Biasi 24S - Inconsistently Requires Resetting

My Biasi 24s has given me it's fair share of grief for the 20 months that I have owned it (since moving into my first house). The previous problem being the ripped switch diaphragm.

Having had that problem fixed I now have a new problem.

It seems that on certain occasions my boiler is not starting up from cold, whether it be CH or for hot water. The first time this happened I pressed in the tiny reset valve and when turning on the CH it worked.

This has been happening for a few weeks and I have coped by just having a small nail to use next to the boiler ready to reset it again and again when I get in from work or first thing in the morning. Also, on a few occasions it has switched off without me knowing whilst it has been on for an hour or so(only when my feet started to feel a bit cold I checked and realised it had gone off).

It is quite inconsistent and so seems hard to diagnose. Could it be overheating and resetting itself, but why do it sometimes and not others.

Any help appreciated, 'cause the plumber must think I've got a thing for him.

This is an "overheat stat" !

It trips to protect the boiler from overheating. The good news is that if you reset it more than a few times its calibration is upset and it trips at lower and lower temperatures.

You need to work out just when its tripping. take HW and CH and see if the temperature gauge goes above 70*C and 85*C respectively.

If both the above tests are ok then its probably tripping AFTER HW use. \there is no pump over run on this model and IF the primary heat exchanger is scalled up then the latent heat boils the water and trips the o/h stat. This only happens if the HE is scalled.

Just VERY unusually the o/h stat can be faulty but thats a 1 in 100 chance, scalling is a 99 in 1000 chance !!!

The HEs CAN be descalled by anyone who has the relevant knowledge but not many "plumbers" do. A boiler engineer is a better bet!

Tony Glazier

Thanks for a great and informative reply.

Appreciated.