Safety BC licensed gas fitter, and larger repair or replacement work on the boiler itself may require a permit through Technical Safety BC depending on scope. CANRO Heating & Cooling holds Technical Safety BC gas licence LGA0043870 and boiler licence LBP0046450, and our Red Seal certified technicians handle both the diagnosis and any permitted repair work, so you are not left coordinating separate trades or permit paperwork.

When It's More Than Just a Leak

Occasionally, chronic low pressure combined with other symptoms, banging or gurgling noises, the boiler locking out on a fault code, or radiators that never get fully hot, points toward a bigger issue than a single leak, such as sludge buildup restricting flow or a boiler nearing the end of its service life. If your boiler is older and repairs are becoming frequent, it is worth asking your technician for an honest assessment of repair versus replacement. High-efficiency boiler replacement in Metro Vancouver typically runs $7,500-$15,000+ installed before rebates, depending on sizing, venting requirements, and gas line work; a full new boiler installation runs $6,500-$14,000+. Current programs like the interest-free Canada Greener Homes Loan (up to $40,000) and the Oil to Heat Pump Affordability program, along with CleanBC, BC Hydro, and FortisBC rebates, may apply depending on your situation — program details and amounts change, so it's worth confirming the current figures at quote time rather than assuming a specific number.

Frequently asked questions

Reviewed by the CANRO Heating & Cooling service team — June 30, 2026. CANRO holds Technical Safety BC gas licence LGA0043870 and boiler licence LBP0046450, employs Red Seal certified technicians, and carries WorkSafeBC and liability insurance. 4.9★ from 85+ Google reviews · serving Metro Vancouver since 1999 · (604) 359-1081 · info@canroheat.ca.