25 years as a volunteer firefighter, almost half as an officer / Chief.

Don’t do it.

When the fire department puts a fan in the door, or opens windows, or cuts a hole in the roof it’s for specific reasons, and there’s one thing in common on all of them: the fire department is on scene!

If we’re putting a fan in the door it’s either to exhaust the smoke after the fire has been extinguished, or it’s to try and clear smoke while the attack team is putting the fire out. The latter is an advanced technique that isn’t practised by all departments, because if the fire isn’t out you’re giving it air and making it bigger.

Cutting holes in the roof is normally done to try and get the smoke out while we’re putting the fire out. The location of the hole is important and the fire will move towards the hole, as someone mentioned “the chimney effect”. We only cut the hole when there’s water on the fire, or it’s ready to be. Again, it will make the fire grow so we have to be in a position to deal with it. Removing the smoke cuts down on the chance of a flashover or a smoke explosion (“backdraft”).

When the fire alarm goes off in commercial buildings it should be cutting the HVAC so that It doesn’t move toxic smoke through the building. (a) it will kill the occupants, and (b) it then makes it more difficult to find the source.

If your whole-house fan isn’t on when your smoke detectors activate then leave it off. If it is on, and you can tie it into your smoke detectors, then it would be a good idea to turn it off. If the fire department get there and it’s something dumb like someone burned the toast or, even worse, the bacon!, and there’s a light haze through the house then you can tell the incident commander that you have a whole house fan and can turn that on to get the smoke out.

If you’re reading this and you don’t have a whole-house fan, and you do something dumb like burning toast, or bacon, or another culinary mishap (current jargon for “burned food”), don’t feel bad about calling for the fire department. It happens every day, we’re only too happy to come and do it, we have the tools and the training for it. It;s not really dumb, but it’s still dumb.

Junction box.

Plus conduit, probably, but depends on exactly what you’re going to do with it.

Could be a hole through the wall, with a stub (short piece) of conduit out into the junction box. From the junction box you’d have more conduit or conduits, or glands to weather seal single cables.