Parent's Ferroli Modena 102 - erratic CH operation / turn-on

[lauraeva]

Hi, I'm new here and hope someone can help.

The fault first appeared last week when I was visiting my folks but I live 200 miles away from them.

It initially seemed you had to turn the CH temp pot higher and higher to get the boiler to ignite, eventually hold it with light pressure fully clockwise or the flame would go out the moment you let go or eased the pot back. Eventually it stopped working altogether and now when set to 70 degC-ish comes on randomly, usually cuts out before the rads have warmed, and has left my parents without heating for whole days.

DHW works perfectly, and all external demand inputs are OK (checked), so rules out a whole lot of things. Also CH pump turns on with demand, and indicators are good.

A heating engineer my folks use was called out but couldn't isolate the fault but said he would price up a new CH flow thermistor, and new PCB. He went off on holiday before being able to do further work, but I phoned him before and he confirmed that the CH thermistor had measured @ 10kohm ambient which seemed OK if a little high.

Am I right in thinking that a failed thermistor would be obvious and certainly not be intermittent ? So pointless spending £26 to replace (come as a CH/DHW pair) ?

I'm hoping the plumber checked for loose connections on the PCB in his first visit but am I right in thinking its indicative of a PCB failure, which although pricey hardly needs a heating engineer to swap out. Also as there is no room thermostat in the system (I intend to fix this over Xmas) my folks have been using the CHW temp control to turn heating on & off for nine years ... so could just be that the pot on the PCB is on last legs ?

Any advice appreciated.

(P.S. I'm an electronic design / electrical control engineer so competent but not qualified for any work that is serious plumbing or gas safe)

Thanks,

Laura

 

[Billy Bob Bob]

10k ohms is around normal for a thermistor, it decreases with temperature so i dont know what the engineer means by a little high,is there a timeclock fitted to the front of the boiler?

 

[lauraeva]

Manual says 20k at 10degC, 10k at 25degC, so ambient with no heating (17ish degC) should be about 15k ?

Meant reading high in temp not resistance - my deduction, engineer said "about OK" !

Time clock is bypassed permanently ON ...

 

[Billy Bob Bob]

i would still try a ch thermistor,corrosion normally sees them off and can give weird resistance readings are you sure the mi,s ask for 20k? seems very high,and are you certain the pump is ok? the modena will give dhw if the pumps suspect

 

[lauraeva]

20k is at a low 10degC ... manual gives a table.

I know the combi doesn't use the pump for DHW, but it seems OK.

What happens is central heating LED goes from flashing (standby) to solid (on) when requested, pump comes on, system waits and will usually ignite the flame which will then turn off very quickly and put system back into standby.

 

[lauraeva]

But usually the boiler just sits in standby when turned on, CH deman turned on, and CH water temp control turned to a high setting.

Can see a thermistor giving a crazy reading but can't see it being intermittently OK (the heating did apparently work OK yesterday but not at all again today !).

 

[SimonJohns]

Thermister and poss earth fault

 

[Billy Bob Bob]

gas pressures not cross lighting could be one explanation if its going to lock out,but from your description its not going to reset?

 

[lauraeva]

As I say domestic hot water is OK, boiler fires on demand. Not sure what lock out is, does it mean that you have to reset to get it working ... and isn't there a lock out "Boiler Shutdown" LED which is not coming on ?

An earth fault would likely affect DHW too, and if the thermistor reads 10k or thereabouts when system is cold its OK, and shouldn't prevent fire-up ?

 

[Billy Bob Bob]

An earth fault would likely affect DHW too, and if the thermistor reads 10k or thereabouts when system is cold its OK, and shouldn't prevent fire-up ?

yes and yes to the lock out light i would still try the ch thermistor as i said before corrosion causes issues and weird faults working one minute dead the next,are you competent to try plugging a new one in? its cheaper than a pcb

 

[stevetheplumber]

this is similar to the fault i had on the biasi boiler cut out reset lights flashed but didnt go to reset in that case it was a board problem

 

[willys warmer]

I totally agree with gas man, dirt in system effects the thermistors causing them to give false readings, normally when you remove c/h thermistor you find because its in a wet pocket the thermistor is black with oxide. easy to replaceand cheaper than a board, to many times do people jump to pcb. hope this helps. my firstpost !!

 

[Billy Bob Bob]

I totally agree with gas man, dirt in system effects the thermistors causing them to give false readings, normally when you remove c/h thermistor you find because its in a wet pocket the thermistor is black with oxide. easy to replaceand cheaper than a board, to many times do people jump to pcb. hope this helps. my firstpost !!

hi willy welcome to the forums are you a engineer or have a backround in the trade/industry?

 

[jncalderc]

Certainly I too would replace the thermistor first, Its a tenner against a £120 pcb and as Willys has said too many people jump on the pcb. You could always change the heating thermistor round with the DHW one and try it then, IF the DHW is ok then the thermistor is most likely good on it. Worth a try

 

[jncalderc]

Sorry £200 for this boiler for a PCB

 

[willys warmer]

hi gasman im an engineer working for housing assosciation who have many ferrolis for there sins.

 

[Billy Bob Bob]

hi gasman im an engineer working for housing assosciation who have many ferrolis for there sins.

yes fun aint they,dont mind the modenas though its the new optimax and domicompact tat i dont like

 

[willys warmer]

fitted loads of 25cs and 31cs, 25c slightly better than 31. work have finished contract with ferroli. customer service gone right down the pan since bringing out 31c, now currently fitting worcester 30 si.

 

[tamz]

With a dodgy thermistor the boiler will light for a few seconds then shut off. It won't show any faults.

Sounds more like the pot is knackered especially if it has been used to turn the heating on and off for the past however many years. It is not designed for that.
If you are into electronics you could probably source one and fit it to the board.

I hope your folks have some other kind of heating as it is still 4 weeks (gawd is that all) to christmas!

 

[Billy Bob Bob]

i mainly fit potterton heatmax/BAXI duotec 1 not fussed on the newer netatec/duotec2 cracking service from BAXI though next day visit if in the worx scheme fitting a worcester camray oil boiler today there technical staff are useless and unhelpfull

 

[Billy Bob Bob]

With a dodgy thermistor the boiler will light for a few seconds then shut off. It won't show any faults.

Sounds more like the pot is knackered especially if it has been used to turn the heating on and off for the past however many years. It is not designed for that.
If you are into electronics you could probably source one and fit it to the board.

I hope your folks have some other kind of heating as it is still 4 weeks (gawd is that all) to christmas!

good call tamz could be the pot never seen one fail though,mind it is a ferrolli though TBF the boards Honeywell seen loads of the resets break

 

[tamz]

If it has been turned up and down a few times a day for 9 years it is bound to be worn.

 

[Billy Bob Bob]

If it has been turned up and down a few times a day for 9 years it is bound to be worn.

yes it could,good call

 

[lauraeva]

Thanks for the replies.

Unfortunately I'm over 200 miles away from my folks so can't really do anything myself. I agree that the pot is highly likely to be shot, but sourcing one to fit the PCB and the knob spindle could be a nightmare !

The thermistors come in pairs at £26 inc VAT, PCB £203 inc VAT (Plumbase cheapest local supplier), so even speculative swapping of thermistor would be a fair bit of wasted cash if it didn't solve the problem.

If I were there I'd unplug the thermistor connector and jam a 10 or 15k resistor into the contacts to see if that would fire things up (obviously there would be no burner modulation and would have to switch off before overheat / limit thermostat cut out !). Would a boiler engineer do that sort of thing for diagnostics ?

Seems like my folks have found a heating engineer, colleague of an electrician they know, so hopefully he will solve the problem without ripping them off or fitting unnecessary parts ...