Baxi 105e kettling for ~30s after pump & AAV replacement + Fernox F1 – worth adding Fernox F2?

[micpwelsh]

Hi everyone,
Looking for a bit of advice on my Baxi 105e.
I recently replaced the pump and the automatic air vent (AAV) diaphragm, and added a can of Fernox F1 Express inhibitor into the system. Since finishing the work a couple of days ago, the boiler kettles/makes a noisy kettling sound for about 30 seconds right when it fires up, but after that initial half-minute, it settles down and runs completely fine.
I’m wondering if it's worth dosing the system with Fernox F2 Boiler Noise Silencer? I know F2 is designed to reduce kettling and plays well with F1, but I'm unsure if that's the best next step or if there's something else I should check first.
Could this just be lingering trapped air in the primary heat exchanger following the drain-down and pump swap, or does it sound like a flow issue / loosened debris after adding the F1?
Appreciate any thoughts or suggestions!

 

[CarloHeatingEng]

I’d not wait a week on that. Thirty seconds at every firing isn’t air working its way out, that fades after a few cycles. Something isn’t right since the pump went in.


Is it like for like? Same model, same speed setting, same way round? A stronger pump, or the same one set a speed higher, behaves differently on an old system to the one that came off.

 

[micpwelsh]

It was exactly the same pump I took out just a new one . Luckily the noise has stopped completely over the last couple of days . Hopefully doesn't come back

 

[micpwelsh]

Quick update on this...
Everything was running fine for a bit, but I had a new electric smart meter fitted today, which meant the main power to the house was cut off during the installation. Once power was restored, the noise came back.
Looking at it closely now, I don't actually think it’s kettling like I originally thought. What's happening is that in short bursts (around 5 seconds at a time), the flame on the burner suddenly gets much stronger and roars louder.
I reset the boiler and it goes completely back to normal—and stays normal—until the power to the boiler is cut again. Once power is isolated and restored, the issue comes right back until another reset.
Given that it only acts up after losing power, I'm thinking the PCB might be failing or losing its stored settings when off. What do you guys think? Has anyone seen flame surging like this triggered by a power cut?

 

[Daviesmith]

Replace the PCB (£40 on eBay) just make sure all the settings are the same . If it's not that it might be the gas value and you'll need the engineer for that !

 

[micpwelsh]

Is changing the PCB as easy as it looks ?

 

[CarloHeatingEng]

Unless you're Gas Safe yourself, I'd hold off fitting a PCB. A flame that surges and roars in short bursts is a combustion problem until it's proven otherwise. I'd stop using it until a Gas Safe engineer has checked the gas pressure and combustion with an analyser, and keep a working CO alarm near it meanwhile. Tell them it only starts after the power's been off. That's a useful clue.

 

[micpwelsh]

It was serviced 2 weeks ago

 

[Daviesmith]

Looking strictly at the clues in your update, the PCB is definitely the prime suspect over a mechanical or gas fault:
Triggered Exclusively by Mains Power Loss: Physical issues like a blocked heat exchanger, bad gas valve, or system sludge don't appear out of nowhere solely because the main power to the house was switched off for a smart meter install. The fact that it worked fine right up until the outage directly points to an electrical boot-up or memory issue.
Instant Resolution via Reset Button: If there were a physical gas pressure problem, mechanical blockage, or severe combustion defect, simply resetting the boiler wouldn't instantly "cure" it and allow it to run completely normally until the next power cut. A manual reset clears the microprocessor/memory state on the board, which forces a clean cold-boot.
5-Second Flame Surges: That brief 5-second ramping/roaring right after power restoration indicates the PCB is failing its initial ignition/modulation startup routine when boot-loading after a hard power drop, causing the gas valve to hunt until a hard reset forces the board back into its normal run state.
If the issue disappears completely after a reset and only returns when power is isolated, the board is failing to initialize its software parameters properly on boot-up. Swapping out the PCB should solve it.

 

[denso13]

Swapping out the PCB should solve it.

Perhaps, but it isn't really a DIY job.

 

[DP]

Quick update on this...
Everything was running fine for a bit, but I had a new electric smart meter fitted today, which meant the main power to the house was cut off during the installation. Once power was restored, the noise came back.
Looking at it closely now, I don't actually think it’s kettling like I originally thought. What's happening is that in short bursts (around 5 seconds at a time), the flame on the burner suddenly gets much stronger and roars louder.
I reset the boiler and it goes completely back to normal—and stays normal—until the power to the boiler is cut again. Once power is isolated and restored, the issue comes right back until another reset.
Given that it only acts up after losing power, I'm thinking the PCB might be failing or losing its stored settings when off. What do you guys think? Has anyone seen flame surging like this triggered by a power cut?

Great boiler but will need gas engineer who knows the model

 

[CarloHeatingEng]

Serviced two weeks ago is a reason to ring whoever did it, not a reason to rule the burner out. Ask them back and tell them exactly what you told us: since the power went off for the meter, the flame surges and roars for about 5 seconds at a time, and a reset clears it until the next power cut.

The good news is they can make it happen in front of them by switching it off at the spur and back on. There's nothing like an engineer seeing the fault live, so it should be straightforward with them there. The PCB may well be the cause, but a roaring flame needs ruling out first, and they'll check the board while they're at it.

Until then I'd leave it off.