We smelled gas and got Fortis out to investigate: carbon monoxide level was 500 ppm each time gas boiler cycled on. First company thought it was manifold or gas pressure problem which he couldn't fix without ordering parts or a new boiler, while plumber who knew system suggested it might be the gas jets needing cleaning. We saw B&B truck in our neighbourhood and phoned to ask for emergency consult. Rahim came next day, diagnosed three problems, took boiler apart, cleaned it thoroughly, cleaned vents and found a loose piece of metal that had come unwelded from vent hood, identified its purpose even though it wasn't listed in the manual or parts diagram and figured out how to reattach it so we could get our gas boiler running today instead of waiting for a new part (we'll get part later). Sherlock Holmes of Gas Boilers. He's awesome. I'd highly recommend his services.