To be precise, you should turn the hood on about 10 mins or so before you turn on the burner to get the air flow working. Have I ever done this? Heck no, but I had a crummy old OTR MW that was loud and didn't work well. (I only learned you're supposed to turn it on before the range when i started appliance shopping early last year.) I got used to only turning it on well after I should have when smoke and steam where billowing out everywhere and I was willing to listen to the annoying hood.

You don't even need a burner on to utilize the hood. I remember a thread last year over in Kitchens where people talked about turning the hood on when chopping onions. No burner, just onion fumes. Apparently, if you have a decent hood, this trick works well.

The products of properly burned natural gas are carbon dioxide and water so the gunk on your vent hood cannot (or st least should not) be the result of just turning on the stove.

I don't turn my hood on early and, yes, I have used it for chopping onions!

-Stooxie

" nasty gunky buildup "
= what looked like steam, from food cooking, but was really a combination of steam and grease.

I have wondered about the necessity of the turn-the-fan-on-very-early rule.

Given that a blower should reach the set motor speed in well under a minute (its not a windmill, after all) and that the inertia of the entire volume of air in most houses is slight (the weight of air is ca. 0.08 lbs/cu ft), and that the speed of air molecules is very high (near the speed of sound) so pressure differences are minimized quickly, I don't think it takes on the order of 10 minutes to establish steady state flow.

Air replacement in a 20,000 cubic foot house would only take 20 seconds at 1000 cfm. (By replacement I mean that a volume equivalent to the house volume is removed. The dilution of the original air is actually exponential.)

However, one does want flow to stabilize before the effluent (or onion vapor) is generated to keep it from spreading. A minute or two would seem to me to be plenty.

kas

I tried the fan for onions!! I thought it was my idea ... ;) Actually, it didn't work as well as some other tricks, IME, slicing more properly, removing "plug", mostly just getting your head away from over the stuff. And, of course, working quickly....

Kas - very interesting, and of course breezy (lol) and stooxie and everyone else who's evidently known of this prescription for a long time - I hadn't thought about setting up, basically to this unschooled thinking, must be a vortex of air drafting upward and out. Makes sense to establish it first, makes sense also that it needn't be really far in advance. And I'm betting you could crank it to top at first but once established after a minute, profitably dial it back.

But big point is, there's no "unseen" gunk that's needing to be sucked away, what needs to go is obvious-enough. I'm sort of vaguely suspicious of gas odors and unburned stuff and I hear you saying "nope". You get gone what you see. Or something.

Personally I have never felt the need to start my vent fan early. Sounds like people are talking as if they are lighting a fire in a colonial house, sticking a candle up the flue to get the air currents moving!

I think a modern vent fan (mine is 1200cfm) will be up to speed in about 2 seconds and be sucking air in the right direction about 5 seconds after that.

-Stooxie

In some houses it takes a lot of "effort" to get air currents moving. Make-up air has to come from somewhere too. It IS similar to getting the chimney (fireplace) to draw. It is a good idea to start air flowing before you need it. When you need it, it's too late. There is a lot of inertia in the still air of the duct.

There are no unburned gas products if your flames are blue. The by-products are CO2 and H2O, which are not toxic, dirty, or greasy. But you may not want them accumulating in your home. If your flames consistently burn yellow or orange, you should service your burners, as incomplete combustion is occurring. In the bad old days, gas stoves had pilot lights which burned continuously, and may not always have burned cleanly. But new residential stoves have electronic ignition so the only flames are the ones you see.

I don't always use my vent fan, and I NEVER start it in advance of cooking, but I have a real make-up air system. If I am cooking something really simple with low heat, like an egg, or slow simmering a pot of soup/stew, I might not vent. But if I am using more than two burners, or sauteing, frying, or searing, you betcha.

Sorry, davidro, but, if by getting the chimney to draw, you mean with a fire in a fireplace, it's not similar. Getting a chimney drawing is a fairly complicated process involving temperature and pressure changes and, sometimes, reversing an existing flow.

The whole point of a ventilation system is that you have a powerful fan and a duct. Fan moves air; duct directs it. Once the motor starts up, that air is moving. And no, there is *very little* inertia in the air in the duct. As kaseki pointed out, inertia is a function of mass, and the mass of air is very low indeed.

Where make-up air is coming from is a more interesting question, but that's not a time issue. Kaseki's recommendation of a minute or two seems more than generous.

And yes, the gunk will be from food, not gas. At least in the old days, gas would sometimes produce a sort of yellowish or brownish residue.

My Makeup air system is the out-of-doors. I'm in SoCal and some window is open always. The secondary backup makeup air system is our poor builders: they left cracks everywhere!

The yellowish-brownish residue was *all over everything* in my house. And my ignitors didn't work at all either ... wonder if we were all breathing gas day and night? In fact, the CPSC actually visited my house regarding some play equipment once and the lady ended up spending hours scrutinizing the stove instead! Never did hear back from her though). I just scrubbed some of that residue from a kitchen scale for hopefully the last time.

So OK, take-home message is: turn on fan soon before or even at time of burner starting. 10 minutes is excessive; moments might be helpful. MUA matters. Gunk is from food, not gas, so boiling water requires no ventilating at all, only heating grease and the like does. i.e., anything with smoke or smell? Is that a good rule of thumb? So, oven would want ventilating too, though perhaps not immediately.

Natural gas has something added to it so you smell it if it leaks. If your stove was leaking gas you would have known it.

Always turn your range hood ON when using your stove. Kaseki is right, it's recommended to turn ON the hood 5-10 mins prior to cooking to establish a steady air flow.

This is blowing my mind. 5-10 minutes to establish air flow? For a powered direct vent system? How long is the duct, 500 feet?

Could I trouble you to cite a source for this? I have never heard of such a thing.

Thanks!
-Stooxie

The gas additive is one of the mercaptans. Ethyl mercaptan is produced naturally by skunks.

One issue with the onions is that they don't have an upwelling effluent plume (unless heated), but instead the vapor tends to diffuse in all directions.

In systems with dynamically controlled MUA, the time constant of the MUA system could be a factor in how many seconds early one should start the hood blower.

Oven ventilation is more difficult because ovens aren't designed to provide a vent to an exhaust system. Even commercial systems with eyebrow hoods cannot capture everything that pours out of the oven when the door is open. So one is left with having the regular hood, or an auxiliary ventilation register above the oven, try to collect the effluent over time.

kas

It was not me who recommended that amount of time.

kaseki

Back in the old days when I had a hood, I'd turn it on to cook just about anything. Habit. Reach for the light, reach for the on switch. Sometimes I'd forget or I'd walk in on guests/the spouse cooking without the hood and I'd switch it on. Guess what. That steam would visibly go up the hood instantly.

I declare that any preventilating is a bunch of hooey!

Go ahead. It's your first hood. Get some water boiling so you can see steam and turn things off and on and move the pot around and pretend bacon grease moves the same as steam (of course it doesn't quite but close enough for us).

The gunk on the old cabinets was cooking gunk. Cuz remember, smells are made of particles. And particles aren't nothing.

I turn on the hood before I take out a pot, just to have the habit of turning it on before putting something smelly in the pot. I have turned it on, then been called away or had some forgotten prep or something, and I can't tell any difference with starting it 10 minutes early or right away. There is a big difference between starting it before I put anything on, and starting after something is already putting out a plume, though my hood is strong enough it can suck that back to a great extent too. :)

I use the hood for steam too. Who wants condensation all over?

First to answer aliris19's question, yes, it is usually a good idea to always turn on your hood. If you've got a speed control on it, you can turn it on half way for boiling water and such. The reasoning is, venting the steam and heat from the home is a good idea in general. In humid areas, excess moisture in houses can be a real problem possibly leading to mold outbreaks which can make people physically sick. Also, if you have AC running while cooking, adding moisture to the air will increase the heat load of the house more than just the heat produced by the range. Now in cold arid areas, it's probably ok to boil water without any ventilation. Another reason to use the hood is that combustion is never perfect and although incomplete combustion is unlikely to deposit gunk on your ceilings and walls, it does produce CO (carbon monoxide). In a correctly functioning gas appliance, there should only be trace amounts of CO produced but still not a bad idea to vent what you can.

With regard to pre-starting the hood, there is a reason behind it. It is mostly about setting the air mass of the kitchen (and even house) in motion rather than just the hood spinning up. To reach maximum efficiency, the hood should create a constant laminar flow of air around the cooking area. If the source of make-up air is far from the kitchen, then some portion of that mass between the hood and source must be moving in order to produce maximum venting. I'm not referring to any specific MUA system but just whatever window or "leaks" around the home that will pull air from the outside as the hood vents.

All of that said, I do think 10 minutes is a bit excessive. Typically, I find a couple of minutes more than enough to generate enough airflow for typical home cooking. Also, I usually crack or open a window in or near the kitchen so it doesn't take long to establish a steady flow. Ultimately, the hood only has one chance to capture the effluent. If vapors escape the capture canopy of the hood, the grease from that vapor will deposit on your kitchen surfaces rather than get vented. That is the reason for the pre-start recommendation. Take it for what it's worth.. a recommendation..

kaseki, your calculation is a bit off.. 20000 cu-ft space would take a 1000cfm blower 20 minutes (not seconds) to displace.

Oops!

kas

Very easy mistake kas. I've made it myself numerous times when doing calculations in my head. Just easier to think of things as "per second" since all SI units are usually per second. :)

I'm glad not to be the only one screwing up such calculations. An old physics-friend said it was all in the units; got through bazillions of years of physics on the units alone ... or so he said.

But the point would seem to be establishing that flow, not total displacement, anyway - right? Wouldn't opening a window near the vent disrupt that flow? Or only temporarily, perhaps.... I guess providing constant, steady new flow is most important.

I can say that every time I open my refrigerator or the front door, even (further away but presumably more input of air), the simmer ignitors on my burners click. All that air may be invisible, but it's still doing something. A stupid comment, no doubt, but one that's hard (for me but I'm guessing others as well) to appreciate intuitively when you don't work with 'air' all the time.

The important thing here for efficient ventilation is airflow but creating that airflow is actually about displacing enough of the air to create a constant motion. You are correct that opening a window can disrupt airflow which is why I don't usually like opening the window right next to the range in the kitchen but rather the ones we have toward the other end (from the range) of the kitchen in our dining room. That way if a wind blows through the window it doesn't disrupt the airflow. Also, it's best to open the window prior to cooking so the consistent airflow can be established.

If you want to see this for yourself, buy some incense and light 5-10 of them and place them around the perimeter of the hood. Then observe the smoke and where it goes. Move them so that they are 6-12" outside the edge of the hood at counter level. Without the hood on, you'll see that the smoke drifts toward the ceiling and around cabinets. turning on the hood, you'll see the smoke start to get drawn in. If you repeat this a couple of times by shutting the hood off and moving the incense out a bit each time, you can see that once the hood gets going after about a minute, it does a better job of pulling smoke in from a wider area. Now you can try opening various windows to see which is the best to use.

Restaurant ventilation systems sometimes have a speed matched MUA blower that blows air from below the range so there is an upward flowing curtain of air around the range and person doing the cooking. It's pretty impressive to throw something like shrimp into a red hot oiled wok and watch the billows of smoke hit this curtain and get sucked strait up. It's like an invisible force field holding the vapors in.

It all sounds like something from a James Bond movie ... or Pink Panther; we just watched them spritzing some vapor or other into the (old) alarm sensors path to make their transit visible last night. Very cool stuff.

I'll file away this fun-sounding plan for once everything's pretty much all done. It will be a little present of sorts, to figure out which window will best aid movement of air up the vent. I'm guessing those screens I just put on aren't going to help the effort any.

Thanks, amcook.

A few days ago colin3 commented on the differences with chimneys.
Here is another thread where antss described it all in a few words.
He made no comparison to chimneys.

Here is a link that might be useful: push pull air to overcome static pressure pipe and bends

It's hot enough now that I don't want to have the xtra heat and humidity in my kitchen when boiling water for anything. If I don't remove some of it with the vent, I'll end up removing it with the AC.