My cold air returns have vents that can be closed but the warm air returns can't be closed. Is this right? Barry

Depends on the homeowner, but yes, technically. It's your personal preference though.. and you should have that right in your own home! Mine can't be closed, but I wanted them that way to prevent my furnace/ac from shutting down, which it will

It's usually the other way around. Cutting off the returns can cause the high limit switch to trip since there is too little air flow and the plenum temperature rises too much. Plus, you tend to get hot and cold spots. Personally I'd switch them since you can regulate the flow as needed.

Just to get this staight in my own mind, are you asking about cold air RETURNS and warm air SUPPLIES. Ive never heard of warm air RETURNS. Unless you have RETURNS on the floor and on the ceiling to open and close from summer to winter.

If you are asking about returns and supplies, I agree, returns should not have dampers, and supplies usually do, but are not necessary if the system is sized correctly. A correctly sized furnace and duct system will operate most efficiently with all returns and supplies wide open.