Ken,
For general purpose ventilation, you can probably get by with 2-3 air changes per hour without freezing yourself. In your case this would amount to between 320 and 480 CFM. This won't clear your shop out very fast, but for intermittent welding or running an engine you should be okay.You need to check your heat source to see if it can keep up. Let's say it's -35 deg. outside and you heat the shop to 65 deg. It will take 108 BTUs for each CFM of air that comes into the shop. For a fan that exhausts 320 CFM, it would take 34,560 BTUs to heat the air plus heat that is lost through the walls. If your heat source cannot put at least this much heat out, then expect the temperature in the shop to drop as the fan runs. Also as I said before, since you have a furnace, make sure you open the window when the fan runs to allow the exhaust flue to operate properly. (Unless you have a furnace with a "sealed combustion chamber", which will have two pipes to the outside. One for combustion gases and one for makeup.) If you want to get fancy, you can mount a switch on the window to keep the fan from running unless it is open. Hope this helps.
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