Expansion Tank Service
Servicing diaphragm and bladder expansion tanks including air charge verification, sizing calculations, waterlogged tank diagnosis, and replacement procedures for hydronic heating systems.
- Diagnose a waterlogged expansion tank using pressure gauge readings and symptoms
- Check and adjust the air charge on diaphragm and bladder expansion tanks
- Size replacement expansion tanks based on system volume, temperature range, and pressure settings
- Properly install expansion tanks with correct orientation, location, and isolation valves
Lesson 1
Expansion Tank Function and Types
Why Expansion Tanks Are Necessary
Water expands when heated. In a closed hydronic system, this expansion must be absorbed somewhere or the system pressure will rise uncontrollably, causing the relief valve to discharge. The expansion tank provides a compressible air cushion that absorbs the increased volume as water heats up and returns it as water cools down.
Expansion Tank Types
Diaphragm tanks have a flexible rubber diaphragm that separates the air charge from the system water. The air side is pre-charged at the factory. As water expands, it pushes against the diaphragm, compressing the air. These are the most common type in residential systems.
Bladder tanks use a rubber bladder inside a steel shell. Water fills the bladder, and the air charge surrounds the outside of the bladder. The bladder prevents air from being absorbed into the water, maintaining the air charge indefinitely. Bladder tanks are preferred for larger systems.
Plain steel tanks (obsolete) - Older systems may have a large unpressurized steel tank mounted high in the system with no separation between air and water. The air cushion gradually dissolves into the water, requiring periodic draining and air recharging. These are being replaced with diaphragm or bladder tanks during system upgrades.
Diaphragm/Bladder Tank (Modern)
Air-water separation: Physical barrier (rubber)
Air charge maintenance: Excellent - air stays separate
Size: Compact (2-15 gallon typical)
Location: Near boiler, on supply or return
Maintenance: Annual air charge check
Plain Steel Tank (Obsolete)
Air-water separation: None - air sits on top of water
Air charge maintenance: Poor - air absorbs into water
Size: Large (15-50 gallon)
Location: Highest point in system
Maintenance: Frequent draining to restore air
Diaphragm and bladder expansion tanks use a physical barrier to keep the air charge separated from system water - the air pre-charge must equal the system cold fill pressure (typically 12 psig) and should be checked annually.