My daughter and her husband live near Edmonton, Alberta.  Winter temperatures are frequently -30 and even -40 degrees F.  When we were making some changes to the duct work (they converted 3 small bedrooms into 2 larger ones), I noticed that there was a 5" duct bringing outside air into the return plenum.  Is this a standard practise, and is it necessary?  It seems to me that enough fresh air leaks into the house without having to add more.  Or is this meant to replace the leaks so that the house does not seem drafty.  If this duct is required (my house which is also heated with a gas-fired hot air system does not have this fresh air intake, nor did my previous house) on what basis do you adjust the damper that controls how much fresh air comes in?  What is the disadvantage of eliminating this duct?  If the fresh air is required, any suggestions on how to adjust the damper in the fresh air intake duct?  Would taping a plastic sheet over a window and watching whether it is bellowing in or out be a guide when adjusting the damper?
My daughter lives in a village far from any towns where expert help might be available.

Modern building codes require a certain amount of ventilation. Ashrae 62.2 -2010 is the latest standards for residential construction.

A blower door test will tell you how leaky the house is. You can take the CFM measured at the standard 50 pascals depressurization differential and divide it by 18 or 20 to get a crude approximation of how much the house will leak in normal winter conditions, with stack effect and wind figured in. That is a very crude approximation, though, but it does give a relative idea of how leaky your house is. If it is somewhat tight, like 3-4 air changes per hour (ACH) at 50 pascals, then mechanical ventilation may be appropriate to ensure fresh air for occupants. Bear in mind that in mild, windless weather there will be almost no leakage of outside air into the house. Worse, relying on leaks to provide fresh air gives you no control over how much fresh air you are getting at any time, in any weather.

For Edmonton, bringing in fresh air via an HRV would be appropriate, as the air being exhausted warms up the incoming air, to lessen the energy penalty of heating the cold air being brought in. With that device, you can control the fresh air rate. Any new house in a cold climate ought to be constructed so as to be exceedingly tight, for energy conservation, and that tightness requires mechanical ventilation to provide fresh air for the humans inside and to keep humidity down (even in winter).

Yes it is necessary. This is indeed your make up air and has been required by code since at least the '97 ABC but I think it was one of the '80s. This is used for when the cloth dryer, central vac, bathroom fan, kitchen range hood etc. comes on. It than draws the air through the return plenum and distributes it where need through the house. There will also be a pipe dropping right next to the furnace. This is for combustion air. As Dick explains an HRV can limit the heat loss but is much easier to install in a new home than an existing one.

Has there been any studies done to see if utilizing indoor plants helps with the amount of "fresh air"?  Of course that does not negate the need for mechanical ventilation but I would be curious to see how much oxygen they can provide.

Thanks for all the advise and information, it is greatly appreciated. 
The home is a small, modest farmhouse built in 1955.  It was well built, but not to today's standards for air tightness and insulation.  I can see that when the clothes dryer and/or bathroom fans are working, extra intake air would be important. 
Any suggestions on how to adjust the damper in the fresh air intake duct?  Would taping a plastic sheet over a window and watching whether it is bellowing in or out be a guide when adjusting the damper?

Lbear,
There are definitely plants that filter indoor air and improve the quality of indoor air. See http://www.treehugger.com/green-food/best-air-filtering-house-plants-according-to-nasa.html