Ventilating a Workshop
I am in the process of building a house and workshop. It is a single 32′ x 48′ building with a partition wall down the middle dividing it into two 24′ x 32′ spaces. The partition wall doesn’t have any doors or windows. You have to go outside to get from the living area to the workshop. The construction is 12″ double stud walls with 16″ TJI rafters. Both sides of the exterior walls are being air sealed as much as possible, so I expect that the building will be fairly air tight. Each side will be heated and cooled with separate mini-splits. I am in SW Wisconsin, zone 6.
On the living area side, I am installing 2 pairs of Lunos HRV’s. My question is, what to do for ventilation on the workshop side. It’s hard do define how much ventilation is needed. Seems like it could vary from less than 1 ACH most of the time to 20 or more on occasion. Activities could include woodworking, metal fabrication, and welding. Workshop volume is 8000 cu.ft. over a 684 sq. ft. floor area.
I hesitate to install an HRV because the payback on the amount of heat recovered seems pretty long. I’m considering a variable speed wall fan with about 1500 – 2000 cfm maximum and running it on low speed and/or intermittently most of the time and turning it up as needed. I would also install a passive air intake with a damper sized for low air flows and plan on opening a window for higher air flows. (I did read the article about passive air intakes not working well, but I think it may be okay in this application since it is a single room and the intake can be well sheltered.)
I would be interested in any thoughts, suggestions or experiences on this. I have been lurking and learning on this forum for the past few years while designing and building my project. The wealth of knowledge here has been invaluable!
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Lunos have no place in a new build, in my opinion. Very expensive for very low flow rate and no distribution. Installing duct work in a brand new build is very easy. Those two pairs will give you a maximum of 44cfm, for about $3000. For a third of that price you could get a good quality, ECM HRV capable of 120cfm. 44cfm is only enough for two people, and only if you never have any visitors either. A good rule of thumb is to calculate the ASHRAE minimum, and then double it. Then try to get one bigger than that. Make sure it's got multiple speeds and has a wall control to adjust. You might want to consider an ERV instead of an HRV. Most climates are better suited to an ERV.
If you're looking at 1500-2000cfm for the shop, then any kind of energy recovery is probably off the table. It's going to be awfully cold in there during the winter when that exhaust fan is running at full. I suspect you'll need an awfully big window to limit the depressurization. Someone with more knowledge will probably weigh in on that.
You've actually got two or three different design conditions for the workshop: 1) Occupied, but doing non-toxic work, 2) Occupied doing toxic work, 3) unoccupied. I would use an HRV for the shop, ERV for the house. The HRV is cheaper and less likely to get clogged with sawdust, but you still need a big, high-quality filter on the shop inlet side. The shop HRV should be sized for the expected number of people working in it, times 2 because there's always going to be some extra "stuff' in the air. Boost mode would be helpful there. You can probably shut it down, or at least make it intermittent when the shop is not in use.
You need source control (vent/fume hoods) at your welding and finishing areas. This removes the concentrated nastiness of those activities. When these hoods are running, you will need LOTs of makeup air. Opening windows works, but your shop is going to get cold. You can also install makeup air fans that vent through linear emitters very close to the hoods, so it's mostly outside air that is getting sucked out through the hoods. There are commercially available systems for the hoods and makeup air systems and if you are using them a lot, you should consider this.
You will also need an air cleaner hanging in the shop to take the fine particulates out of the air without exterior air exchange. Again, there are commercially available units that do a good job of this off the shelf, or you can cobble something together with fans and MERV-13 filters. Even MERV 8 filters will help a lot.
Wall mount mini split is not the best idea for a dusty shop. The filters on them are mostly cosmetic.
You want a ducted unit mini split. This can be a slim unit you hang on the ceiling or a floor mounted multi position air handler. In either case, you don't need to do any fancy ducting, just figure out how to bolt the largest filter you can to the intake.
Seems to me that when you start putting the plumbing and wiring into this building lots of it will find its way into your center wall once all that is in place it may not be as air tight as you have imagined.
General welding shops needs to be separated from woodworking shops from machine shops. The sparks and flames from grinding and welding tend to start fires in the saw dust that fills most wood shops. The saw dust from the wood shop tend to become a gummy mess when it settles on the well-oiled equipment in the machine shop. Welding in a air tight building does not sound like a healthy idea.
Walta