Door type between shop and garage (with DC in mind).

[Chris Harper]

My shop is a third bay addition to an attached two car garage. There is a 7' wide by 8 1/2' tall opening between the double bay and the shop where I want to put doors.

There is a Reznore NG heater in the double bay that does a very good job of heating all three bays. I have some electric radiant cove heaters to provide supplemental heat to the shop once doors are installed.

I'm going to exhaust at least dust outdoors and will likely have a cyclone in a shed just outside the shop so I need to be worried about makeup air, pulling carbon monoxide from the NG heater, etc.

I also need to be able to roll my table saw from the shop to the garage and also need to occasionally be able to drive an ATV with a plow between the shop and the garage.

My first choice was to use one of those cheap accordian doors without the bottom track but I'm concerned that I'll be pulling too much make up air from the garage. The accordian doors gives me more height and closes up and out of the way nicely.

So I guess the most important question I have is how tight of a seal do I need between the shop and the NG heated garage if I'm going to exhaust my DC outdoors? To me being able to move tools, ATVs and door height are less important issues that can be dealt with later.

On Bill Pentz's site he writes "Finally, Unless we provide a way for makeup air to enter our shop, blowing the air outside can suck deadly carbon monoxide backward through our vents. Most need to just open a door or window and they the risks of pulling the carbon monoxide out of our fired appliances vanishes." http://billpentz.com/woodworking/cyc...austingOutside

I'm not sure what to make of this. Does this mean I need as much or more area for return air on the shop side as would be found around the entire door to the shop?

Thanks for any input.

[Bruce C Garland]

exhausting it outside doesn't work very well. I've been doing that for a few years now. I am now in the process of installing a clearvue cyclone in the shed and exhausting it through the filters back in the shop. you will suck all the heat out of the shop when you have it on and it takes forever to recover that heat or AC. you would need about 10" diameter hole to the outside to provide makeup air.

For the door, I have a slider like "Norm" has. How much snow do you get in the shop that you need to move it out with a plow? ;-)

[Dan Friedrichs]

Depending on your climate, exhausting outside can work very, very well, or only so-so. Remember that most heat energy is stored in things with mass (not in air, which has a low heat capacity), so sucking "hot air" out of the shop isn't as bad as it seems, assuming the tools are warm to start with.

To answer your question, you need an inlet air source that is at least as big as the exhaust, and should probably double that to be safe. I just crack open a door.

[Jim Neeley]

I've played with the numbers (specific heat, density, etc.) and offer up a rule of thumb for *dry* air.

If we assume the humidity outside is zero, the make-up heating (or cooling) comes out mighty close to the fillowing formula:

(X cfm) * (Y degrees difference in temperature between shop and make-up air) = Z in BTU/hr.

From this, 500cfm * 20 degrees difference [example, 70F inside, 50F outside] = 10,000 BTUs/hr.

At the other extreme, 2000 cfm and 90 degrees difference = 180,000 BTHs/hr

This calculation errs by giving you a lower number than you will likely see. It can vary a bit depending upon the temperature of the air (affects density) and would go up dramatically if the outside air were high humidity.

I did it to prove to myself why my 1100 CFM Jet, when dumped outside in Alaskan winter (say 20F) drops my shop temperature down near freezing in a few minutes, even with my 40,000 BTU shop heater.

This load would be in addition to any load caused due to heat loss through walls or windows, if the furnace could keep up.

Jim

[ian maybury]

This is one I've wondered about too Jim. The stock formula for sensible heating used in HVAC design is 1.08 x cfm x temperature inc in deg F = Btu/hr, so that gels.

By that 1000cfmx30 deg comes out at around 9kw (3412btu/hr = 1kW) - which fairly clearly isn't if run continuously an option. Against that, if it's not that cold outside, you only run the dust system when you are using a machine, and you add in heating, heat gain from yourself, lighting, motor inefficiencies and the fact that many buildings have lots of heat capacity then it becomes less of an issue. Even more so if your dust system is only moving the say 400cfm typical of many. It probably all depends.....

Not 100% sure on how much open area is required to eliminate any significant drop in pressure in a workshop, but there's an oil boiler in mine and the dust system exhausts to filters in a room next door. It's probably running round 1000cfm and while it's never been tested with a CO monitor this seems to work fine (mesh in place of lower door panels - the top panels are glass):

air return to shop via door mesh.jpg

ian