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Boilers · Pressure ControlBoiler Expansion Tank Replacement
Boiler expansion tank replacement is needed when the tank is waterlogged: the gauge reads normal with the boiler cold, climbs toward 30 psi as it heats, and the relief valve starts to drip. A tank that has lost its air charge or torn its diaphragm can no longer absorb the water's expansion. A new diaphragm tank, pre-charged to match the system's fill pressure, falls within the $150–$800 range for typical boiler repairs and stops the pressure swing at its source.
Water expands by a few percent between room temperature and boiler temperature, and in a sealed system that extra volume needs somewhere to go. The expansion tank is that place: a steel shell divided by a rubber diaphragm, with system water on one side and a cushion of air on the other. When the air is gone or the diaphragm tears, the tank fills solid with water and every heating cycle drives the pressure up until the 30 psi relief valve opens. The mechanics are laid out in our boiler expansion tank glossary entry.
Two quick checks separate a dead tank from a tired one. Press the air valve on the tank for an instant: water spitting out means the diaphragm is torn and the tank is finished. If only a weak puff of air comes out, the charge has leaked down and the tank may be recharged, but the pre-charge can only be measured with the tank isolated from the system and its water side drained to zero. A gauge on the air valve while the tank is still under system pressure simply reads the system.
A new tank ships at 12 psi, which matches the 12 psi cold fill that suits most two-storey houses. A three-storey pre-war house with radiators on the top floor needs a higher fill pressure to lift water that far, and the tank's charge has to be raised to the same figure before it is connected. Size matters as well: a house full of cast-iron radiators and large-bore pipe holds several times the water of a copper baseboard loop, and a tank sized for the smaller system runs out of room on every cycle.
Older hot-water systems, the kind found in pre-war brick houses in Hamilton and Old Oakville, may still have a steel compression tank strapped between the basement joists. It has no diaphragm; air sits directly on the water and is slowly absorbed, so the tank waterlogs every few years and the cure is draining it to restore the cushion, not replacing it. Automatic air vents should not be added to such a system, because they bleed off the very air the tank depends on. When a compression tank is finally retired, we fit a diaphragm tank together with an air separator as part of boiler repair.
Signs the expansion tank has failed
- Pressure near 12–15 psi cold that climbs past 25 psi as the boiler heats, then falls again as it cools
- The relief valve drips or discharges only while the boiler is hot
- Water, not air, at the tank's air valve
- A tank that feels heavy and sounds dull along its whole length when tapped
- The automatic fill valve hisses after each cycle, replacing water the relief valve just dumped
How it works
Note the pressure cold and again when the boiler is hot, and tell us both numbers. A swing of more than a few psi points at the tank before we arrive.
The technician isolates the tank, drains its water side and reads the pre-charge. A torn or undersized tank is replaced with one sized to the system's water volume and charged to the cold fill pressure.
The system is refilled, air is purged, and the boiler is run to full temperature to confirm the pressure now rises only a few psi. For a visit, contact the shop.
Common questions
Can I just add air to my expansion tank?
Only if the diaphragm is intact, and only with the tank isolated and drained on the water side. Pumping air into a tank that is still open to the system changes the gauge reading without restoring the cushion. If water comes out of the air valve, no amount of air will fix it; the diaphragm has failed and the tank needs replacing.
Why does my relief valve drip only when the heat is on?
Because that is when the water expands. With a working tank the pressure rises a few psi from cold to hot; with a waterlogged one it runs up to the valve's 30 psi setting. The drip stops as the boiler cools, and the fill valve then tops the system up with fresh water. Why boiler pressure gets too high covers the other causes.
Is a boiler expansion tank the same as the one on a water heater?
No. They work on the same principle, but a water heater tank is built for drinking water and for much higher pressure, and a boiler tank is not rated for either. The two are not interchangeable. The potable version is described under water heater expansion tank installation.
Should the relief valve be replaced with the tank?
Usually. A valve that has been opening every cycle has passed rusty water and grit across its seat and often will not seal fully again, so it keeps weeping even once the pressure is under control. We check it after the new tank is in and replace it if it drips — see boiler pressure relief valve replacement.
How long does an expansion tank last?
There is no fixed figure. Diaphragm tanks slowly lose charge through the rubber and the air valve, and a tank that is checked and topped up during an annual boiler service lasts much longer than one ignored until the relief valve drips. High fill pressure and an undersized tank both shorten its life.