boiler on then off ,on then off, on then off

[brianfrost]

Ok so I asked this a bit back but am sill having probs
boiler keeps switching on and off about every 5 minuets, had a man in to look at it and he thought it was a block, so i have replaced a lot of pipe work, that is all i can get too, so no blocks on the cold feed from tank
and outlet from boiler up to valve switch. Draned system and refilled it.
and i am now at a loss. Now i am thinking pump, boiler thermistat. CAN ANY ONE HELP, ONLY THREE DAY OF HOL LEFT AND I NEED ONE DAY FOR FISHING.

 

[ChrisR]

But boilers do do that - heat up to thermostat temp, switch off, cool and repeat. How hot do the flow /return pipes get?
Pump on highest speed?
What's yer actual problem?

 

[bster]

is the bypass (if fitted) fully open - if so crank it down and see if this solves it.

 

[brianfrost]

back filled system got two out of three rads third furthest away from boiler cold

 

[bster]

does it heat your cylinder alright or same prob? - are rads that are hot staying hot or is boiler clicking on-off-of-off (short-cycling) still?