I plan to air seal my attic and put down additional insulation this fall. Before I do that, I’d like to resolve the matter of the range hood in my kitchen. It vents up into the attic, then the ductwork makes a 100-degree turn toward the gable end of the house, running at a slight upward angle about 20 feet before stopping six inches from the gable vent. I know it would be better to skip the turn and have the ductwork go straight up through the roof, but I already have four holes in the roof on that side of the house (two plumbing vents and two bath-fan vents), so I’m reluctant to add a fifth. The house is a simple, one-story ranch.
As far as I can tell, my options are these: (1) don’t change anything, (2) put another hole in the roof, or (3) buy a ductless range hood (Vent-a-Hood makes a good one, so I’m told, but it’s expensive). I don’t like any of these options. Anybody have another idea?
Replies
Holes/Roof
Nothing wrong with roof penetrations if they are done right. If you don't know how to do one yourself, hire a competant roofe to do it for you.
IMO range hoods that do not completely exit the building envelope are a reciept for trouble. I have been in enough attics with similar hood vents and bathroom vents stubbed up in them to know the moisture and grease accumulation is down right scarey.
That 100 degree bend and long horizontal run means the any grease and moisture laden exhaust has lots of time to cool and condense inside the pipe. A straight, or nearly vertical pipe, is aided by a natural draw that assist the fan trying to push the crud out. With your desciption you don't get that assist, and the gable vent could partialy block the exhaust flow. It may even stop it if the prevailing wind is entering through that gable vent.
I can almost bet you that the inside of you current pipe is coated with grease and the back side of that gable vent has a layer on it also. I can also guess that the hood and exhaust install were not part of the original home construction, unless the home was never inspected. It is a code violation in most places.
Ditto.
Through the roof
Thanks for the advice. I punched the two holes in the roof for the bath fans myself, so I can do one more. Those fans had been venting directly into the attic insulation—another code violation.
I'm sure you're right about the grease inside the ductwork. One day last winter after my wife had done a lot of cooking, I noticed water dripping from the range hood. Evidently, the steam from the stovetop had condensed in the pipe and was rolling back down!
If you are doing any kitchen remodeling and don't mind opening up the wall, consider venting the range hood down and out. Up and out too often results in cold downdrafts to the vicinity of the range in winter, and as you've noted condensation runs back down. Going down and out reduces the tendency of the duct to fill with cold air and leak into the kitchen. It's like an air trap, with the cold, denser air down below the lighter warmer air up by the range. Of course this works best in a fairly tight house, without a lot of air leakage from the house into the attic.