Fired up for the season- drained, powerwashed, re-filled, opened valves, bled water out of all 4 sand filters. Pump on- good flow. Checked for more air in sand filters and voila- 3 of 4 never seemed to get all the air out, even after letting the air/water mix bleed for an hour. 1 of 4 has never filled with air. Ran pump for week and everyday same 3 of 4 have air trapped. Water levels have never dropped below skimmers and vac lines have been valved off. What's strange to me is when the pump is off, I can bleed all air out, but as soon as pump is on, air comes back. I've backwashed several times and forced air out, but back to filtering and the air returns. Next things I plan to check were leaking/sucking backwash valve and to flush the vacuum lines...what else should I look for? Also, one return/effluent line has been blowing quite a bit of air for the same duration into the pool.
FYI- 163,000 gal, Im in MD, 7.5 Hp (older then dirt) pump, liquid chlorine, pumps/filters all below waterline.
The filters are Swimquip High-rate HRP-30's. The sand was replaced 3 seasons ago.
The head guard doesn't remember the trapped filter air last year (which was her first), but does remember lots of air blowing out one return/effluent line (my predecessor told her not to worry about it...).
Thanks for any direction!