Boiler cuts out before getting fully hot

[Honeymonster]

My Glowworm micron 60FF runs fine from cold but once the system is warmed up (but not fully up to temperature) it keeps going around the cycle: turn on, fan starts, gas ignites, runs about 10 seconds, turns off, waits 10-30 seconds, repeats. This is all with the central heating thermostat still calling for heat.

So, is this indicative of a failing themistor? In other words is it indicating that the boiler water temperature is hot enough and hence not allowing the boiler to run? The boiler stat is turned full on.

Also, when it is running it does a good impression of a steam engine .... chuff .... chuff .... chuff.

[Roger Mills (aka Set Square)]

Do you any means (e.g. IR thermometer) of measuring the *actual* flow temperature when it starts cycling? What evidence do you have that it's not "up to temperature"?

[Cicero]

================ Buy a couple of these:

think they're also stocked by Wickes, B&Q etc.

They're reasonably accurate and if one is placed on flow and another on return they will show the difference between flow and return temperatures which is probably more significant than a maximum temperature.

The manual suggests that the '....chuff ....chuff ....chuff ' ( technically 'hunting' ) is a sign that the circuit board needs to be replaced.

Cic.

[Fentoozler]

What colour are the LEDs when on (reset & burner) - orange? If so, replace PCB. I must have changed 100s of these! The new ones have green LEDs. Look on the underside of the PCB, there'll be a scorch mark on it.

Someone calls up with a Micron misbehaving, I turn up with my toolbox and a PCB.

Angus

[Andrew Gabriel]

Stick them both on the same pipe to start with, and make sure they read the same (if you are going to be interested in the differential temperature). A large dollop of heat sink compound on the back of the thermometer where it contacts the pipe considerably helps with responsiveness and accuracy.

[Honeymonster]

It's not up to temperature because the pipes in and out of the boiler are warm but not hot. I would guess no more than 50 degrees C.

[Cicero]

=============== Your room thermostat could be faulty. Try it on a much higher setting and see if it makes a difference.

Cic.