Ravenheat cs185 HW okay, CH on-off: any ideas?

[gmh20]

I would appreciate some help with a Ravenheat csi85, 2yrs old. The HW works fine but the CH is malfunctioning.
On selecting CH the pump and fan run, the burner lights then the fan stops after 5secs and the burner goes out 4secs afterwards. This cycle repeats indefinitely with the burner switching on for 7-9secs then going out for 10-15secs.
The timer/room thermostat/valves are all calling for heat (linked out), the pump runs fine, fan runs normally, no obstruction in the flue, both CH and DHW sensors register about 20k ohm cold, the overheat thermostat has continuity across it (0 ohms). The APS tubes and venturi are clear, APS switch clicks on blowing tubes. No air in the system. Condensate drain clear (and no sensor anyhow).
Whilst running the fan is getting 240v and the APS has an input of 15vDC and an output of 0.8vDC with the fan off and 1.4vDC with the fan on.
I don't understand how the HW works fine on demand (burner and fan on coninuously) but something is obviously switching the fan and burner off when the CH is activated. I've tried rotating the venturi collar 90degrees (previous post) but no change.
I tried ringing Ravenheat on friday - engaged all afternoon. Any ideas much appreciated. Thanks

 

[gas4you]

Fan modulation board usually needs changing.

 

[sooey]

Dam, I never saw that post, I would have said that.

 

[gmh20]

Much appreciated help, thanks.

 

[Agile]

I don't understand how the HW works fine on demand (burner and fan on coninuously) but something is obviously switching the fan and burner off when the CH is activated. Thanks

The boiler works on DHW because the power is high and the fan is on full.

On CH a lower power is required and the fan fails to power back.

Not mentioned on our visit but the failure might also occur if you run a hot tap and slowly turn the flow down. Can you try that for us?

Tony

 

[gmh20]

The HW also failed over the weekend - burner and fan switching on-off whatever flow rate of HW demanded. I replaced the fan control board this AM and the thing kicked into life again and seems to be functioning normally (after a few resets whilst it sorted its issues out).
The fault finding flowcharts in the service manual are poor (considering how often Ravenheats seem to develop faults) and the fan control board is not even mentioned.
Thanks to those experienced resident pros who have helped me out.

 

[Slugbabydotcom]

Technical and illustrative data are not binding and can be modified without prior notice.

Says that on the front of the book!

The fault finding flowcharts in the service manual are poor (considering how often Ravenheats seem to develop faults) and the fan control board is not even mentioned.

One desirable effect of this from the manufacturers point of view is that it's a deterrent to the incompetent.
It could be that they added these to keep us in work after they got over their thermistor problem.
More realistic is the probability that they were are asked to reduce power consumption by the eco freaks. Their target electrical power consumption has to come down from 170 watt to 100 watts

If an RGI was on the job he would call Ravenheat if he couldn't suss it out.
Considering they claim 22% of the condensing combi market I think their failure rate is fairly low.

 

[Agile]

Considering they claim 22% of the condensing combi market I think their failure rate is fairly low.

I dont think they were saying that two weeks before.

Sales much have jumped just prior to Christmas!

We also have the advantage of the 24 hr tech help line!

Tony