What can cause a boiler to smell of “gas” but not be gas?

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I have a Worcester Bosch Greenstar 24i system boiler that was installed in our utility room in 2015. For the last 2-3 years I have had a very feint smell of what I thought was gas but apparently isn’t. I have had National Grid out twice and they have investigated all over the house and tell me I don’t have a gas leak, the last engineer said he could smell what I could and said it’s not gas. I have had British Gas also investigate during the last two annual services and they say there isn’t a problem with the boiler.

I only get the smell if the CH is on and only if the boiler is alight when the programmer switch’s off never when the thermostat switches off, I now have a Wiser smart programmer/thermostat and I have the same smell when the CH turns off. The smell isn’t apparent every time, just occasionally. The utility door is always closed and when opened if there is a smell it dissipates very quickly. I’m stumped!
 
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when someone carries out a gas tightness test, they are only testing up to the gas valve of the boiler, once the boiler lights you may have a gas leak on the outlet of the gas valve , hence why you only smell it when the boiler is running, all joints after the gas valve need checking with leak detection fluid when the boiler is running
 
Could be rich mix and some gas unburnt and expelled. Try a different gas man for your next service. I know a british gas serviced boiler smelling like that.
 
when someone carries out a gas tightness test, they are only testing up to the gas valve of the boiler, once the boiler lights you may have a gas leak on the outlet of the gas valve , hence why you only smell it when the boiler is running, all joints after the gas valve need checking with leak detection fluid when the boiler is running
The smell is only when CH has been on, never with the HW. It also only happens occasionally, maybe twice a week, and only if the programmer turns off the CH when the boiler is running. The National Grid engineer used some sort of gas detector and even with the smell present the detector didn't register any gas.
 
What things do you think, I have had the boiler checked twice now with no issues found.
Twice by whom? National Grid? I had one not so long ago where a customer’s carbon monoxide alarm was sounding, I attended after another engineer had been out and found a problem with the flue in heating only mode.
 

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