Baxi Solo 30HE - Overheat Lockout/blocked pipes?

[ISM]

Right, went to look at a job last night where 3 rads where not achieveing tempurature at all, albiet the nearest of the 3 to the boiler was hotish on the flow tail pipe (31c to be precise and the trv on their is fine), the return was around 25c (lockshield valve fully open and reletivly new). On this particular stretch of pipe work (which t's of the main flow and return running through the centre of the house) two radiators work fine, nearest to the boiler and are before the 3 rads which are cold. There are 18 rads in all and balanced accordingly. I turned off all rads bar the cold rads in question and the boiler went into Overheat Lockout. I also turned the two rads on which are working fine and the 3 cold rads on, which is on this particular stretch of pipwork, again the boiler went into Overheat lockout.

Its all thoery at the moment, but my gut feeling is that there is a blockage or air lock between the return of the 2nd (hot) and 3rd (cold) radiator. I am also replacing the last two rads on this section of pipework as they are old and have the all in one lockshield and man rad valve on one side!!

Can anyone else put some thoery my way for me to trial and error when I change these radiators.

Ross

[fixter1]

i would take the 3 rads off and open the lock sheilds see if im getting any flow at them, sounds like a blockage

[steveb]

i would do that as well, and flush the rads through at the same time.

[stevetheplumber]

easier to take the pin out the vent and open each valve seperately to check if theres a blockage

[danielp]

could also try putting the flusher on and the boiler at the same time with some chemical in there