Filling and purging the heating loop

What is going on

Fill and purge the heating loop before the first start. Air left in the loop gives noisy piping, no flow and cold radiators.

Likely causes

• New install, loop filled for the first time
• Loop drained for service
• Air trapped after a leak repair or pressure loss
• Expansion tank precharge not matching system pressure

Check in this order

1. Make sure the expansion tank pressure matches system operating pressure before filling, because a wrong precharge causes pressure swings later.
2. Open the manual fill valve left of the domestic water inlet, or use a pressure reducing/automatic fill valve between the heating and domestic loops, and fill to 16 psi.
3. Loosen the 1/4 inch cap on the automatic air vent to let air escape while the loop is pressurized.
4. Power the unit to start the De-air sequence, which cycles the primary pump on and off for 14 minutes.
5. Fit a 3/8 inch I.D. drain hose to the manual air bleed valve barb and turn the valve counterclockwise at least three turns to bleed air and water.
6. Watch the gauge during de-air and top up to 16 psi if it drops.
7. Retighten the air vent cap once the loop runs quiet and clear.

More boiler guides

• 0-10 VDC input to modulate firing rate (Mode 5)
• Annual maintenance and inspection
• Blank display
• Boiler pressure keeps dropping / FILL shows
• Boiler shows FILL and will not fire
• Cleaning the combustion side of the heat exchanger