Celtic FF Intermittent Pilot

[whitegold]

Celtic FF 2.20 of 15 years vintage has recently developed an intermittent Pilot failure.

Symptoms: Only after prolonged DHW usage, eg 10mins shower the pilot will sometimes extinguish. After the main burner shuts down, for the next 20-30 seconds, the pilot gradually fades, so TC cools until eventually the Tev drops out. (The 105deg overheat stat is not opening.)

Has recently had new TC and 105 stat (C&M assembly complete)& Tev.
TC mv output is >spec.
Heat exchanger is clean & clear.
Pilot & gauze clean & clear.
Fan runs Hi/Lo as expected.

Next check will be to pull fan assembly, although blades appear not to be dirty, it's not easy to see if they are clogged.

Past experience with this beast have led me to expect some weird and unexpected fault,
Many thanks for all suggestions, and thanks for reading this far!

 

[HarrogateGas]

Is the return filter clear? That would be the next thing I would check

 

[whitegold]

Is the return filter clear? That would be the next thing I would check

Return Filter is clear.
CH section runs fine and as expected.
It's actually only after a prolonged run on the DHW that the pilot fails.

 

[HarrogateGas]

How have you proved the heat exhangers are clear?? If its a chaff celtic ill place money that they are not.

 

[whitegold]

How have you proved the heat exhangers are clear?? If its a chaff celtic ill place money that they are not.

Sorry, should have written "Primary Heat Exchanger", is clear.
Have inspected the fins externally with pocket lamp and mirror.
The boiler 1/3 & 2/3 control valves cycle according to DHW demand, and the primary circuit runs reasonably quietly & smoothly.

Checking secondary Water/Water exchanger is another matter, of course.
However, DHW throughput seems acceptable, and system has been Ferox'd occasionally, when individual rads have been added/replaced.

 

[pannierstan]

How have you proved the heat exhangers are clear?? If its a chaff celtic ill place money that they are not.

Sorry, should have written "Primary Heat Exchanger", is clear.
Have inspected the fins externally with pocket lamp and mirror.
The boiler 1/3 & 2/3 control valves cycle according to DHW demand, and the primary circuit runs reasonably quietly & smoothly.

Checking secondary Water/Water exchanger is another matter, of course.
However, DHW throughput seems acceptable, and system has been Ferox'd occasionally, when individual rads have been added/replaced.

You need to check the temperature differential between the primary flow and return and then those against the tap temperature.
My bet is that the secondary heat exchanger or calorifier is not transfering heat.

Diagnosis murder

 

[cross thread]

clean the braided hose from the pump it gets restricted in the bend this can cause probs

 

[Agile]

When "we" talk about the primary heat exchanger being "clear" then "we" are referring to the water ways rather than the gas side fins!

Its likely its scaled up after so long.

However, your fault seems to me to be typical of an opening o/h stat.

Please describe how you are so convinced thats not the problem???

Tony

 

[whitegold]

Gordongas, & Pannierstan
Thanks for the replies, I'm going to descale again, as I've been concerned about scaling / low flow in the Primary circuit, but I'm not sure that this is the core problem.

Tony:
1. By direct observation of the Pilot flame, which dwindles and eventually fails accompanied shortly afterwards by the click from the Tev releasing. This occurs about 30-60 seconds after the DHW demand ceases (hot tap closes)

2. Temp at the OH stat junction at this point is typically less than 75C.

3. Continuity measured at OH stat immediately after trip shows stat closed.

4. To lay that to rest for once and for all, I've just tried to replicate the trip with the OHstat clear of the outlet pipe, but Sod's Law has intervened, and the intermittent fault refuses to occur on demand.

BTW Temp drop across main boiler due to the Secondary heat exchanger is around 66C in / 86C out.
A quick note regarding the Flue, it's just straight out through a cavity brick wall, appears to be in exactly the same condition as the day it was fitted.

 

[Agile]

I dont understand your temperatures. It sems you have more coming out than going in! They both seem rather high but what are the corresponding DHW temps.

I would tend to agree about the o/h stat but something is causing the pilot to go off. I would repeat the test you tried under surveilance.

Tony

 

[whitegold]

Tony, thanks for your reply.
The temperature measurements refer to the points here on this sketched plan.

I'm going to try lifting off the OH stat again and running it (strictly under surveillance, as you so rightly mention) until it either trips, or I get fed up with watching.

Cheers!
Simon

 

[Agile]

The primary temperatures seem rather high as if the secondary HE is not very efficiently transferring the heat.

More to the point perhaps is that most boilers have the flow through the secondary HE in the other direction, that is hot in-hot out. Your model may be different though.

Tony Glazier