Combi Boilers--how they work and start-up problem

[2cautious]

1) Been trying to understand the functioning of combi boilers, I`ve got the basic idea, but do feel that I do not fully understand the linkage of the thermostat and the thermister.
Does the thermister control the "on/off" of the ignition , with the temperature first being set by the " temperature contol knob"on the boiler casing, with the thermostat being the "last resort safety device" set to a higher temperature than that of the max. temp.setting of the control knob on the casing .

2) I have a problem with my boiler after cold start-up. Sometimes it turns off after a few minutes (well before the rooms are warm)and then restarts and works for say twice as long and then turns off, then repeats and then gets to its` "normal" cycle.
On the next cold start-up, it might behave properly and go into its "normal" cycle straight away.
My thinking is that the thermister is faulty , can you experts help with this problem.

Thank you

2

 

[DP]

A thermistor is an electronic component that exhibits a resistance depending on its temperature. E.g. at 20 degree C, it might read 10,000 ohms and low of 1,000 ohms at 70 degree C. A thermostat on the other hand is a mechanical device that flips over from on to off (or off to on) at selected point. While a thermostat acts as an on/ off switch to control say a gas valve, a thermistor more commonly, by use of electronics, controls the gas valve to give varying burner flame resulting in refined boiler operation.

Next part. Boiler make and model would help as many boilers go through a self check which checks prime sensors for acceptable values long before they actually take part in system operation.

 

[2cautious]

DP

Thank you for the reply.(The combi is a Halstead Finest Gold--9yrs. old) I do not think boiler is doing self check as (from cold start--7 am ish) it works for say 10--15 mins. then turns off , just as rads. are getting warmish ( not hot ) It then stays off for say 10--15 mins. and then starts up again and basically repeats the above,for one or two times more, but stays fired up for slightly longer time each time until it starts to behave "normally"It then will function as normal all day.
Any ideas?

Thank you

2