DHW fine, central heating not working at all

[Cohen]

Hot water from the tap works with no problems but the radiator side does not come on at all. Things I've checked:

* Boiler is in DHW + CH mode

* Radiator temp control on the boiler panel set to max

* Room thermostat is calling for heat (set to 30c)

* Digital programmer is calling for heat (tried Timed, Advance, Boost and Constant)

* Pressure needle in in the green zone.


Boiler is an Alpha HE CB33. Can anyone suggest any ideas what could have gone wrong?

 

[Cohen]

Here's something that may help: Previously, if I was to cycle through the modes on the digital programmer (Timed, Adv, Boost etc) I would hear a light click noise. This is not the same click noise as when the boiler ignites as that is much louder. When the CH stopped working this light click noise went away when cycling the modes on the programmer.

As such, I thought the digital programmer might have failed and so took it out and put in the analog clock to control it instead. This analog clock came with the boiler but it was never installed as I still had this digital programmer left over from my previous boiler. The analog clock is wired up correctly and I can hear it ticking away. Setting it always on did not turn on the CH nor did timing it so that it should be on at the time of testing. That rules out the digital or analog timer as being the problem.

 

[Cohen]

Anyone have any clues as to what might have failed?

 

[twgas]

Sounds like the diverter is not operating, can you check that power is getting to it, or not.

 

[Cohen]

Looking at the wiring diagram for this boiler doesn't show the diverter valve. The nearest thing is the DHW flow switch which is plugged into the diverter valve.

I will get a multimeter and check the connections to flow switch if that's what you mean?

 

[Agile]

It sounds as if you have an old water operated diverter valve. They are not usually used on HE boilers as they have been superceded by motor driven diverters.

If its the old brass type then your fault would be caused by a jammed manifold section. However, the reality is that does not normally ever happen like that.

Can you measure the resistance of the temperature sensors?

Tony

 

[Cohen]

Hello Agile thanks for the response and apologies for the delay in the getting back to you, I went away on a short break.

Anyway, I measured the resistance and got these readings...

DHW sensor: 11.92
Primary sensor: 8.97

I'm keeping my fingers crossed that they should both be the same value and as such these figures would indicate one or the other has failed

 

[Agile]

For each sensor to be at the same temperature the boiler needs to have been off for some time.

Thye should be at the same reading if at the same temperature.

They are best replaced as a pair ! They can age in use subject to the thermal cycling. They also seem to shift resistance after being subjected to over heating ( when water pressure has fallen to zero! ).

Tony

 

[Cohen]

Hello Agile,

I had the oppurtunity to test the temperature sensors again first thing this morning having not used the boiler since 10pm yesterday evening. Here are the readings:

DHW sensor: 15.11
Primary sensor: 14.97

Is the difference still sufficient enough to warrant replacing the pair of them? Or are they within margin of error (or least accuracy of the cheap multimeter I used)?

 

[Agile]

I dont know!

Those resistances are very close to each other.

However, there is also the tracking to take into account. i.e. what would they read if both at 70° ?

I have experienced sensors which read apparently the same resistance like that but they still regularly generated the fault codes whereas replacing them solved the problem.

Tony