Ferroli Modena 80E DHW gets hotter and hotter

[2kool4skool]

Hi,

I have a Ferroli Modena 80E, and I recently had the Venturi replaced because the gas kept firing on and off every few seconds. At the same time the Engineer also replaced the thermistors, but they were copy parts and not Ferroli.

Now when I have a shower the DHW is getting hotter and hotter, so I have to keep adding more and more cold water to keep the water at an acceptable temperature. At the same time the cold water flow is increasing while the hot water flow is decreasing until the hot water flow becomes so low that the boiler switches off, and the water goes cold.

I've had 2 Engineers look at it so far, and neither could find a fault. They seemed to think it wouldn't be the thermistors, because they'd been replaced.

Has anyone got any suggestions? Thanks.

 

[Agile]

the thermistors seem so unreliable that I would say it could easily be them again.

No competent engineer will make any asumption because a part as just been replaced!

Its so easy to measure the resistance of a sensor!

Tony

 

[2kool4skool]

Thanks for the suggestion. I've had the original thermistors put back in, and the hot water is still temperamental. I've measured the temperature of the hot water using a digital thermometer, and noticed the temperature gradually increasing 1 degree every 30 seconds or so until it got to 67 degrees celcius. At this point the temperature rapidly decreased, and the water became cold. The boiler had switched off for a short while, and then the water started heating up again. When I carried out this test the DHW regulator on the boiler was set to minimum.

I say the problem is temperamental, because on another occassion the hot water remained fairly constant within the range 48-50 degrees.

Does anyone have any other suggestions? Thanks.

 

[Agile]

There can be different causes of similar seeming faults.

Yours could even be a gas related fault ( which we dont advise DIYers about ).

Your boiler does not have a plate heat exchanger? But if it did then it would be blocked!

Tony

 

[DP]

OP stop looking at the boiler and investigate why water flow through the boiler is decreasing! If stipulated water flows through the boiler, then you will get the correct temperature of water. Also, water will be very hot when you get the initial slug of hot water at the tap. Check water flow at other taps, if temperature OK, then boiler and contrrols OK.

 

[2kool4skool]

Although my initial posting is true its not the real issue, and is a bit misleading. The real issue isn't related to flow. Because the behaviour is so erratic its been difficult to diagnose what the issue is. However, I think I now understand what it is.

The issue seems to be that the DHW is gradually getting hotter and hotter, reaching 67 degrees celcius over a period of maybe 10 minutes. At this point on sunday the temperature rapidly dropped and the water became cold, and then started heating up again. This evening it reached 67 degrees and the temperature slowly reduced until it reached 54 degrees, and remained fairly constant. All the time the flow seemed fairly constant. The DHW regulator was on minimum.

So the issue seems to be that the temperature of the DHW is constantly changing, and sometimes eventually becomes constant after a long time.

I read another posting that said this boiler doesn't have a plate heat exchanger. Any other ideas? Thanks