Water in heating system boiling

[Kelvian]

I have added new radiators recently and had a leak at the valves, on the return side, I fixed these and bled the system, and due to the location of the new rads increased the pump speed on my Grundfos UPS15-50 from 2 to 3.

Thought all was well as all evening good heat all round, heating came on following morning and started boiling as the noise was incredible and the radiators upstairs were absolutly boiling. but cold radiators down stairs.
I have since turned pump down to 2 as I thought the additional pump speed would have generated cavitation and air back in the system. Is this correct and how do I balance betwen higher pump pressure to get new rads hot downstairs and boiling water in system? also should i only bleed pump when heating is off?

 

[2face]

yes bleed when the system is off,

you have an air lock most probably .. turn the rads off that are working and the cold ones should come hot then let them run for 1.2 hr or so.

balance the system.

 

[bengasman]

are you absolutely sure all is piped up correctly?

 

[Kelvian]

Thanks for the advice,
checked routing on new pipes all looks okay followed hot water as rads warmed up.
Boiled again this morning, I am going to try the bleed idea, shutting upstairs off and running only downstairs.
I noticed also that the expansion tank is churned as the water has change to a rusty chocolate looking colour.

Thanks again

 

[Fluffster]

I would suggest you have a possible blockage check where the cold feed meets the primary flow as this is the most common place use a magnet to see if it sticks.

Pete