Radiators Constantly Need Bleeding After Boiler Replacement All aspects of plumbing questions and answers, help, tips and information

[Romany4]

My friend had a condemned Baxi Boston boiler replaced with a Worcester Bosch Greenstar 15-Ri4140676, together with a new Grundfoss UPS2 Type 15-50 /60 pump. Since boiler change 3 radiators are constantly getting air in them – these are bled but air returns over a few days!
The heating engineer suggested that the likely cause was either a leaking coil in hot water cylinder or leaking pipes under the concrete floor? He subsequently replaced the hot water cylinder which has not solved the problem – the next step would be to find a leak in the underfloor pipes and repair or replace pipes. There are no obvious signs of leaking water anywhere in the system! One radiator was replaced when new boiler was installed but the remaining radiator network and piping is over 20 years old but in good condition. It is NOT a microbore system.
The mains water supply to the loft expansion tank was turned off for 2 weeks to ascertain whether water level dropped – signifying a leak. Periodic checking of tank water level over this period actually shows a significant RISE in water level, which in my view can only be caused by air displacing water in radiators and forcing the excess back into expansion tank?
Can anybody please suggest a likely cause for air getting into the system?

[ericmark]

Likely down to pump speed, often there is a auto bleed in pipe work to let air out, but if pump running too fast it can do reverse and suck air in.

You say Grundfoss UPS2 Type 15-50 /60 pump not a plumber but if there is a speed control turn it down, I tried to find details but seems discontinued so could not find out is speed is adjustable.

Above the tank there is often a pipe to allow water back into the tank looped over the top, that is where it can if pump running too fast suck in air.

However this is all rather basic, so why has plumber not corrected it already? If being done DIY would understand, but plumber it should be first thing he looked at. So may be not what I am thinking?

[Romany4]

Thank you ericmark for your reply. The Grundfoss UPS2 Type 15-50 /60 is a current model. My initial thought was, like you, that the pump might be running too fast and as far as I remember I did check that it was set to its lowest setting - I will however double check! I should have mentioned in original post that the building being heated is a bungalow and the only high level pipework is the run in loft to link new boiler location with existing heating / hot water pipe network.