Quick answer
A sealed hydronic system only loses pressure when water leaves it, or when the expansion tank stops doing its job. Look for a leak first — radiator valves, circulator flanges, the relief valve discharge pipe, a damp patch under a pipe run — then check the expansion tank, which is the single most common cause of a gauge that keeps falling.
A hot-water boiler is a closed loop, so the water in it should stay in it. Most residential systems are set to roughly 12 psi cold and climb into the high teens or low twenties once hot, and that swing is normal. Pressure that keeps falling means water is escaping — or that the air charge behind the expansion tank's bladder has gone, which produces pressure loss with no puddle to find.
Start with the expansion tank
The expansion tank absorbs the volume change as water heats. Inside is a rubber bladder with an air charge behind it, factory-set to match the system fill pressure. When the bladder tears or the air leaks away, the tank fills with water and has nothing left to give: pressure spikes when the boiler fires, the relief valve dumps the excess down its discharge pipe, and once everything cools you are short again.
- Tap the tank. A healthy one rings hollow near the top and sounds solid at the bottom; a waterlogged one sounds solid all the way up.
- Press the pin on the Schrader valve underneath — the same fitting as a tire valve. Air means the bladder is intact. Water means the tank is finished.
- Check the pre-charge with a tire gauge on a depressurized tank; it should match the cold fill pressure the boiler was commissioned at.
- Look for drips at the relief valve or inside its discharge pipe. Chronic weeping there usually traces back to the tank rather than to the valve itself.
Replacing an expansion tank is routine work and sits in the middle of the bands on our boiler repair cost page. Leaving it takes out the relief valve next, and eventually the boiler.
Where hydronic leaks hide
- Circulator flange gaskets and pump seals. A rust stain under the pump is the giveaway, because a slow hydronic leak evaporates off hot iron and leaves the mineral behind rather than a puddle.
- Radiator valve packing and bleed vents, especially on old cast-iron radiators that get bumped by furniture or bled every autumn.
- Automatic air vents. The small float vents at system high points fail open as they age and start venting water instead of air. They are cheap, and they are often the answer.
- The relief valve itself, once a bit of debris has passed across its seat and stopped it sealing.
- Threaded joints and unions near the boiler, which get disturbed by every repair the system has ever had.
- Embedded PEX or copper in a slab. In 2000s-and-newer homes in Alton Village and Binbrook with radiant basement floors, a slab leak shows up as pressure loss with nothing visible — finding it takes zone-by-zone pressure testing and sometimes a thermal camera.
- An indirect water heater's coil, where the boiler also makes domestic hot water. A pinhole there pushes system water into the potable side, so you lose pressure with no leak anywhere in the building.
The auto-fill valve that hides the problem
Most systems have a pressure-reducing valve on the cold-water feed that tops the boiler up automatically. It is convenient, and it is also how a small leak goes unnoticed for years while it quietly destroys the boiler, because the fresh water it admits carries oxygen and, on Halton and Hamilton's hard lake water, minerals that scale the hottest surfaces.
There is a simple test. Close the isolation valve on that feed for a week and watch the gauge. If pressure falls with the fill valve shut, you have a leak the auto-fill has been covering. That test is also why we ask about top-up habits when a boiler shows corrosion damage well ahead of its expected service life — it is rarely bad luck, and usually years of make-up water.
Pressure changes you can safely ignore
- The gauge climbing while the boiler runs and settling back as it cools. That is the expansion tank doing exactly what it is there for.
- A small drop over the first week after a repair or a round of radiator bleeding, as trapped air works its way out through the vents.
- A seasonal dip when the system has been sitting cold since April and is fired up in October.
- A reading that differs from another gauge on the same system. Cheap gauges drift, and several psi of error is common.
When to stop topping it up and call someone
Refilling a boiler takes two minutes, and doing it once at the start of the season is fine. Doing it weekly is not. If you have added water more than twice in a heating season, if the relief valve has discharged, or if pressure will not hold overnight with the fill valve closed, the system needs a technician rather than another top-up. The same goes for pressure that climbs past the relief valve setting — typically 30 psi on a residential boiler — which means something is feeding the loop uncontrolled.
If the boiler has locked out on low pressure and the house is going cold during a January stretch, treat it as a no-heat emergency. Frozen pipes cost far more than the repair ever would, and how to keep pipes from freezing when the heat fails is worth reading before you need it rather than during.
ZK Mechanical pressure-tests hydronic systems, replaces expansion tanks and relief valves, and tracks down the leaks that do not announce themselves. We answer the phone around the clock with no overtime premium at (647) 801-1252, and you can request a visit online any time. Our boiler repair service covers this work across Burlington, Oakville, Hamilton and the wider GTA.