dy Biasi boiler brainteaser :-/
It's a 24S combi. CH works, except that the 3-position switch (ON|OFF|timed)
is broken and stuck in the centre OFF position. On DHW it seems to require a
lot of flow* to get the boiler to fire up at all and when it does the boiler
comes on full bore for a second or so then modulates down to a much lower
burn rate, resulting in lukewarm hot water.
Does the same for summer or winter position of that switch. Temperature
control know is turned up to max: if it is turned down even a little the
boiler stops firing.
The diverter valve seems to be working: voltage across DHW flow switch
contacts changes from 12V to 0V when the HW flow rate reaches the point that
the boiler fires up, and with a judiciously placed mirror (rather than
dismantling the machine as per the service manual
http://www.rads.uk.com/Biasi_tech_2003/303.pdf ) I can see the actuator shaft
of the diverter valve moving out and back operating its microswitch as a hot
tap is turned on and off.
* seems to me, but I don't know how it was when it worked OK or what other
samples of this model are like.
There are two thermistors: on the first picture in the manual both are
labelled 'D.h.w. temp probe'! but on the wiring diagram there's a 'Domestic
hot water NTC' and 'Central heating NTC'. Both have about 8V across with
nothing happening, dropping a volt or two when the boiler fires up on DHW
At this point I ran out of clue. Anyone got any to offer?
If a job's worth doing, it'll still be worth doing tomorrow.