Ferroli Modena 102 CH wont come on after HW has been on.

[grill]

Guys,

Was called out to a Ferroli Modena where the customer said the DHW was staying hot for 20 secs and then going cold. From reading through the forum this would suggest a faulty thermistor. Boiler hasnt been serviced for 3 yrs. 18 mbar working pressure at gas valve on CH. 12 mbar for HW and 7mbar for CH on outlet.

I went round tonight for a recce. When i got there pressure was sitting at 2.5 bar so i nearly had to unscrew the PRV the full way out before it started to drop (this is how it says to drain it in the manual). Needless to say we now have a small drip from it so it will need replaced.

When the HW was cold the Therm was reading 14 K Ohm and when i got it going hot it read 2 K ohm. So it appears to be okay.

Anyway when i went to put on the heating after running the HW it would only stay on standby. I had to reset the boiler to get the CH to come on and then it ran away merrily. Then turn on HW, boiler goes off, cuts back in for HW and it is fine, turn of HW and CH goes back to standby again.

Im going back round tommorrow so what do i need to bring???

G

 

[Slugbabydotcom]

A CH thermistor

There is a delay before it lights for CH after a HW demand

 

[grill]

Sorry forgot to mention. I checked the resistance of this as well and it was dropping when heating up also. Now i know these need to be replaced as a pair as they give bother. Could it be as simple as that???

G

 

[grill]

Also i waited for about a minute for it to kick back in. How long does it usually take??

G

 

[Slugbabydotcom]

It is best to renew as a pair. Resistances FWIW are in the MI's on the fault finding sheet. Sometimes testing doesn't prove much as the fault can often be temp sensitive or just plain sensitive. For the price of them I tend to swap them on these and Ravenheats as if they are not the cause this time they will be the next. Leads can sometimes be swapped over to prove or disprove a problem.

IIRC Anti cycling delay is 3 mins but often this can be adjusted on the pcb.

 

[grill]

Cheers for that,

Reckon it must be the prob on the HW as sometimes it runs and is piping hot and then the next time it runs and goes cold.

So if a Therms resistance is changing it still may not be okay. Just when i thought i was getting the hang of my multimeter!!! More product specific knowledge required!!!

Is the PRV handy enough to change on this one?? The water was only dribbling out of it when nearly fully opened which concerned me. Would these suggest it being blocked with crap??

G

 

[Gasguru]

Sometimes thermistors just go o/c at a particular temperature and the Modenas must have the poorest quality sensors out there. In addition i've probably changed half the boards in these boilers (with allsorts of bizare problems) since they are usual Honeywell carp. Venturis/pitots disintegrate, flue restrictors riings are often missing, electrodes mis-aligned, ign pressure/min/max rates incorrectly adjusted. Apart from that lot they work ok.

As the the PRV it's very difficult to remove the valve head without adjusting the spring rate when replacing it...there's no room. Count on removing the whole brass manifold. Don't use the PRV as a drain-off unless you can replace it esaily.

 

[grill]

Yeah i was looking at the PRV and struggling to get my head around how it works. It didn't seem to be spring loaded like some of the other ones i have come across. It is only literally dripping slowly now but i know this wont get any better. Will get the whole manifold and just unscrew the various fittings out of it and replace.

I ahev had teh debate bfore about whether or not to use the PRV. This manual and a Chaff and Maury specifically say to use the PRV to drain down. To me this is ridiclulous as they always bloody leak afterwards!! But the other theory is this is a safety feature and should be checked.

So on these boilers im looking for Therms and ignitor problems?? Ill add that to my list!!!

G

 

[Gasguru]

.......... Will get the whole manifold and just unscrew the various fittings out of it and replace.

It's normally a Caleffi PRV but not the common type. Often there is a small plastic rectangular lever fitted (but often missing). Prizing off the outer red cap will reveal the brass pin.

If you just replace the valve head be very careful not to adjust the relationship between the inner alloy threaded part an the outside plastic housing otherwise the pressure setting will be affected. Best to just replace the whole brass manifold complete.

Have a look here to see why. //www.diynot.com/forums/viewtopic.php?p=692445#692445

 

[bengasman]

If I remember correctly, there is not much you can do about the prv than replace the whole manifold. Expensive too.
This model can develop really strange faults if fan/venturi/flue is not quite correct

 

[grill]

Any of youguys got any idea of price/part number for manifold?? Surely you will have to screw the PRV into the manifold anyway and will have same possible adjustment problem??

G