Ferroli 901 short cycle

[NEILC25]

good evening, I've got an issue with my ancient however well maintained ferroli 901 boiler, it short cycles, it stays on for 35 seconds, then shuts off for approx 10 seconds and repeats all day, this never happened last winter, just this winter since turning it back on.

When turned on is fine, however after approx 10 minutes when up to temp issue starts

This is what I have done so far,

2 new thermisters.
New pcb
New flow meter

Water runs hot / cold, however this is due to the short cycling

I have been told as soft water area unlikely to be heat exchanger.

All rads are warm all over,

No airlocks in rads or taps.

Pump is working well because rads get hot quickly.

Any ideas please, I would appreciate any advice,

I appreciate ferrolis aren't the best
Thanks

 

[oz-plumber]

Changing the thermisters was probably your best stab.
If that didn't work, I wouldn't have changed the PCB or Flow Meter.

I would have started to look elsewhere and determined what was causing the problem.
Gas rates, pressure switches, flame rods, flame sensors, pump capacitor, gas valve, gas valve modulation, heat exchanger.
Most of these things can be checked within a very small amount of time.

 

[Gasmk1]

Pump ? Is it performing correctly

 

[SJB060685]

What’s the ΔT across the boiler after this ten minutes?

 

[NEILC25]

ive figured it out, however I need a bit of advice, it's the pump, it's got to the point now where it's not even starting up, I've tried to remove the old pump, however the switch and casing come of but how do you get the actual fan bit out, thr shaft with the rotating part won't come out of the cast iron casing, how do I get it out?

 

[NEILC25]

ive figured it out, however I need a bit of advice, it's the pump, it's got to the point now where it's not even starting up, I've tried to remove the old pump, however the switch and casing come of but how do you get the actual fan bit out, thr shaft with the rotating part won't come out of the cast iron casing, how do I get it out?

 

[ShaunCorbs]

Have you drained the system?

 

[NEILC25]

yep, systems empty, is this part supposed to come off, I don't want to have to buy the whole cast-iron part unless I have to, can I tap this with a rubber hammer, surely this should come out, there's nothing on backside of It securing it. The replacement part has it all as one piece

 

[NEILC25]

Here is the specific part

 

[SJB060685]

It could be a weak capacitor, alternatively the whole pump could do with replacing.