hot water problem

[vmr]

Hi,
I have a problem with my Ferroli combi 77FF. CH is O.K., but when using hot water for 5-10 min the boiler overheats and cuts off totally (pilot flame is also off). It is same when shower or kitchen tap is used. The problems started year ago during the winter and there was almost no problem during the summer, but in cold weather the boiler cuts every day. The hot water flow is slightly reduced, but about 10l/min. Three engineers looked on the boiler, but no help up to now.
Any suggestions, help,
Thanks

 

[sifu]

Most likely the how water thermistor is faulty, but it could be a scaled up heat exchanger. Is the water very hot just before it cuts out. Can you see if the flame reduces to a low flame as the water gets hotter? If you have a multimeter you can test the resistance of the thermistor. Let me if you need to know how.

 

[vmr]

Many thanks,
The water is very hot before the boiler switches off, but the flame doesn't reduce - I'll check it again for any changes, but in any case the boiler switches off totally. I have got a multimeter and will appreciate to tell me how to check the thermistor. Is it possible to increase the water flow within the hot water loop? The tap is new and fully open.
Many thanks again,

 

[oilman]

You need sifu's help to check the thermistor, but as for the heat exchanger, the scaling occurs on the cold water feed side and so increasing the flow is only possible by asking your water supplier to raise the water mains pressure, or if they won't agree to this more than reasonable request, descaling the heat exchanger, unless you replace it.

 

[sifu]

Ferroli 77F Thermistor readings: 890 ohms @ 10 C, 1.25 kohms @ 80 C. Try swapping the wires of the heating and h/w thermistors. They are identical, so if it then modulates you know it's the thermistor. If these tests show the HW thermistor to be ok then try adjusting potentiometer P2 on the PCB (this controls HW temperature). If this has no effect then I suspect a new PCB.

 

[vmr]

Hi Sifu, Oilman,

Thank you very much for your help. I've checked the thermistor - it measured 915 Ohms before turning on the hot water (~20C) and 1.020 Kohms at the moment boiler cuts off - unfortunately had no proper thermometer. Also tried adjusting P2 - it was at the maximum clockwise, I've turned it 1/4 anticlockwise, but the boiler still cuts off. I'll try to adjust P2 again, but it seems there is another problem: I've measured the hot water flow again and it was 8 l/min, which is low and the water overheats. The water pressure seems to be O.K. - the cold water flow is much stronger about 24 l/min. The reduced HW flow seems to be a result of scaling as Oilman suggested. Any advise on descaling procedure? I found some chemicals, but they are for CH loop-radiators use. I suppose the HW loop should be treated separately, but have no idea how to do it. Please, help.
Thank you very much

 

[spence]

hello vmr,

just been reading your questions on your boiler problems.i have simular problems with my boiler,low rate of hot water flow.have been advised that the heat exchange unit needs descaling..........so next question is how do it?any help would be great! Cheers Spence

 

[oilman]

I use CH descaler, you could use kettle descaler. Try to make sure it flows somehow, and it'll take a couple of hours.

Or you could get a plumber to descale it for you.

 

[sifu]

If adjusting P2 DOES NOT affect the gas rate then check the voltage on the modureg. This should vary between 4 and 20VDC depending on temperature. If readings do not vary then you have a faulty PCB, if readings are ok then you have a faulty modureg/gas valve i.e. a CORGI job.

 

[europa]

I know this is an old thread - but I have a similar problem with my Ferroli 77 - how much does it cost to have a new heat exchanger fitted - I know they cost about £300, but how much would fitting be?

Also, what's involved in 'de-scaling' the existing heat exchanger - costs, time etc?

Any help appreciated!! Thanks

 

[Softus]

I know this is an old thread...

Well start a new one then!

 

[europa]

Helpful - thanks

 

[werewolf]

low flow rate could also be due to blocked/restricted water filter on inlet to water flow switch.

 

[markejon]

Not sure if your boiler is the same, but after several years, I finally bottomed the problem on my Modena 80, which kept shutting down when running DHW.
The overheat stat on the upper left hand side of the heat exchanger was operating at around 50 deg, instead of 100. It's actually a mechanical device operated by a bi-metal pad - The excess force of the spring clip used to mount it pushed in the face plate in slightly, causing it to switch off at a much lower temperature than it should.
A Ferroli engineer suggested that the heat exchanger might be blocked, but a new stat, and modified clip has the boiler running much better than it ever did.
Careful when renewing this - It operates at 240v.