Circulating Fluid, Flushing, and Purging
What the exam tests
This domain covers fluid selection, antifreeze concentration, flushing, purging, and fill. Expect questions on freeze protection, corrosion inhibition, flow velocity, air removal, and disposal.
Core rules
- Water is the best heat-transfer fluid but freezes; use propylene glycol, ethanol, or methanol where freeze protection is needed.
- Mix antifreeze to the lowest expected loop temperature with a margin; too weak risks freezing, too strong raises viscosity and pump energy.
- Use inhibited HVAC glycol, not automotive antifreeze.
- Flush each loop to remove construction debris, then purge air at adequate velocity; 2 ft/s or more is a common target.
- Fill from the bottom, vent high points, and check pH and inhibitor.
Worked example
A 1.25-in IPS line has about 0.0104 ft² of area. At 10 gpm, flow is 0.0223 ft³/s, so velocity is 0.0223 ÷ 0.0104 = 2.1 ft/s. That meets a 2-ft/s flushing target. If the same loop needed 3 ft/s, flow would need to rise to about 14 gpm.
Traps
Skipping loop-by-loop flushing; using a 50% glycol mix when 20% is enough; leaving air in the loop; using automotive antifreeze; discharging flush water without permits.