DHW was perfect, then erratic, now useless, though CH seems to be OK. I
don't understand what is going on and I don't want to replace bits at
random (!) - if anyone could shed some light I would be very grateful.

The hot water runs for a few seconds, then the boiler shuts off. A
'Reset' gets things going again. The length of the run time has steadily
diminished over the last year or so.

The boiler is a Ferroli Domina, approx 10 years old, and the Ferroli
help line suggested replacing the DHW thermistor. I've bought a
replacement thermistor but plugging it into the connector in 'dangle
mode' seems to kill the DHW completely, yet another thing I don't
understand!

Any ideas from the gurus?

Thanks
Tim

On Jun 1, 3:42*pm, Tim wrote:
DHW was perfect, then erratic, now useless, though CH seems to be OK. I
don't understand what is going on and I don't want to replace bits at
random (!) - if anyone could shed some light I would be very grateful.

The hot water runs for a few seconds, then the boiler shuts off. A
'Reset' gets things going again. The length of the run time has steadily
diminished over the last year or so.

The boiler is a Ferroli Domina, approx 10 years old, and the Ferroli
help line suggested replacing the DHW thermistor. I've bought a
replacement thermistor but plugging it into the connector in 'dangle
mode' seems to kill the DHW completely, yet another thing I don't
understand!

Any ideas from the gurus?

Thanks
Tim

I'm far from a guru on this, but no-one else has replied. It just
strikes me as symptoms that would fit with a failing divertor valve.
Poor flow would cause rapid overheat.


NT

On Jun 1, 6:43*pm, Tabby wrote:
On Jun 1, 3:42*pm, Tim wrote:





DHW was perfect, then erratic, now useless, though CH seems to be OK. I
don't understand what is going on and I don't want to replace bits at
random (!) - if anyone could shed some light I would be very grateful.

The hot water runs for a few seconds, then the boiler shuts off. A
'Reset' gets things going again. The length of the run time has steadily
diminished over the last year or so.

The boiler is a Ferroli Domina, approx 10 years old, and the Ferroli
help line suggested replacing the DHW thermistor. I've bought a
replacement thermistor but plugging it into the connector in 'dangle
mode' seems to kill the DHW completely, yet another thing I don't
understand!

Any ideas from the gurus?

Thanks
Tim

I'm far from a guru on this, but no-one else has replied. It just
strikes me as symptoms that would fit with a failing divertor valve.
Poor flow would cause rapid overheat.

NT- Hide quoted text -

- Show quoted text -

Yes I agree, it sounds like the diaphram in the divertor valve has
perished.
It is possible to get spare diaphrams for some boilers, if not the
whole valve will need to be replaced.

In article ,
Tabby writes:
On Jun 1, 3:42*pm, Tim wrote:
DHW was perfect, then erratic, now useless, though CH seems to be OK. I
don't understand what is going on and I don't want to replace bits at
random (!) - if anyone could shed some light I would be very grateful.

The hot water runs for a few seconds, then the boiler shuts off. A
'Reset' gets things going again. The length of the run time has steadily
diminished over the last year or so.

The boiler is a Ferroli Domina, approx 10 years old, and the Ferroli
help line suggested replacing the DHW thermistor. I've bought a
replacement thermistor but plugging it into the connector in 'dangle
mode' seems to kill the DHW completely, yet another thing I don't
understand!

Any ideas from the gurus?

Thanks
Tim
I'm far from a guru on this, but no-one else has replied. It just
strikes me as symptoms that would fit with a failing divertor valve.
Poor flow would cause rapid overheat.

Also, secondary heat exchanger blocked on the primary side, usually
with rust from elsewhere in the heating system, such as the radiators.
That would explain how it's slowly got worse too. Does the system have
inhibitor in it which is replenshed when serviced? If it's a sealed
system, do you have to top it up often (which will reduce effectiveness
of the inhibitor)?

Less likely, but scaled up secondary heat exchanger on the secondary
side might result in too little heat transfer, causing primary to
overheat. (This more often causes the running water to alternate hot
and cold as the boiler cycles on and off.)

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

In message , Tim
writes
DHW was perfect, then erratic, now useless, though CH seems to be OK. I
don't understand what is going on and I don't want to replace bits at
random (!) - if anyone could shed some light I would be very grateful.

The hot water runs for a few seconds, then the boiler shuts off. A
'Reset' gets things going again. The length of the run time has
steadily diminished over the last year or so.

The boiler is a Ferroli Domina, approx 10 years old, and the Ferroli
help line suggested replacing the DHW thermistor. I've bought a
replacement thermistor but plugging it into the connector in 'dangle
mode' seems to kill the DHW completely, yet another thing I don't
understand!

First question is

What's the DHW flow like?

In other words, have you got a scaling problem?

The progressive deterioration would be consistent with this, especially
in DHW mode where you are continually refreshing the supply through the
heat exchanger

Can you measure the resistance of the old and new thermistor to check
one against the other at room temp and 70 degrees?

Others have suggested the diverter valve diaphragm, but the progressive
nature of the problem would point me towards heat exchanger scaling



--
geoff

On 01/06/2011 15:42, Tim wrote:
DHW was perfect, then erratic, now useless, though CH seems to be OK. I
don't understand what is going on and I don't want to replace bits at
random (!) - if anyone could shed some light I would be very grateful.

Thanks for all the suggestions - they all worked. All of them.

Following up the tips, I realised that I had been diagnosing a DHW fault
by fiddling with the CH sensor. Connecting the DHW lead to a dangling
replacement sensor seems to have fixed the problem; this weekend I will
do the job properly.

Humble apologies..

--
Tim