Boiler problem- cutting out

[pma]

My boiler gets it's supply from the programmer (L, N, E). so power for HW and CH curcuits!
Boiler will cutout without warning although there is still demand for HW /CH.
However, it will only run for 4 minutes if demand for HW only, but if demand for CH and HW or CH only, boiler can run up to in excess of 25 minutes until cutting out.

Why could this be if technically boiler can differentiate between HW and CH?

In order to get boiler to fire up I have to turn boiler off for 15 minutes before it will fire up again- usually making sure no demand for HW.

Any ideas?

 

[baxpoti]

Ideas? yes..
overhaeting
tired pump
incorrecttly wired
faulty thermostats
blockage
motorised valve(s)
bypass

More info please

Read //www.diynot.com/forums/viewtopic.php?t=6026

 

[pma]

Was working fine until 2 months ago.
Been living with this problem since then.
I'm confident that system wiring is in order.
Had a new motorised valve when loft conversion done 4 years ago. Original pump. I can hear it running, but not unduely noisy. Prior to loft conversion pump was on high speed, but at the time plumber told me he'd reset to medium speed, but as I say it's been fine for 4 years like that.
Boiler has had new fan and pressure switch in last 2 months.
Interestingly, been though fault diagnosis sheets: Neon I3 lit (power to boiler), but I2 (thermostat calling for heat) not lit when wont fire although there IS demand. Removed NTC sensor wires from PCB. Still no I2 neon. Checked supply to and from pressure switch-OK!
I run out of steam at this point.
I haven't checked operation of NTC stat or burner stat- not sure that would help??
If I switch off boiler, leave for some time, then switch back on NeonI2 will illuminate and boiler will fire.
But as I said before, runnig time before it cuts out again will depend on whether the demand is for HW, CH or both.
Any help?
Thanks