Worcester Bosch 280 RSF - Hot water problems

[Waffty]

Hi,

I've had an intermittent fault on my 280 RSF for around 4 years now. Every time the British Gas engineer shows up, the fault doesn't 'show' itself, until they leave.

The fault itself is related to water temperature i.e. the water comes through luke warm, then goes cold, luke warm, then cold etc. When I check out the boiler, the demand light is on & the main burner lights for a minute or so, then the tone of the burner changes (kinda like a rushing noise), then goes out (the pilot light stays on). Then there's a click and the burner fires up again, runs for a minute and the whole cycle starts again.

The H/W works fine when the heating is on, so it appears to be something specific to the hot water circuit.

To date, the thermistor has been changed twice, the spark has been rest(?), the cylindrical water to water heat exchanger has been replace (with a square finned assembly) + it's been cleaned more than my new borns bottom.

I've spoken to Worcester direct and they advised it sounded like an overheat fault but then 'offer' that is could be just about everthing in the boiler except the thermistor, given that it's already been changed twice.

I do have an MP4 of the above in action, if this would be usefulm then let me know and I'll post the link to it (when I've got round to uploading it).

Any ideas?

Thanks
Darren.

 

[Bryan King]

definately sounds like the hot water temp sensor

 

[Waffty]

Bryan,

Many thanks for your reply

definately sounds like the hot water temp sensor

By hot water temp sensor, I assume you mean the picture below, if so, I'm on my third one now i.e the orginal one was swapped with a new one, this new one was once again swapped with a new one & neither of them made any difference to my issue.

Here's a photo of the new heat exchanger (square as opposed to round).

Plus here's the actual video of the fault, which incidently ran for over 10 minute before I got bored, so here's the edited highlights both in the light and in the dark i.e I turned off the lights in an attempt to get a better image.

(Click on the picture to watch the video)



Anyways, hope you can help.

Regards
Darren.

 

[ollski]

I would be checking the low gas rate setting and then suspecting either the modureg or modulating board.

 

[ChrisR]

Lgr setting certainly. The intervals between high and low (off!) are a bit short sometimes too though...

What's the hot water flow rate while it's doing this Darren? Is it a tap half-open?

Have you (had)checked the things referred to in section "O" of the manual?

I didn't know they'd changed the dhwhe type on these .

 

[HarrogateGas]

That looks to me like (and sounds) like a a relay dropping in and out on the pcb. Has the PCB been changed?

 

[Waffty]

What's the hot water flow rate while it's doing this Darren? Is it a tap half-open?

To be honest, when it's in 'fault' mode, the flow rate doesn't really make any difference i.e I've tried the tap in wide open, the mid point & trickle (just enough to trigger the demand light). I've then tried tap off, cycling the mains power to the boiler and trying again.

Have you (had)checked the things referred to in section "O" of the manual?

Is that the 'flow questions, diagram' in the back of the installation manual? If so, I've read it a couple of times with limited success i.e the fault I appear to have doesn't seem to fit in any of the diagnostic 'routes' on the diagram.

That looks to me like (and sounds) like a a relay dropping in and out on the pcb. Has the PCB been changed?

There is deffo a click before the burner goes out & then another click when it restarts, so it's either a sensor/fault condition reseting the relay, or a faultly relay on the PCB. And in answer to your question, no the PCB hasn't been swapped out.

Many thanks again.
Darren.

 

[scatmanjohn]

Without wanting to encourage you to dabble but has BG engineer tried running it with the front case partially open to see if it still does it.

 

[ChrisR]

Fair one.
If thet's not helpful you need to know whether gas valve is being told to go high low or on off, and which if any input signals are making the board say that. Needs a manual and a meter and some knowledge.

 

[Waffty]

Without wanting to encourage you to dabble but has BG engineer tried running it with the front case partially open to see if it still does it.

I'm guessing not, seen as in the last 4 years, none of the BG engineers have 'admitted' to seeing the fault in person i.e I'm guessing they've only changed the items above to keep me happy, with the £6 thermistor being the easiest and cheapest item to do so (hence why it's been changed twice).

In respect to the water to water heat exchanger, this was changed after the thermistor and in defence of the engineer, it was full of limescale. Since then I've installed an electronic water conditioner around 5 to 6 meters away from the boiler itself.

I may give the board a swap out, as I may be able to 'borrow' one for a week or so. So in nothing else, I can rule that out.

Thanks again.
Darren.

 

[Waffty]

definately sounds like the hot water temp sensor

Now here's a thing, the temp sensor has been changed twice, however in prep for my impending main PCB change out, I thought I'd have a look at the connector cable layout etc.

Anyways on tracing the cable runs etc, I noticed that the two wires to the H/W sensor were very tight. So I cut the cable tie, loosened the wires + removed the connector to the main board and just checked it for continuity.

I put it all back together and waited for the replacement main board to be delivered, only to notice that the hot water didn't miss a beat i.e the burner never cut out once.

So hopefully I'm not tempting fait but at leas at this moment in time, I've got hot water and all I've done it remove the 'tension' on the two wires and re-seated the connection to the main board.

If this is the cause, then it could explain why the BG guys missed it i.e they usually take the cover off and have a 'fiddle' before getting started.

Anyways, I'm happy, so thanks again for all your help.

Regards.
Darren.

 

[pannierstan]

Without wanting to encourage you to dabble but has BG engineer tried running it with the front case partially open to see if it still does it.

I think Scatman is on the right track.
Although its somewhat difficult from your video it looks to me like the flame is baffling, The click you are hearing is the relays and gas valve dropping but only because either the air pressure switch has dropped or the pilot has baffled and stopped rectifying. In fact it has to be the rectification otherwise the pilot would go out completely and ignition would reattempt.
If you look carefully you can see the pilot egtting badly disturbed just as the main burner goes out then what appears to be after the products a are cleared the pilot then rectifies again.
I would be looking at flue ways and fan.

Stan

Just had another look at the video. The flame is definalty baffling. Get BG to replace the fan or see if it runs O.K with the front off.