Greenstar CDi Conventional Overheat

[mattltm]

I've run out of ideas for getting our CH system working. Our regular plumbers have been here for nearly 2 days and we still have a non working system.

The boiler seems to be stuck in an overheat cycle. It heats, overheats and shuts down for 2 minutes and then repeats.

The pump, 3 way valve and air separator have all been replaced. The AAV in the boiler has been replaced and the system has had mains water run through it to wash it through. The water came out black and was run until it came out clear. It seemed to flow fine with no restriction.

Originally it was though that there was a blockage in the CH system and the plumbers still seem to think that a Powerflush is the way to go. My only comment is that the boiler still overheats if we isolate the CH and ask for HW only so if there is a blockage it must be on the single return pipe to the boiler but this pipe does get warm when HW only is on.

I think there may be an issue with the boiler. Heat exchanger maybe?

Can anyone give me any ideas where to investigate next?

 

[picasso]

Try servicing the boiler especially cleaning out the heat exchanger which is prone to blocking with condensate residue.

 

[mattltm]

Thanks Picasso.

I've cleaned out the heat exchanger which was full of stuff that looked like hard limescale. The top half of the heat exchanger was bone dry so I wonder if the water has been blocked from up there and that may be why it's been overheating so fast.

All cleaned and put back together but it looks like it's still doing the same thing.

Burn for about 30 seconds
Temperature shoots up to the 60s
Burner off
Temperature continues to climb to the 70s
Temperature starts to drop back town to the 40s
60 seconds later, Burner on again and around it goes...

Plumbers want to power flush it tomorrow.

 

[picasso]

surprising how many times pump valves have been left partially shut causing all sorts of problems.

 

[mattltm]

All valves checked and double checked. Still misbehaving.

 

[picasso]

The temp rise is a bit too gentle, You sure you got all the air out of the boiler ?

 

[mattltm]

I think so. AAV stopped hissing and the magnaclean on the return is bled.

 

[picasso]

Looks like your having a powerflush then.

 

[mattltm]

Looks like it. Thanks for the help Picasso.