Problem with Potterton Suprima 100L fluctuating water heat

[andythur]

I have an 8 year old potterton 100L suprimal boiler.

Open system, with 3 zones, Water cylinder, Upstairs and Downstairs all zone valve controlled ( 20 Rads )

with all zone vales open, with the cylinder water and Radiators cold, the boiler starts and runs for about 80 seconds the water in the boiler outlet gets HOT ( very ), the inlet still has cold water entering. The boiler then switches the burner off, this lasts for about 130 seconds by this time the water is only Warm coming from the Outlet. The burner then comes on again for the 80 seconds and the Water at the outlet gats HOT again.

This cycle carries on, with the water getting Hot then Warm but the average is only Warm and not hot so the Rads and Water take a long time to heat up and heat the house.

The radiators are only generaly Warm at the Top.

The system has

1, New Pump
2, New Potterton PCB ( NO Red light cutouts happening )
3, the Zone valves have been checked.
4, the Cylinder pipe work is right next to the boiler, so no distace to travel

Note About 1 year ago I had an under floor water leak that had been going for probaly 2 years or so, now fixed but caused Rust on Iron oparts in the system,.

Any one got any thougts, ( blockage , sensor etc )

thanks

Andy T

 

[warrington]

when did you have your pcb changed? was it replacement kit ?

 

[andythur]

About 6 months ago, yes it was a replacement it was the correct type because you cant use the newest type on the 100L as you need a new fan motor, I used to get the redlight lock out with the old board burnd tracks etc but not with the new one, the haeting has been not right for a while ( the under floor laek caused all sorts of issues ) just cant get the constant heat output.

 

[warrington]

If boiler not locking out , just cycling i would think you may have sludge/scale build up or boiler thermostat failed.

 

[andythur]

Would the sludge build up be in the Heat exchanger as when heating the water cylinder all the pipes are 28mm and its a total of about 20 foot of pipework, the only thing else that would restrict the flow would be the pump ( new), the isolating valves for the pump or the Zone valve for the cylinder heat coil.

could the heat excanger rust up ( if its iron ) can the haet exchanger be cleaned?

cheers

Andy T