Biasi 24S safety thermostat tripping on CH

[DreamingOfWhiteHorses]

I have recently taken on the ownership of a somewhat aged (and troublesome) Biasi 24s (Year 2000).

Shortly after getting to know each other the boiler started overheating and tripping the safety thermostat on the DHW side while blowing steam out the top like a whale. This was rectified by replacing the DHW heat exchanger, which was very clogged with black debris.

Having solved this first problem, we appear to have revealed a second issue, this being that the safety thermostat now trips on the CH side after c. 1 hr of operation. Can restart CH after boiler has cooled and safety thermostat can be reset.

At the point that it trips, the radiators are very hot to the touch. Suprisingly hot to me, as they each have a TRV, which I would have presumed would have shut them down before they got so hot. A few other points that may help in diagnosis are that

- I have checked the resistance of the primary NTC - when boiler was cool resistance was c. 14k (its chilly here today) and just after the safety tripped it was c. 800 rising rapidly above 1000.
- lowering the CH temperature regulator dial when CH is on leads to a reduction in the flame height; and
- the main circuit flow switch shaft does not protrude out perhaps as far as it could but slight adjustment of the microswitch cantilever against which it pushes seems to have addressed this, by reducing distance shaft needs to travel to trigger microswitch.

Hope the above is helpful in assessing what might be going awry. Having just been given a quote for c. £5k plus VAT for new boiler, gas piping and powerflush, spending a bit to rectify the issue(s), could be attractive.

Thanks in advance

 

[BARNY78BG]

if overheat stat hasn't been changed after tripping so may times then it could now be tripping at a lower temperature(nuisance tripping), try changing this, if not then pump could be worn(reason for spindle not rising fully) or restrictions in boiler, as you've said plate was dirty.
p.s. manipulating the switch arm is cheeky

 

[petertheplumber12]

The overheat stats are usually set to go at about 90DegC so if it is going earlier you need a new overheat stat, about £20 fitting £30 Buy it on line if you need one and then get someone to come and fit it.

 

[DreamingOfWhiteHorses]

Thanks for the thoughts.

Annoyingly the temperature gauge on the central heating temperature and pressure gauge is not working. Most of the time it reads zero other than when the temperature spikes when it rises be a few degrees before the safety thermostat trips. This makes it hard for me to tell how hot the CH is when the safety trips.

Mind you, according to the temperature v resistance chart in the manual, at 100 degrees Celsius the resistance of the NTC should be c. 1000 ohms. When I measured it just after the safety had tripped its resistance was c. 850 ohms, which would seem to imply that the safety thermostat is not tripping at too low a temperature.

 

[BARNY78BG]

the readings will drop quickly after tripping as pump overrun occurs to drop temps in boiler for safety. burner cuts out immediatly after overheat, pump continues to run to disperse heat into system, thus dropping temp+ resistance readings.
ps. if temperature+ resistances rising after overheat, then poss pump worn or restriction in boiler

 

[Agile]

the readings will drop quickly after tripping as pump overrun occurs to drop temps in boiler for safety. burner cuts out immediatly after overheat, pump continues to run to disperse heat into system, thus dropping temp+ resistance readings.
ps. if temperature+ resistances rising after overheat, then poss pump worn or restriction in boiler

The 24S does not have any pump over runs!

But the tripping would often occur during a demand and that would stay and run the pump.

OP, what is the boiler doing when it trips?

Do you realise every time you reset it it will trip at a slightly lower temp the next time? Best to get it sorted out sooner!

Tony

 

[DreamingOfWhiteHorses]

Tony,

It trips while the burner is operating not after it has shut down. If the latter, I understand that this could indicate a scaled main heat exchanger. Mind you having seen the state of the hot water heat exchanger, I would not be surprised if the main one was also suffering from scaling.

When the CH first comes on from memory the flame is high and this then modulates down. I cannot recall it shutting off at any point prior to the safety tripping but could watch a cycle, if this would be important to know.

Thanks for your help.

 

[Agile]

Obviously some problem!

But rather inconclusive from the info you have given!

There is obviously nothing terribly seriously wrong but probably a combination of one fault and a series of mis-adjustments. They are very reliable boilers if set up correctly.

The more often you press the o/s stat to reset, the lower the temp it operates at! If you press it too often it will need replacing!

Tony

 

[DreamingOfWhiteHorses]

Tony thanks again. I will bring in some professional help (not of course to imply that you are not professional!). I have grown rather fond of my troublesome new friend and spending c. £6k with VAT for a replacement so soon after being introduced just seems rude and somewhat pricey.

Out of interest, do you know how multiple resets of the overheat stat leads to the stat trigger temperature dropping?

 

[Agile]

Its designed to fail saf( er ).

To replace the boiler would be about £1750 and not £6k

They are totally repairable!

Tony

 

[DreamingOfWhiteHorses]

Most of quote was not for the boiler (which alone makes up a small fraction of the total figure c. 1k-1.3k ex VAT depending on make and size as far as I can tell). Bulk of costs arise, I am told, due to need to fit a new wider diamter pipe from meter to boiler (its a terraced house so much drilling and lifting of boards to get pipes to boiler and gas hob) and a system flush.

 

[Agile]

Still seems totally excessive!

Power flush is about £130 from a boiler install quote! But rarely needed compared with a chemical or mains water flush!

Tony

 

[Gasguru]

Get the pressure gauge fixed/unblocked.

Get the max and min burner pressures checked.

Range rate the heating output.

Replace the overheat stat.

Don't have TRVs on all the rads, take one of the heads off.

Then see if there's still a problem.

And whoever qouted for that boiler replacement is ripping you off so find someone that can repair the boiler instead.