Expansion tank and air elimination

What is going on

What I usually see wrong on these jobs is air and the expansion tank. Closed-type systems depend on a correctly sized tank and a cushion of air in it, and that air has to be kept out of the piping. Bad sizing or an automatic air vent bleeding the system down gives you noisy pipes, pressure swings and relief valve discharge. These rules cover how to size and connect it.

Likely causes

• New closed-type expansion tank system being sized
• Tank tied to the wrong side of the air separator
• Automatic air vents installed that bleed off the cushion
• No micro-bubble air elimination, so air stays in the loop

Check in this order

1. Size the expansion tank for total system volume, including all piping, fixtures and the boiler. Heat exchanger volume is 2.0 gal (TRX150) or 2.7 gal (TRX199/199C).
2. Connect the tank to the air separator only if the separator is on the pump suction side.
3. Install the system fill connection at the same point as the expansion tank connection.
4. Install a micro-bubble air elimination device on every installation. Automatic air vents alone are not acceptable.
5. Do not install automatic air vents on closed-type expansion tank systems. Air must stay in the system to form the air cushion.
6. Check tank precharge against fill pressure before filling.

More general guides

• Combustion air and leak protection checks during install
• Install and test the CH and DHW relief valves
• Low water cut-off wiring (field supplied)
• Primary/secondary piping and 3-way valve for heating-only use
• Removing the boiler from a common vent system
• Wall mounting the boiler