Good question James. Some time ago, I asked this question of a State code official and the reply was: "Flexible connectors are permitted to extend through an appliance housing where the connector is protected by means of a factory equipped grommet". A local gas company official responded with the same. So I assumed it was allowed, but then: I have found a few furnace installation instructions that state what you found. The installation instructions of one popular brand of connector states: "DO NOT conceal connector or run connector through enclosed outdoor BBQ pits, walls, partitions, floors or appliance panels. So the answer to the question of connectors passing through a furnace or boiler cabinet is yes and no.