Water expands about 4 percent heating from cold to 200 degrees F, and without somewhere to go, pressure would shoot up and open the relief valve. Diaphragm and bladder tanks hold a precharged air cushion that should match fill pressure at the tank, commonly 12 psi for a two-story house. A waterlogged tank, with its air charge gone, gives pressure spikes on every firing cycle.
Tap the expansion tank, if it's full of water top to bottom the bladder's gone, that's your dripping relief valve.
Check the precharge with the tank isolated and drained of water pressure, or the reading is meaningless.
Also heard as
- ex tank
- X-tank
- expansion vessel
Related terms
Point of no pressure change
The place in a closed hydronic loop, where the expansion tank connects, whose pressure the pump cannot change.
Relief valve
The pressure-relief valve on a hot water boiler, rated in Btu per hour and set at or below the boiler's maximum pressure.
Pop-off
Shop slang for the safety valve on a steam boiler or the relief valve on a hot water boiler.
Air separator
A hydronic fitting that pulls air bubbles out of circulating water and sends them out through an automatic vent.