Diagnosing a wonky boiler

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

Reply to
Tim
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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

Reply to
Tabby

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.

Reply to
chudford

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.)

Reply to
Andrew Gabriel

In message , Tim writes

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

Reply to
geoff

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

Reply to
Tim

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