"John Grabowski" wrote in message
...


A few years ago a customer asked me about replacing her ancient hood which
had a duct going through the cabinet upwards. I went up in the crawl
space above the stove to see the duct so I could figure on the proper
fittings. I couldn't see it. I went outside and looked on the roof for
the cap and could not find one. So I asked her to cook something smoky
and turn on the exhaust fan. When she did the smoke starting coming out
of the eaves. Not through vent holes, but through gaps in the wood over a
wide area. The duct through the cabinet was just poked through the
ceiling and stopped.

That's the way it used to be done.

The same for bathroom fans.

The primary reason to have it vented directly outside is to keep moisture
from condensing on the underside of the roof. That's the potential of the
"downside." In most climates, most of the time, it's a non-problem.

Most of the grease should be stopped by those metal mech "filters" which can
be cleaned in a dishwasher (somewhat, anyway).

Needless to say after about 30 years it was ugly up there. Thank goodness
she never had a fire.

Unlikely. Even in busy restaurants, such fires are rare.

The hood vent piping isn't installed like a flue; it often is directly
against combustible surfaces. If a stove fire gets out of control to the
point where the vent is hot enough to damage your 4" alluminum dryer vent
material, you have other problems.

The "flex vent (metal)" shouldn't be used mainly because it picks up crap
from the air flowing through. Sections of galvanized vent are dirt cheap
and are better for longer runs.