Can anyone help me with this please...
Suprima 60 with what seems like a thermostat problem
When boiler reaches it's preset temp it turns off correctly, [red on -
green flashing] but then 10 secs later red goes out and it re-fires.
30 secs later it again realises it's up to temp and goes off.
10 secs later Red goes out and green flashes [call for heat] but
this time it does not re-fire but goes into what I now realise is an
"anti cycle timeout" of about 5 mins. [green flashing all the time]
After the 5 mins, it suddenly decides to fire up and then it repeats
all the above again.
The temp at which it goes off for the first time, varies as you would
expect with the temp control setting.

I have checked the pump speed - four speed pump, - it is working well
and the speed makes no difference. [Pottertons theory was the pump
was too slow.]
I have replaced the thermistor - a waste of £11.

I have monitored the PCB around the voltage comparator [U5] which does
all the temp measuring/control, and it seems to be working well. The
o/p of the chip goes high when the two inputs match and drops a 10
secs later as the water temp drops. The red led follows the chip o/p.
I am beginning to wonder if this is a design prob, and so I would be
very grateful if anyone with a suprima could do a couple of tests for
me....

Set the temp to minimum [50deg approx] and fire it up with a constant
demand.
When the boiler reaches temp the red should come on and the green
flash
Q1. Does the red stay on all the while the boiler is up to temp? - or
does it go out after 10secs?
Q2. How long does it take for the temp to drop sufficiently to re-fire
the boiler?
Q3. What is the temp drop needed to make it re-fire - mine refires
when my thermometer shows about a 3 deg drop - ie almost immediately.

I am loath to buy a new PCB without being sure that it is faulty-
answers to the above questions would help a great deal

Thanks in advance

Phil