alpha 240eco..no heating

[paolo1969]

hi guys,i have an alpha 240eco which has developed a no heating fault.
hot water is fine but when i call for heating..zilch.symptoms as follows

1,selector switch to heat/dhw neon is lit

2,thermostat to max HEATING neon **UNLIT**

3,boiler then continues to carry out normal functions,pump runs,fan runs burner lights as if i had turned on hot tap.

following the fault finding flow chart, external control calls for heat,

240v across terminals 1 and N,

continuity between terminals 31&32 on pcb m7 connector,

HOWEVER pcb m11 connector 20v across terminals 48&52 is absent(no volts).

the book says to change pcb but as you all know these pcbs are bulletproof,so my question is...is there any other component which can fail and give the no volts reading?because the first fix battlecry of "change the pcb"is made far too easily when,in actual fact its usually another component.if not i shall just change pcb.
any input would be greatly appreciated

 

[ChrisR]

burner lights as if i had turned on hot tap.

eh?

So what isn't working??

 

[paolo1969]

MAYBE I DIDNT EXPLAIN MYSELF IN THE FIRST LINE OF MY POST WHEN I SAID I HAD NO HEATING

 

[gas4you]

1,selector switch to heat/dhw neon is lit

2,thermostat to max HEATING neon **UNLIT**

3,boiler then continues to carry out normal functions,pump runs,fan runs burner lights as if i had turned on hot tap.

But you say in 3 that the burner has lit?

 

[paolo1969]

thats right,i'll try to explain better.
imagine boiler completely switched off and isolated from mains.
switches set to..
selector switch at dhw and heating
thermostats to max
clock calling for heating.....
switch on at mains.
***heating neon unlit (should be illuminated)***
pump runs
primary flow switch operates
fan runs at max speed
air pressure switch proved neon stays off
burner lights
burner output modulates
burner goes out
fan reduces speed
pump continues to run.

this all happens but the rads do not heat up, the only rad which gets warm is the one directly above the boiler.

so in following the fault finding flow chart in the installation and servicing manual, when i come to the box marked thermostat to max,heating neon illuminated yes/no?
i follow the no string which then leads onto the pcb m11 connector which should have 20v DC across terminals 48&52.but it does not.
the manual says replace pcb
all i wanted to know was ...could another component in the boiler be faulty and produce wrong readings in the pcb,making me think that the pcb was at fault,
because there are loads of posts on here where the pcb was changed but the original fault still remains.i just wanted to be absolutly sure before changing pcb.
cheers paolo

 

[gas4you]

From your last post I would say that your pcb is probably not faulty.

It sounds to me as though the heated water is not getting away from the boiler so the boiler is just shutting down as it should, hence no volts where you are testing.

Basics, are the F & R isolators under the boiler fully open? Are all the rad valves/trv's open?

If above is yes, then could possibly be ch thermistor or possibly blockage or even dv stuck in HW position.

Without being there it is hard to diagnose this one.

 

[paolo1969]

hi,thanks for speedy reply
i have carried out the basic checks you suggested and my initial thought was the dv.how can you tell if it is stuck at hw?
i think it could be the flow valve rod is stuck.it is pushed up against the microswitch.is this the open position?if this is so does the diaphragm in the flow valve need changing? or the complete dv?

 

[gas4you]

Usually on these you will need to replace the complete DV valve.

I always fit new, but some prefer to buy the recon kit and strip out, clean and rebuild.

Replacing will be quicker, but make sure you ask for new seals 'o' ring pack as well. This is the most expensive option for you though.

 

[paolo1969]

thanks again