Intermittent boiler burn

[rss91]

I think there may be an issue with my boiler. It's a Baxi Megaflow 18 System Boiler.

The burner only stays on for about 30-40 secs and cuts out when the temperature reaches low 80's. The temperature is set to 75.
The burner then remains off for 3 minutes (timed 3 cycles of this behavior) before firing up again. During that period the temperature drops to around 40 and hovers there for a while. I believe the boiler remains pumping throughout the cycle.

Lukewarm rads after being on for close to 3 hrs and UFH is only at about 30 degs after being on for 4 hrs.

Any ideas? Thanks

 

[CBW]

Sounds like it’s possibly overheating - check the circulating pump and motorised valve(s).

 

[rss91]

Is that something best left to the plumber?

Is it safe to leave the boiler running in this state or shall I just leave it off?

 

[CBW]

Unless you know what you’re doing or what to look for then yes best left to a plumber/heating engineer. If it’s used in an unvented cylinder then you’ll likely need a G3 qualified engineer.

 

[rss91]

I wouldn't have a clue! It is an unvented cylinder.

As an update I switched off the stats and left the system to cool down for about 30 mins before turning one again.
The boiler fired up to 75 and this time stayed there for about 10 mins before very slowly creeping up, probably another 5 mins before hitting 80 and then cut out. Rads do get hot in those 15 mins but obviously not sustainable, as the time the burner remains on for subsequent cycles is only 30-40 secs before cutting out.

Now I think about it, it certainly does seem like overheating.