I have now changed the central heating pump twice in the last week on my grundfos pump plan installation using grundfos ups 15 50 heads . The problem both times is that they worked perfectly and totally silently for the first 5 hrs then developed a sound of swarm of bees . The system has been flushed out and inhibitor added. The header tank is full and no air in the system . CAn anyone help before I go insane! Regards Keith

Keith, March 2013

Thanks for your help ltd I think your onto something with your first reply the pump plan left motor for the hot water has an arrow on the casting underneath pointing left the arrow on the right central heating pump points to the right . The pumps I used to replace was just a bare pump with no casting so other than knowing that both pumps have same rotation of anticlockwise (the old pumps the plumber took away with him as scrap ) and that's when the problem started so I'm gonna have to check with grundfos technical when they re open after bank hol . Not certain about air in system tho as all rads are red hot , plenty of hot water to just the constant buzz. Thanks for your help tho its appreciated.

Keith, March 2013

Have you notice flow arrow on pump what direction when took out ?

ltd, March 2013

There is air in system, undo big nut in middle of pump left open for 1/2 hrs , open air vent on radiator one by one ,open drain out point let water comes out slow flow when pump be come quite than you notice no air ,close all operation it depends who the pipes are run some time it takes 4 to5 hrs to eliminate air from system while you do all that switch of boiler from thermostat which is on boiler

ltd, March 2013