As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
UPON USING APU BLEED AIR, THE FLIGHT DECK WAS INUNDATED WITH ICE PELLETS AND AIR COMING THROUGH THE VENTS AT APPROX 5-10X NORMAL VOLUME AFTER TWO CYCLES WE TURNED OFF APU BLEED AIR AND CALLED MX. GROUNDED - YES. REMOVED AND REPLACED 2 EACH WATER EXTRACTOR ON L/H AIR COND PACK OPS AND LEAK CHECK GOOD, NO ICE PELLETS NOTED IN COCKPIT REF B737-800 AMM 21-51-40-00-801.