[Backyardmech]

I'm in the prosses of setting up a shop/welding area in my attached garage. So ive got an exhaust blower fan up and mounted on the wall, and got the hole cute..I'm planning on running 6 inch ducting from it to an exhaust hood that will hang over my future welding table. My qeustion is i started it up the other day and got some huge negative air pressure in the garage...looks like i need some kind of air make up set up...has any one had this problem? Up here in Canada its pretty cold...i could feel the cold air being sucked right through the garage door..

[Poland308]

Pretty common problem for any HVAC system. There are lots of ways to remedy it. If you don't want to get an air to air exchanger /lots of $. Then try gravity dampers that are meant for a chimney pipe or a gravity bypass damper set up from a supply house. But unless you reheat that air your still bringing in outside temps except your controlling where you bring it in at.

[Backyardmech]

Yeah I was a little worried about this one...as soon as I turned on that blower. I could feel the garage temp dropping fast, it like -20 Celcius here today...Im not sure exacly how im going to tackle this new problem..just know with the amount of money ive already put into this garage...my fiancee wont be impressed. But I'll figure something out..

[Tom Osselton]

I am planning on doing the same thing come summer mine is made for a multiple bay so I was thinking if I plumbed the outside air through it it could provide enough vacum for fume extraction using a y fitting, a valve on the outside air would make the suction adjustable. Any thoughts?

[ldbtx]

I am planning on doing the same thing come summer mine is made for a multiple bay so I was thinking if I plumbed the outside air through it it could provide enough vacum for fume extraction using a y fitting, a valve on the outside air would make the suction adjustable. Any thoughts?

You thinking a venturi effect -- like an underwater dredge setup? You might be onto something there.

[Poland308]

If you start to restrict air flow to a squirrel cage style or any cycntrifical fan strange things begin to happen. Over a ping the motor is most common result. Better if you restrict the discharge. But there is still the opportunity to over amp doing this as well. If you have an amp clamp you can restrict the air flow down till the draw on the motor is about 80% of name plate max. Anything over that and you will start to cause problems. Unless it has a duty factor of 1.1 or greater.
Every cubic ft of air you exhaust will be replaced by an equal amount from outside. The easiest way to control the temperature loss is running your discharge air pipe through the center of your intake air pipe. I.E. 5 in diam pipe inside an 8 in diam pipe. Then slow down your fan as much as you can with speed taps. If that isn't enough then use one of those power regulators like for a drill to slow ot down more. You will need at least 15ft of pipe in a pipe assembly to give you enough surface area for heat transfer. It will help but not solve your temp issue unless you add a reheat unit of some kind to the intake air.

[Tom Osselton]

My thought is if the main outside air is passed through the blower and directed back outside. The suction created by the y fitting would balance out the air requirements through the blower as the outside air valve is adjusted. I can see your point when using different size piping working the motor more but can also have a min /max set up on the valve.

[Poland308]

Yeah there are lots of simple ways to restrict the flow. But no matter how you pull the air out more air will come from wherever it can find an opening. Otherwise your shop will collapse like a garbage bag hooked up to a shop vac

[Backyardmech]

I should have relized this would happen, where I work as a millwright we have many ventilation and air make up units. I work in a plastics and fiberglass plant. I see first hand what can happen when you draw out to much "dirty air" and dont replace it..we have old make up units..not near big enough for the sq ft of our plant.
Any ways i guess being a smaller garage throw me off..id like to try the advice given to me about the heat lose by Poland308. I dont have a dischange air pipe. The blower is mounted directly on the wall , and exits through a 12×12 hole with a fan shutter on the outside. Might just have to deal with the cold when im welding, im worried about gasses entering our home with the kids here and the future wife. ...brrrr -40 windchill in our forecast.