Alpha 240 - HW won't heat without CH

[michaelcs]

Help please for a newbie on this forum.

For no obvious reason (boiler's been working fine for a good few years) the HW on this Alpha 240 combi will only fire up if CH has been running/heating for at least ten minutes. Otherwise, demanding HW brings up the relevant green light but doesn't ignite boiler, and in fact switches the whole boiler off if the CH demand has had it running for less thn 10 mins.

I have checked (removed, circuit-tested, replaced) the microswitches to the primary and DHW flow valves and both are working fine and I've even switched them over. Cables carefully reattached.

I've changed the DHW diaphragm (using kit - which included brass nut which forms the spindle for the pin). Also cleaned and replaced the primary flow diaphragm. (There was no visible wear/damage to either.)

Both the primary and DHW pins are moving freely. When heat/CH demanded they emerge easily far enough to operate the microswitch levers. The springs are strong (pins don't push back easily with a screwdriver).

Thus we have done (quite carefully, yesterday) all the easy checks/remedies recommended in earlier threads in this forum. What next - is it likely to be a problem in the diverter valve itself? Can that be tested before buying a new part (expensive)?

Many thanks in advance.

 

[Agile]

If, and thats a big if, you have correctly tested what you say then I would be considering the temperature sensors.

But I suspect the actual problem is associated with what you say you have already tested.

I suggest you measure the voltage across the CLOSED switch contacts of both microswitches during operation in DHW mode.

The problem for me is that when fault finding I do each test and then move on to the next according to what the last told me.

Tony

 

[gas4you]

Basics first.

Does the HW light come on, on the front when you run a hot tap?

 

[michaelcs]

Agile and gas4you - many thanks for this advice/support.

gas4you: yes the DHW green light comes on when DHW demanded, but it doesn't fire up.

Agile: I removed the two microswitches and tested them with a bulb and battery (and could also hear each one click as the bulb came on). I don't have a voltage tester but am going to get one and carry out the test you suggest across the closed contacts in DHW mode and will report again. Obviously I would love it to be a microswitch fault as that would be the cheapest/quickest to fix.

Would need further advice if I need to check out the temperature sensors - I guess start with the DHW one first? - and whether just to replace or can it be tested?

Michael

 

[gas4you]

As the green light is on, then the dhw microswitch is ok and by default so is the diaphragm.

Test the primary (ch) ntc, or replace it to eliminate it.

 

[Agile]

You can test the resistance of the NTCs when cold.

Should be about 13k.

Tony

 

[michaelcs]

Success!

Retested the microswitches and they were indeed fine. Tested CH temperature sensor, also fine. But the DH one proved defective. Impossible to resistance-test it in place because of the angle/location (couldn't be more awkward). Took it off and it was totally dead. New one fitted (cost: £6) and hey presto, the system works.

I would like to thank both Agile and gas4you for your great help in getting to this solution.

 

[Agile]

You could have followed the wires to the PCB plug and detached it and then measured the resistance from there. That also tests the leads and connector.

Tony