I have a return in my basement. I also have return on the first and second floor of my house. On a mild warm day I may run the fan to circulate from the basement to the second floor. The cooling effect is limited. The temperature in the basement will be around 68 degrees. I may be able to get the temperature down one degree (79 to 78). Most of the relief come from the wind chill effect of the cooler air blowing on your skin. The set up does not work when the temperature is in the 90s and the sun is beating down on the house. You also run the risk of bringing more moisture into the house form the basement. This will make it more uncomfortable.

You don't mention where the furnace and duct is located but being in a cold basement or especially an attic where cold air naturally falls, you could have some migration. In addition you may have a fresh air make up duct that is letting even colder outdoor air into the return duct from the outside. This you should check, some are wide open to the outdoor intake duct and can have a manual damper while some have a motorized damper that should be controlled with blower operation. Make sure the return duct doesn't have any leaks and check connections and cabinet for leaks and cracks for air to infiltrate. Most furnaces have lousy gaskets on the blower doors and do not seal well. You might try taping the seams with a good aluminum duct tape. The problem with covering the return is having the furnace turn on without removing the cover could result in unit control and/or blower damage, including overheating and fire.

ok, I'll be less cryptic. 1st, I'm no expert. 2nd, I do know that an equal amount of air has to come inside as goes out the vent hole. 3rd, it seems you are in a climate that makes you want not to lose the conditioned air (which is understandable to me). 4th, I would arrange for an equal amount of unconditioned air to be pumped inside and sent to a grid rimming around the cooktop or range, and let that air be "the chosen" air to be pumped out of the house. This is because I always feel comfortable building my own apparatus to achieve my goal. If you are the same, and you make your own air curtain, in so doing you isolate the rest of the kitchen from the fine grease particulate matter that spreads everywhere whenever a bit of grilled salmon smoke rises up and escapes the hood canopy capture area. An air curtain curtails air movement. It directs the rising steam and grease particles to continue rising in a straight line to the canopy. You create a stream. You pull air in and you pull air out. One motor (blower) for each. The unconditioned air makes a funnel around the cooking surface and channels rising air to follow it to the hood canopy area and get pulled out of the house. Without this channel, the air that gets pulled out of the house can come from any direction and that lets rising steam and grease rise up and slip away in the opposite direction. Very disorganized. HTH

I the United States the code states return vents must be at least 10 feet from any "open combustion" appliance. I don't know what Canada's code is but the key word is "OPEN" and if you're sure both the hot water heater and furnace are sealed combustion units then you are good to go with the return vent. The return vent will act just like an exhaust fan in a bathroom or kitchen and could cause a backdraft and you may die. Below is a nifty 61 second video simulating a backdraft on a hot water heater when they turn on a small exhaust fan.