It all comes down to the basics, ID - capping it off, it would have to be an immediate danger to life or property, ie gas escape, spillage, incomplete combustion etc you all no the drill, anything or than immediately dangerous situation at the time of testing is classed as At Risk. Just because you don't know the ventilation requirements, the burner pressure or gas rate doesn't make it a life threatening situation. You cant just cap appliances off because you don't feel comfortable leaving it on. Under gassed or over gassed appliances are At Risk as long as it doesn't effect the appliance operation ie flame lift or delayed ignition, undersized or no ventilation again is At Risk so long as flue flow & spillage & general conditions are OK.