Biasi Combi troubleshooting 36v instead of 24v

[DanDIY]

Hi. I have a Biasi Riva advance 32HE.

It's quite old and over the years j have become quite familiar with fixing it.

This week it has been locking out with a flue temp ntc alarm. I think this must be a common alarm, as I've had it before and actually had a spare probe. With the spare probe (which was used, but the resistance checked out ok) fitted I'm still getting the alarm.

I have ordered a brand new part. But I'm not convinced this is the root cause.

I was doing come more checks today on the PCB. And where the manual says I should have 24v at the fan, I have 36v.

Has anyone seen this before. Is the manual usually correct. Could Biasi have ran out of 24v transformers and interchanged with 36v. Or could a fault cause this.

I believe the flue probe ntc alarm means that the boiler is finding the flue gas temp to be incorrect. (not sure if too high or low).

What I'm thinking is; if the fan is seeing too much voltage, would it spin faster. Could this make the boiler unable to control the combustion temperature. Or would voltage not effect fan behaviour?

 

[MeldrewsMate]

The measurement you get may depend on the method used to measure it. 24vdc or 24vac? Is the quoted value with the fan running?
My gut feeling is that you are barking up the wrong tree.
The flue gas alarm is most probably there to shut down in the event of a high flue gas temperature. They are often called dry-fire sensors, because the flue gasses come out much hotter if there is no water in the heat exchanger to take their heat away.

Suggest you gather some evidence...
Is the Riva a combi boiler? Does your hot water exhibit the same fault?
IS THE CIRCULATING PUMP WORKING?

 

[DanDIY]

Hi. Thanks for the reply.

I have been measuring in DC. The manual didn't say either way, but I assumed 24v would be DC?

I agree the flue is probably too hot, which is why I thought fan speed might be a factor.

The pump is spinning. Central heating gets warm pretty fast (before tripping) and it's not very noisy. It was only changed a couple of years ago. So I reckon this is ok.

The other possibility I guess is whether the primary heat exchanger could have been blocked and not enough circulation to cool the flue.

 

[DanDIY]

A bit more info. The manual says "under normal CH or DHW operation" the voltage should be 24v.

What I have found is that the red and black wires that it suggests to test across have 36v DC at idle, which drops slightly to around 30-33v DC when the fan is running. There are actually 5 wires to the fan, but the circuit diagram doesn't explain the others.

I guess that the other wires must control the on, off and speed as the red / black seem to be permanently energised.

 

[durhamplumber]

36v DC at idle, which drops slightly to around 30-33v DC

As said...I think you are being side tracked

 

[DanDIY]

As said...I think you are being side tracked

Where do you think the real problem might lie then? I assume that you also don't think its just a dodgy probe?

Today I had the the underfloor heating running flat out (heating is in two zones) and it ran for several hours without tripping. I expect this might have reduced the return temperature, as it is a lot more load. Would this be relevant?

 

[durhamplumber]

Where do you think the real problem might lie then? I assume that you also don't think its just a dodgy probe?

Today I had the the underfloor heating running flat out (heating is in two zones) and it ran for several hours without tripping. I expect this might have reduced the return temperature, as it is a lot more load. Would this be relevant?

Personally I would not be wasting my time on an old Biasi..They are crap new..It is well past its useful days if near 10 year old.If it is a hobby of yours fair enough..From experience, most, but not all intermittent electrical anomolies are main pcb related..Thermistors etc tend to go faulty and stay that way...

 

[durhamplumber]

Or would voltage not effect fan behaviour?

More volts across a fan winding will make it go faster yes...But not always that simple with windings,especially modern variable speed ones with a pcb attached to them.

 

[DanDIY]

Personally I would not be wasting my time on an old Biasi..They are crap new..It is well past its useful days if near 10 year old.If it is a hobby of yours fair enough..From experience, most, but not all intermittent electrical anomolies are main pcb related..Thermistors etc tend to go faulty and stay that way...

It's 13 years old and my brother (gas Safe installer) has been saying it needs replacing for at least 7 years. I suppose it has become a bit of a hobby in proving my brother wrong.
Parts are cheap, one breakdown every couple of years costs me £20-70 to fix. It will take me a while to claw back the cost of a replacement boiler at that rate, although putting a value on the irritation factor when it plays up is difficult. If I'd have had a new one 7 years ago when originally told "biasi's are crap" I guess my replacement would now be nearing the end of its life, it would certainly be older than the Biasi when I was first told it was not worth repairing!

I think I'll ask my brother to get a price on a replacement, as the annoyance factor is significant. But he is unlikely to be able to drop on a boiler swap at short notice in this weather. So I could still do with sorting this in the short term.

I don't think I'd call this problem intermittent. It's quite predictable. Always the same error code after about 1 hour of operation. It worked for longer before throwing the error code with ufh drawing heat as well, which suggests to me that it is getting too hot after a period of operation

 

[ianmcd]

Biasi are not the worst boilers, they got a bad name because they were sold in B&Q and were thrown at the wall by DIY installers