Selgas
He is testing to ensure the flue gases are being pulled correctly up the flueway to vent to atmosphere – it is often refrred to as being a smoke test as some tradespeople use smoke matches. Me? well I use a specific draft gauge – that way I know when it is right.
It is a task that is by far better left to the experts – as a faulty flue can have disaterous consequences – DON’T play about with gas appliances or their components – get the assistance of a suitably qualified gasfitter to ensure yours and everyone else’s safety is paramount. 
Selgas Services Ltd
Craftsman Gasfitters, Plumbers, Electrical Service Technicians
