Boiler expansion tank: what it is and how it fails
The grey tank above this boiler is the expansion tank, and the small brass cap on the piping is the automatic air vent. Between them, they keep the water in your heating system at a steady pressure with the air bled out.
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From one of our jobs
The grey tank and the air vent over the boiler
Right now: Check the boiler’s pressure gauge. If it jumps high every time the boiler runs, the expansion tank may be waterlogged.
Water grows as the boiler heats it. The expansion tank has an air cushion inside that squeezes to make room, so the pressure doesn’t climb. When that cushion is gone, the tank fills with water, the pressure spikes every time the boiler fires, and the relief valve on the boiler starts to drip.
The air vent lets out air bubbles that collect at high points in the piping. Air left in the system is what gurgles in the pipes and keeps a radiator cold at the top.
Quick answers
What people ask about boiler expansion tanks
What does the expansion tank on a boiler do?
It gives the heating water room to grow as it warms up. Without it, the pressure in the system would climb every time the boiler runs.
How do I know if my boiler expansion tank is bad?
The pressure gauge climbs well past where it sits cold, the relief valve drips or spits, or the tank sounds solid and heavy when you tap the top. A good tank sounds hollow up top.
Why are my boiler pipes gurgling or banging?
Gurgling is usually air in the system, which is the air vent’s job. Banging can be air too, or pressure swings from a waterlogged expansion tank.
Is a dripping boiler relief valve dangerous?
It’s doing its job, letting out pressure the system can’t hold. Don’t plug it or cap it. Call, because the cause is usually the expansion tank or the fill valve.
Who do I call for boiler repair in Northern Virginia?
Call SwiftPro at (703) 997-9757. We service and repair gas and propane boilers across Northern Virginia.
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SwiftPro Heating, Cooling & Plumbing
4230 Lafayette Center Dr N, Chantilly, VA 20151
44679 Endicott Dr, Suite 211, Ashburn, VA 20147
SwiftPro Plumber & Water Heaters
4200 Lafayette Center Dr, Suite N, Chantilly, VA 20151
Virginia Class A contractor license #2705192276