Hot Water System Low-Flow Fix (Air Purge + Heater Inlet Filter Cleaning)

PROBLEM Hot water loop flow at the A.O. Smith ATI-540H-N tankless heater (buffer tank retrofit with Taco 006 circulator) was only 0.4-0.9 GPM, dropping to zero until a cold-side valve was opened. The heater needs about 0.5 GPM to activate and 0.4 GPM to stay running, and the manual lists 2-4 GPM through the unit for recirculation. Flow also decayed after about a week. DIAGNOSIS 1) Air trapped at the highest point of the loop. Dissolved air comes out at 140F and collects at the crest; low flow cannot push it down the drop to the heater, so the pump loses prime. 2) Clogged cold-inlet wire mesh filter on the heater, restricting flow. Readings before the fix (hose draining to a bucket, 62-64F in, 136-140F out): 0.6, 0.8, 0.9 GPM. PROCEDURE A. Purge air: open the service-valve hose port on the heater cold side and let water run until air stopped (flow jumped to 1.0-1.2 GPM with the port open). B. Clean the heater inlet filter: power off the heater, close the gas and both water service valves, relieve pressure through the hose port, unscrew the filter housing counterclockwise, rinse and brush the mesh clean, reinstall hand-tight with the O-ring seated, reopen valves slowly, bleed air at the port, restore power. C. Re-test flow and temperatures. RESULT Flow after purge and filter cleaning: 1.4 GPM, then 1.6 GPM (64F in, 138F out, 140F setpoint). Improved from 0.4-0.9 GPM; still below the 2-4 GPM manual target, so follow-up items remain. PARTS ORDERED (not yet installed) Caleffi PLUMBVENT potable automatic air vent (NA502640A), Viega 79590 press tee, Y-strainer. Install order on the loop: strainer first, then the tee with the PLUMBVENT upright at the highest point of the loop (storage tank to heater line). FOLLOW-UP - Install the automatic vent, tee and strainer. - Inspect the check valve on the riser, confirm all valve handles are fully open, check pump cartridge (005-020RP) and confirm Taco 006 is sized for the heater pressure drop (ask A.O. Smith / check pump curve). - Check and clean the inlet filter regularly (weekly until stable, then every few months). - Descale the heater every 2-3 years (soft NYC water); check tank anode at 4-5 years. - Log flow, inlet and outlet temperatures after each service. - Confirm tank thermostat versus 140F heater setpoint and the AngleMix at 120F.