hello all, i am having a recurring problem with losing water from my system which (correctly) shuts my boiler down. i have a mod/con boiler (triangle tube solo 110) with pumps for each zone, which services one floor of staple-up w/plates and two floors with radiators (plus DHW via indirect tank). i suspect that the water comes out of either the air vent on the indirect tank or the boiler. there are no other leaks in the system. i have not witnessed it happening but have seen the aftermath which made me rule out the prv. i inherited this system from the last owner and unfortunately the installer has since retired/moved so i can't go to them for answers. two pieces of info that i suspect may be relevant;#1, at times i see the supply temp going to the top floor is ~20deg warmer than the second floor and ~25deg warmer than the first floor (with return temps ~10deg cooler for all three floors) and #2 i see that the pump for the top floor is different (grundfos up15-100) from the lower floors (up 15-42). the psi readings always stay steady at ~14lbs. once i let (what sounds like a little) make-up water back in, the boiler fires right back up. other than this the system seems to run smooth, the heat demand is met, etc.. any ideas what may be causing the air vents to be going off? can they be letting that much water out? it seems to happen about weekly/bi-weekly. any suggestions or diagnosis much appreciated

System may have not been purged with warm water (boiler running) and after time the flux can jam an air vent open, take an air vent off and look for slim, sand, dirt, you should see trace tell tale water stains on the vent or leak area as well loosing that much water.
Check the boiler core for leak, could be a pin hole and dripping out the condensate drain.
Dan