I own a duplex and my tenants upstairs claim the furnace is blowing out cold air. :eek: I had the thermostat replaced and they said it helped but it still blows out cold air at times. I asked them if they had the heat up high and the furnace needs to cool down and they said no. I'm wondering if the furnace and vents need to be cleaned. If so, can I do that myself or do I have to hire someone? Where do I look to see if they do need to be cleaned?

Cleaning the air vents will not cure the problem. Check to make sure the air filter is clean. You did not give any info on what type of unit,Brand,exact model number so I can be of no further assistance.