Measure room in cubic feet. That's L x W x H. A room that's 12 x 10 x 8 would be 960 cubic feet for example.
Select Exhaust Fan
Fan strength is measured in cubic feet per minute (CFM). If your room is 960 cubic feet and you want to do an air change every 1-2 minutes (good baseline), you want a 480 - 960 CFM fan.
Exhaust Duct Size
Your exhaust ducting size will be specified by whatever exhaust fan you chose in the previous step. Just read the manufacturer's guide and it will tell you want it needs.
Intake Duct Size
Your intake ducting size should be roughly equivalent or the same as the exhaust ducting size for equal pressure. A smaller intake duct would bottleneck what your exhaust ducting could exhaust otherwise. To compare equivalent duct sizes see http://www.engineeringtoolbox.com/equivalent-diameter-d_443.html.
Select Duct Heater
You know the rough size of what your intake ducting should be now, but your duct heater may have a duct size it specifies. As long as it doesn't bottleneck the exhaust run you may want factor this into the decision, otherwise you'll need some extra duct transition pieces.
Type of Ducting
You can go with rigid or flexible ducting. Rigid is most durable but more expensive. Flexible is easier to work with and less expensive but not as durable in theory. In my first room I went 100% rigid ducting with transitions. In my second room I loosened up a bit and went with a combination of rigid and flexible ducting. The rigid ducting was used around the duct heater for safety reasons. The flexible ducting was used once I got into the attic because it was easier to shape and direct to where I needed it to go.
The duct heater should be built directly into the intake ducting room. Due to the overall weight of the unit you will want to build a mount of some type to secure it. I have mounted these both horizontally and vertically before.
Opening / Closing Ducting
The simplest way to open your fresh air intake and close off your HVAC vents is by manually opening or shutting your registers. In the case of my current cigar room that’s what I do and it only takes all of 10 seconds to accomplish when I want to smoke. When picking your registers out just ensure they have a way to open and close manually.
For my first room I had easy access to all the ducting since the room was a new build. That allowed me to get more sophisticated and put in automatic electronic duct dampers connected to a wall switch that did the opening and closing for me. The HVAC ducting line had an automatic damper in the normally open position while the fresh air intake damper was in the normally close position. When I turned the switch on, the two of them would essentially flip positions and one would open while the other would close.
The intake hood, also referred to as an inlet wall cap, lets air in.
The exhaust hood, or goose neck vent roof cap in the case of my second room, should include a built in back draft damper and let’s air out.
Both hoods should include wire screens to keep birds out.