Worcester Greenstar 432kHz noise?. SOLVED. Honeywell Gas Control Valve, faulty batch.

3 Jan 2024
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United Kingdom
Worcester Greenstar 4000 combi boiler 25kW
Install new September 2021.
Last gas safety check October 3 2023
Combustion reading 0.0003 initial 0.0003 final. (is that very low?)

432kHz Drone, howl, noise from within boiler case.
No pipe / rad knocks.
Boiler located upstairs. External 30mm condensate gravity discharge.
Has been intermittent over last couple of weeks lasting 20-30s during heating/hot water modes fire ups.
Noise last 10s during heating/hot water mode switch off.

Once the noise has stopped, boiler/pump super quiet once in heating or hot water mode.

Taps supply hot water, rads hot and so diverter appears OK, no significant chatter.
Pressure in the green, no rad cold spots, no water gurgle when pump runs.
No fault codes.

Noise went 'nuclear' last night continuous and very loud.
Have had to power off boiler due to the noise which can be heard outside.

Loud enough to wake the neighbours and we are detached by 20 meters !!!

...how do i know its 432kHz?
Last edited:
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Your boiler is resonating, it is still under warranty so get worcester out
yep ...worcester on the way tomorrow...
will report .
...meanwhile i can busy myself fixing the fence that blew down at the same time!!!
and no its not the wind blowing across the top of the flue, been happening in still conditions too. if only.
just a thought, Combustion reading 0.0003 initial 0.0003 final. (is that very low?) can it be too low?
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hi, not sure what you mean. the only 'readings' i got on the Safety Record sheet is see image attached
Screenshot 2024-01-03 at 11.01.43.png
The ratio is pretty meaningless...it's the ratio between carbon monoxide and carbon dioxide. The figure is "designed" to show how safe the combustion is but the relevant figures we need are the carbon dioxide at the high and low combustion settings.

Given the boiler is so new a proper annual combustion check should have been done and the figures obtained entered into the service log at the back of the manual (for compliance with the warranty). Have you checked?
Gasguru, guess what. you say....'service log at the back of the manual (for compliance with the warranty)' ???
Checked the back pages of the Installation/Maintenance book, not a single Service record filled in !
Dammit...my fault, I assumed wrongly that the Gas Safety doc sent to me by the 'service engineer' from the company that originally fitted the boiler, would be the record required. Was not shown this by the company that fitted boiler on commission day.

Called Worcester Bosch direct, engineer arrives 07:00 next day.
We start boiler and 432kHz whine kicks in.
Engineer asks to see past boiler record C0/C02 rates. Engineer opens last page and just says...'Oh'...er...!
I can read his body language straight away.
Checks the levels/ratio and off the scale.
Would have, should have shown up on recent service as this noise had been intermittent before then.
Engineer then went straight to the Gas Control valve (Honeywell?.
Valve failing. Suggested it most likely to be one of a faulty batch supplied to Bosch during the boiler build.
Valve replaced, CO/CO2 back to spec.
Job done.
So there you have it 432kHz whine is going to 99% one of the faulty Honeywell units.
thanks all.
Maybe Worcester need to add the words 'Service Record' on the front of the Install /Maintenance Manual. ?
Now I am off to see the installation company and explain why i wont be employing them in the future suggest they refund the recent service charge.

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