I've got a pit, which most know, I'm thinking your splitting hairs on your concern, for any gas, vapor or anything to settle to the lowest point, takes time, its not like spilling water and watching it run the lowest point, gas takes far longer to accumulate in the lowest point of any shop, unless your fogging the shop in such massive quantities of poisonous gas such as when an old Detroit is puking out massive amount of smoke and fumes from unburned diesel. If your running a Detroit or putting in massive quantities of poisonous gas of another source, the pit is the least of your issues. Aside from that, it takes far longer than just a few minutes for gas to accumulate, like a half hour or hours, with even the slightest air movement in the pit, your circulating the air and no issues of excess gas pocket accumulation should occur. Now unless you plan on circulating the air outside the shop as in a heat exchanger/scrubber/filtration system of some sort, your concern about a pit is what?? your only circulating the air that is already in the shop, if its toxic, I'm not wanting to bash anyone's concerns, but your still breathing the same air where is the concern with the pit come into play?? you've got far larger issues than the pit, as the insurance company rep explained to me, and which made sense to me and still does.
Not wanting to be negative, but the issue with putting air into one end of the pit, and a fan in the other, depending on how large and long your pit is and where the vents are located, why would the air be drawn from one end, when it can suck in from the open top of the pit, far closer to the exhaust fan or suction tube?? You need air inlets and exhaust along the pit length, but before anyone can tell you anything about where or how, you need to assess your dimensions and how air flows and where it will flow, every one is different.
Now let me ask a question here, since I've heard this a couple hundred times before about pit gas's, when your about to crawl under a machine on a creeper, where are your fans located?? how will you survive a gas issue, how do you plan to get out in case you'd spill some gasoline, after all, your at the lowest location in the shop at that point, what was your plan before you crawled under the machine, the air there is stagnant and toxic vapors could be accumulated waiting to kill you??
If you have a loft in your shop and your on the main floor, basically your in a pit of sorts, what's your plan then to survive and circulate the air so in case of gas vapors it won't kill you, do you have fans blowing on your constantly anywhere in the shop. Not sure if you have a house with a basement or not or an attached garage, but if you did, are you concerned with poisonous gas's in your basement every time you start your car or pickup and leave, how about coming back home hours later and entering your home?? Next question is, even if you don't have this scenario how many do you hear dying each year or being injured and hospitalized every year in the world from this issue??
If you were to walk into your shop, and feel sick, or see a fog or low hanging haze near your shop floor, what would you do, close the doors and stay inside or open all the doors and vent the fumes as safely as possible?? A pit isn't the issue, it how you handle the situation your given at any point in time is.
Yes circulation is important, air flows are really important, but its not just for the pit, its for the entire shop and work environment and for that you need someone who's up on those things to walk you through them on every aspect of the planned project. Now if you don't, don't get too shook up, you'll get to later on, you'll have condensation issues, frozen doors you can't open, same as windows you can't see out or open and it'll eventually get done, I'd suggest doing it up front, just keep in mind while doing it, as you add machines, shelves, tools, equipment in the shop being worked on and everything else, air flows change.
You only need a small amount of air movement to circulate gas's and fumes, if they are indeed toxic, you need to vent them somehow, not really sure yet how anyone gets those toxic of fumes into the shop and are ever willing to keep them in by not opening doors, windows or vents and turn on massive fans to blow it all out, but I'm told its been done before and usually where all the statistics come from on legal or deadly accidents.
When we bring machines in, we have a large fan sitting in the loft, keep the door open and turn it on, vent all the smoke and when its clear and fine, we shut the doors back up, can't say the pit was ever an issue or concern and the cost of heating never entered into the equation once at those times. As for gas vapors or any other vapors, whether its in the pit, shop floor or anywhere else, the doors come open, every fan in the shop comes on and the whole shop is ventilated massively, heating is the last of my concern, the pit is far below that on the list.
We do a lot of welding and fabricating in the shop, I need a heat exchanger but have not put one in yet, I have natural draft ventilation in the pit, never been concerned yet with working in it, when someone is next to the pit welding, does that mean mine is perfect, absolutely not, but if I can see, smell or taste something out of the ordinary, fans come on and it doesn't matter where I'm standing in the shop, in the pit or right next to the welder being run.
The last issue to keep in mind is this, your not laying in the pit floor, your usually standing upright, or at shop floor height or above, its not really any different than laying on the main shop floor where your head and nose are at, your not sucking up and breathing vapors laying on the pit floor unless your laying there unconscious for a long period of time. Most don't weld on or near the pit floor, when I'm welding in the pit, its usually on a machine above the pit or above the shop floor in essence, all you need is a small amount of air movement to circulate any air in the pit, is adequate to never have issues, the only better way is use common sense, just like anywhere else in the shop or with any situation. Not really a rant or downgrading anyone's concerns, heath is everyone's concern, just keep everything in perspective and don't let some peoples hysteria deter you from putting in a pit, common sense is still your best friend no matter the issue.