Fresh air return on furnace necessary

you need a fresh air intake if you have stale air, cooking smells, odors, smoking or too much humidity in the Winter. If not then don't bother. Google Hoyme damper if you want to know more about a damper to shut it off in the off cycles.

the fresh air is there because the builder felt they build your house so tightly that it doesn't leak. this is good and bad because your house is energy efficient however if u take a shower, cook, or even just breath in your house the humidity will stay in your house and over time grow mold and mildew and cause paint to peel. the fresh air helps prevent that by reducing the relative humidity. Honeywell makes a very nice ventilation box that mixes the fresh air with already heated air before piping it into the return trunk. Using this makes it less of a shock to your furnace and better ensures the air will be heated properly and efficiently.
http://yourhome.honeywell.com/home/...Efficient+Ventilation+Systems/SummaryList.htm

Modular home? is that the same as a mobile home? The building code in Canada says that all new homes need a fresh air pipe into the RA duct as they are all very tight nowadays and the code just changed to you must have a basic HRV.