faulty thermister?

[bcoleman]

Hi,

I've been having afew problems with my combi boiler (FER FALCON) in that the DHW seems to cycle as follows - hot-very hot-extremely hot-freezing cold-hot again...

I put up a post on this a week or so ago and was advised to try adjusting the DHW control knob on the boiler and/or running a hot tap whilst showering to keep the flow rate up but neither of these seem to make a difference.

Having watched the boiler when my wife was showering I've now noticed that the cause is the boiler turning on and off (accompanied by her screaming from the bathroom!!) even though the DHW light is always on.

Could this be caused by a faulty thermister not telling the boiler to modulate when the appropriate temp is reached and therefore causing the water to get hotter and hotter and hotter until it eventually hits a safety point and cuts out?
If so, can I change this myself or should I get someone out?
If not, what else might it be?
Thanks in advance!

 

[Agile]

Hi,

Having watched the boiler when my wife was showering I've now noticed that the cause is the boiler turning on and off (accompanied by her screaming from the bathroom!!) even though the DHW light is always on.

Thanks in advance!

Tell her not to scream. Boilers are sensitive !

It could well be a faulty thermister. As they are cheap you could just replace and see what happens. The correct procedure would be to measure the resistance at various temperatures.

I might replace the thermistor with a pot to see if the PCB correctly makes the boiler modulate.

If anyone has been fiddling then the gas settings may be incorrect.

Tony

 

[jackecole]

it could be a scaled heat exchanger or incorrect flow rate also through the combi.

Agile - when you measure the resistance of a thermistor how exactly is this done as doing this with power on to the boiler causes the multimeter to blow!

 

[onlyfitidealboilers]

thats the first i new! its only measuring resistance.

 

[petit_pablo]

Agile - when you measure the resistance of a thermistor how exactly is this done as doing this with power on to the boiler causes the multimeter to blow!

turn the power off and disconnected it before you test

 

[Andygasman2010]

I'm 95% sure that your secondary heat exchanger (plate-to-plate or calorifier) is blocked up. This means the primary circuit in the boiler will heat-up too quicky and cut-out before the burner gets chance to modulate down. Once it cools it will fire-up again and cycle like this. Hence the hot/cold temperatures at the shower. Very occasionally it's an electrical problem but not often. Hope this helps!

 

[bellairaphon]

falcon with a plate to plate? thought they where set up like the modena?

Like Agile said thermistors first then pcb but get a rgi in for the board as sometimes the ignition pressures are out.

Nath

 

[Andygasman2010]

Ah Yes, duel-pass heat ex! Thanks, missed that one there... I've known those sensors fail a few times, they're relatively cheap though. Well worth changing before going for a board.

 

[bcoleman]

Thanks Guys - will try to get the thermisters and fit them myself - looks a relatively simple job based on the intructions in the manual.

Also, I've just had the front off the boiler so I could actually see the boiler flame (hadn't done it before as I had to remove a load of kitchen cupboards first!) and it didn't really modulate at all before the flame cut out completely after about 15 secs or so. Only really seems to happen when the taps are half open though (although I checked the flow rate was greater than 2.5 l/m which is the min for the boiler) - could this be another sympton that it is a faulty termister as the water wouldn't have as much chance of overheating and causing a cut-out if it's flowing at a faster rate, right??

Thanks for all your help guys - even if I end up calling somone out at least I now know some of the likely causes.

Ben

 

[nogoodatfaultfinding]

Hi,

I've been having afew problems with my combi boiler (FER FALCON) in that the DHW seems to cycle as follows - hot-very hot-extremely hot-freezing cold-hot again...

I put up a post on this a week or so ago and was advised to try adjusting the DHW control knob on the boiler and/or running a hot tap whilst showering to keep the flow rate up but neither of these seem to make a difference.

Having watched the boiler when my wife was showering I've now noticed that the cause is the boiler turning on and off (accompanied by her screaming from the bathroom!!) even though the DHW light is always on.

Could this be caused by a faulty thermister not telling the boiler to modulate when the appropriate temp is reached and therefore causing the water to get hotter and hotter and hotter until it eventually hits a safety point and cuts out?
If so, can I change this myself or should I get someone out?
If not, what else might it be?
Thanks in advance!

What i find strange about this whole post is that your misses is naked and showering yet you are watching a boilers for diagnostics.

Summit not right there me thinks.

 

[onlyfitidealboilers]

test the thermister 10k at 25 degrees If its an NTC (negative temp coafficant) that is

disconnect the wires from it and put your test leads over the two pins an read your multi mtr

 

[nogoodatfaultfinding]

test the thermister 10k at 25 degrees If its an NTC (negative temp coafficant) that is

disconnect the wires from it and put your test leads over the two pins an read your multi mtr

Are you sure thats wise when shes in the shower

 

[Agile]

What i find strange about this whole post is that your misses is naked and showering yet you are watching a boilers for diagnostics.

Summit not right there me thinks.

Yes, you are right!

He should be enjoying the shower and leave the missus to do the boiler repairs!

Tony

 

[onlyfitidealboilers]

 

[bcoleman]

test the thermister 10k at 25 degrees If its an NTC (negative temp coafficant) that is

disconnect the wires from it and put your test leads over the two pins an read your multi mtr

Thanks but onyl 2 probs with this -
1) I don't have a multimeter (easily fixed I know)
2) the DHW control is crap and doesn't have any actual numbers on it - just an indication that hot is one way and cold is the other

Cheers.
Ben