The Importance of Make-Up Air & A Question

[Heather/Ericaceae]

Hello! I just got my gas-line installed last night and the installer took a gander at my brand-new venting system. It's really basic - 425CFM vented out the top of my basement wall. I had been planning on installing a make-up air vent before starting to torch, but after his demonstration and explanation, I'm DOUBLY motivated to get that sucker installed.

My new work area is about 8-10 feet away from my furnace and water heater. Since it all uses the same gas, I'm not too worried about that aspect. But my gasline installer explained that what I DO need to worry about is the exhaust fumes from my water heater. In neutral air pressure, these fumes rise up and get sucked away by an exhaust pipe up to the roof. There is a 2 inch air gap between the tank and the pipe to suck up extra air and keep the suction constant. However, if that air is getting sucked away by, for example, a lampworking vent, the hot water heater exhaust can get diverted from its pipe into the household air. Which is bad because it's chock full of carbon monoxide!!! He told a cautionary take of a guy in Portage de la Prairie, just west of Winnipeg, who installed a gorgeous new basement hot-tub, added an extra gas water heater, then added a strong vent to get rid of the hot-tub moisture. He left the vent running one night, which drew the water-tank fumes into his house and his wife and children died of CO poisoning.

Okay!!! I'm SOLD on make-up air!!!! Now I just need to know how big my make-up air vent needs to me. The gas installer's cigarette smoke test of my heater duct, with lampworking vent *and* dryer running (they're all in the same room, ha ha... :| ), showed that the air was being sucked up at 1" away but not at 3" away. He says ideally it should suck from 6" away. So, I don't get a downdraft, but I do get an impeded updraft.

I have no openable windows in my basement but I do have an old sealed-up coal chute right next to the heater on the side opposite my work area, through which I was planning to put a intake duct. The duct I've already bought is 4" round but now I'm concerned that it won't be enough. I know there are some folks here who are handy with the calculations - any tips? Should I go for 5", 6", 8"? Thanks so much!

-Heather

[MikeAurelius]

I'm surprised your installer didn't do the calculations for you.

An 8" round duct (50.24 square inches) will provide approximately 350 CFM of free flow fresh air. I would install at a minimum a 10" round duct with a properly sized damper that you can close off when the exhaust fan is not being used.

Because you are "up north", you are going to have some issues with the cold air cooling off your studio area, just through air exchange. There are a couple of ways to resolve this.

First of all, you can use an old hvac installers trick of looping the duct. You bring the duct down from (whereever) almost to the floor, then back up in a "U" bend. This "U" bend will prevent cold air from just dropping onto your floor. Another trick is to dump the cold air duct into a 5 or 7 gallon bucket. The cold air will rise to the top of the bucket, and pretty much stay there until the exhaust fan needs it. I personally prefer the "U" bend method of these two.

My hand's down recommended method is to duct the fresh air line direct to the underside of your bench, then cut a hole in your bench top and use a duct adapter to route the duct through the bench top. This will force the fresh air into your torch space, and will not affect the surrounding room air temperature, because it is almost immediately exhausted.

As long as the coal chute is at least 10 feet from the exhaust duct, and it appears from your picture this is so, you don't have anything to worry about.

The fresh air duct should have a slight bend downwards to it, to keep rain or snow from falling into it, and keep it at least 24" higher than any accumulated snow rises. Put 1" mesh screen on the duct to prevent birds, insects or other critters from entering.

You will have to insulate the ducting, because bringing in cold fresh air will cause moisture to condense on the duct surface, and you will get drips and puddles, and possible freezing. Insulating helps prevent that.

[Heather/Ericaceae]

Hi Mike - Wow, thanks for the detailed response! Would a longer intake duct still privide the same airflow? I had been thinking of just having a duct just long enough to penetrate to the outside, just flowing cold air in, and was expecting to use a space heater, furnace radiant heat and wool socks to keep warm. There will be a 60 degree Celcius/109 degree F temperature difference between indoor and outdoor air for a month or two, though, so I'll try your idea if you don't think the long ducting (maybe 15-20 feet in total?) will affect the airflow. Also, I don't understand the way you recommend the duct to exit through the top of the bench. I made another picture - is this what you mean? Thanks again!

-Heather

[MikeAurelius]

Yes, your picture fairly accurately describes what I'm talking about.

The long duct is the reason I suggested the 10" duct. The area difference between 8" (50.24) and 10" (78.5) is more than enough to handle a long(er) run.

[Heather/Ericaceae]

Make-up air update... My partner and I spent all day yesterday getting our fresh-air ducting in. It's 10" ID insulated plastic expandable ducting, which I know is no good for exhaust venting, but I often read qualified people mentioning that they use the expandable stuff for make-up air - besides, it's very smooth inside when expanded (...and that's all I could find in 10"). I did the U-bend thing, over 3' up and down and maybe it helped a little... but the air is still blasting in, even though we used the full 25' of duct! Maybe the difference in temperatures pulls more air through convection?? It's actually overpowering our vent fan, to my surprise! For now, I'm going to block off half of it. The good news is that I think I'm set for make-up air and could probably safely upgrade to a 500-600CFM fan if I wanted to, now!

It was easier for us to install the duct with the end at head height to the upper left of the torch and the lower left of the vent. We might move it further back, too, to diffuse the air so that it doesn't eddy around the fan. This'll make the room colder for sure but hopefully catch more bad air...

[MikeAurelius]

Wellllll....I'll just say that I'd prefer to see non-flexible ducting for *all* ventilation systems regardless of intake or exhaust. I know that some installers do use flex for fresh air intake, but most do not, especially on critical applications.

If you have air "blasting" in, I'd say you have other issues in the house that are drawing the fresh air in. Fresh air won't normally enter unless it is needed somewhere - there's a flue open, there is a strong breeze creating suction, something is causing the air to flow.

Another thing you can do is add a damper to the duct, and keep it closed when the system is not in use.

[Heather/Ericaceae]

Thanks Mike. I'll have to order a blast gate, 'cause nobody stocks them here at 10"... and most of the places that carrying ducting stuff are only open M-F so I can't get to them. Right now we've got a disc of 3" thick blue styrofoam plugging up the intake hole when we aren't working, which definitely helps! The suction might be from wind in the attic, 'cause we have HUGE leaks in the fascia there and we opened it up to get ready for insulating it once the roof was re-done... but then the roofers haven't come so it's just wide open. It is VERY windy in Winnipeg. If that is having such an effect on our utility room air pressure, then that's REALLY not good or safe, even without a lampworking studio! We'll have to try to seal it ourselves ASAP from the inside. One more thing to try to do before Christmas... :(

-Heather

[sadiesmom]

I'm working on my ventilation system soon, and I am wondering if you are using a blower to bring the air into the house, or are allowing the exhaust vent to draw the air through the air intake hose.

Thanks!!

[Dale M.]

The exhaust blower is responsible for bringing in the fresh make up air... Technically the exhaust blower causes a negative pressure area, and since nature abhors a vacuum it rushes in air to fill the low pressure area... Hence, fresh makeup air....

Dale

[sadiesmom]

Thanks Dale! So I only have to buy one motor for the exhaust. :)

[MikeAurelius]

Yes, in most cases you do want a passive fresh air intake. There are exceptions, but 99.999% we, as lampworkers don't need them.

[Heather/Ericaceae]

What I find is that the air just rushes into my basement through my passive make-up air duct! I was confused by the strength for a while but during research for insulating the attic I found the answer - the stack effect! Generally in a tall house the warm air rises and creates high pressure in the high part (air escaping through cracks and vents) and low pressure/vacuum in the low areas (air rushing in through cracks and vents).

I'm sealing and insulating the attic right now and that will hopefully make my stack effect a little less extreme. Our attic was pretty much a big sieve until now. :) -Heather

[MikeAurelius]

The other thing you can do is to put a "U" bend in your fresh air duct. This can be very useful for people in basements or anytime the outlet for the fresh air is below the height of the intake.

If you run your fresh air just straight down the wall and leave an open duct, try putting two 90 degree bends so the opening now points back UP the wall. This will then act like the U bend in your plumbing does and trap some of the fresh air and prevent it from just falling into the basement.

Another thing you can do is put the duct end into a 5 gallon pail.

[Heather/Ericaceae]

I've got 2 4-foot u-bends. They do nothing right now. I'm sealing and insulating the last sections of the attic today and tommorrow and I think that'll help. My house was the poster child for stack effect.